Honors Chemistry Lab Fall. By: Mary McHale

Transcription

1 Honors Chemistry Lab Fall By: Mary McHale

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3 Honors Chemistry Lab Fall By: Mary McHale Online: < > C O N N E X I O N S Rice University, Houston, Texas

4 This selection and arrangement of content as a collection is copyrighted by Mary McHale. It is licensed under the Creative Commons Attribution 2.0 license ( Collection structure revised: November 15, 2007 PDF generated: October 26, 2012 For copyright and attribution information for the modules contained in this collection, see p. 111.

5 Table of Contents 1 Initial Lab: Avogradro and All That Stoichiometry: Laws to Moles to Molarity VSEPR: Molecular Shapes and Isomerism Beer's Law and Data Analysis Hydrogen and Fuel Cells The Best Table in the World Bonding Solid State and Superconductors Organic Reactions Transition Metals Physical Properties of Gases Attributions

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7 Chapter 1 Initial Lab: Avogradro and All That 1 Initial Lab: Avogadro and All ThatExperiment 1 Objective The purpose of this laboratory exercise is to help you familiarize yourself with the layout of the laboratory including safety aids and the equipment that you will be using this year. Then, to make an order-of-magnitude estimate of the size of a carbon atom and of the number of atoms in a mole of carbon based on simple assumptions about the spreading of a thin lm of stearic acid on a water surface Grading Pre-lab not required for the rst lab Lab Report (90%) TA points (10%) Before coming to lab Read the following: Lab instructions Background Information Concepts of the experiment Print out the lab instructions and report form. You may ll out the lab survey, due at the beginning of the lab, for extra credit if you wish. Read and sign the equipment responsibility form and the safety rules, Ms Duval at nduval@rice.edu 2 to conrm completing this requirement by noon on August 31st Introduction Since chemistry is an empirical (experimental) quantitative science, most of the experiments you will do involve measurement. Over the two semesters, you will measure many dierent types of quantities temperature, ph, absorbance, etc. but the most common quantity you will measure will be the amount of a substance. The amount may be measured by (1) weight or mass (grams), (2) volume (milliliters or liters), or (3) determining the number of moles. In this experiment we will review the methods of measuring mass and volume and the calculations whereby number of moles are determined. Experimental Procedure We will start in the amphitheater of DBH (above DBH 180) for demonstrations: oxygen, hydrogen and a mixture of the two in balloons and more besides. 1 This content is available online at < 2 nduval@rice.edu 1

8 2 CHAPTER 1. INITIAL LAB: AVOGRADRO AND ALL THAT Mandatory Safety talk by Kathryn Cavender, Director of Environmental Health and Safety at Rice. 1. Identication of Apparatus On your benches, there are a number of dierent pieces of common equipment. With your TA's help, identify each and sketch - I know this may sound a trivial exercise but it is necessary so that we are all on the same page. 1. beaker 2. erlenmeyer ask 3. graduated (measuring) cylinder 4. pipette 5. burette 6. Bunsen burner 7. test tube 8. boiling tube 9. watch glass 2. Balance Use In these general chemistry laboratories, we only use easy-to-read electronic balances saving you a lot of time and the TA's a lot of headaches. However, it is important that you become adept at the use of them. Three aspects of a balance are important: 1. The on/o switch. This is either on the front of the balance or on the back. 2. The "Zero" or "Tare" button. This resets the reading to zero. 3. CLEANLINESS. Before and after using a balance, ensure that the entire assembly is spotless. Dirt on the weighing pan can cause erroneous measurements, and chemicals inside the machine can damage it. 4. Turn the balance on. 5. After the display reads zero, place a piece of weighing paper on the pan. 6. Read and record the mass. (2) 7. With a spatula, weigh approximately 0.2 g of a solid, common salt NaCl, the excess salt is discarded, since returning the excess salt may contaminate the rest of the salt - in this exercise, this is not a big deal but in strict analytical procedures it is. 8. Record the mass (1). To determine how much solid you actually have, simply subtract the mass of the weighing paper(2) from the mass of the weighing paper and solid (1). Record this mass (3).You have just determined the mass of an "unknown amount of solid." 9. Now place another piece of weighing paper on the balance and press the Zero or Tare button then weigh out approximately 0.2 g of the salt (4). Thus, the zero/tare button eliminates the need for subtraction. 3. Measuring the volume of liquids When working with liquids, we usually describe the quantity of the liquid in terms of volume, usual units being milliliters (ml). We use three types of glassware to measure volume (1) burette, (2) volumetric pipette, and (3) graduated cylinder. Examine each piece of equipment. Note that the sides of each are graduated for the graduated cylinder and the burette. You can read each to the accuracy of half a division. Put some water into the graduated cylinder. Bend down and examine the side of the water level. Note that it has a "curved shape." This is due to the water clinging to the glass sides and is called the meniscus. When reading any liquid level, use the center of the meniscus as your reference point. Graduated cylinder 1. Look at the graduations on the side of the cylinder. Note that they go from 0 on the bottom and increase upwards. Thus, to get the mass of 10 ml of a liquid from a graduated cylinder, do the following:

9 3 2. Add water up to the 10 ml line as accurately as possible. 3. Dry a small beaker and weigh it (2). 4. Pour the 10 ml of water from the cylinder into the beaker. Reweigh (1). 5. Subtract the appropriate values to get the weight of the water (3). Pipette 1. You may nd either that 0 is at the spout end or at the top of the pipette. You should be aware of how these graduations go when using each pipette. Thus, to get the mass of 10 ml of a liquid from a pipette, do the following: 2. Half-ll a beaker with water. 3. Squeeze the pipette bulb and attach to the top of the pipette. Put the spout of the pipette under water and release the bulb. It should expand, drawing the water into the pipette, do not let the water be drawn into the bulb. 4. When the water level is past the last graduation, remove the bulb, replace with your nger, and then remove the pipette from the water. 5. Removal of your nger will allow liquid to leave the pipette. Always run some liquid into a waste container in order to leave the level at an easy-to-read mark. 6. Add 10 ml of water to a pre-weighed dry beaker (5). 7. Weigh (4). 8. Subtract to get the weight of the water (6). Burette 1. Examine the graduations. Note that 0 is at the top. 2. Using a funnel, add about 10 ml of water. To do this, rst lower the burette so that the top is easy to reach. 3. Run a little water from the burette into a waste container. Then turn the burette upside down and allow the rest of the water to run into the container (you will have to open the top to equalize the pressure). 4. You have just "rinsed your burette." This should be done every time before using a burette rst rinse with water, then repeat the process using whatever liquid is needed in the experiment. 5. Fill the burette to any convenient level (half-way is ne). It is a good technique to "overll" and then allow liquid to run into a waste container until you reach the appropriate level so that you ll the space from the top to the tip of the burette. 6. Dry a beaker and weigh (8). 7. Add 10 ml of water to a pre-weighed dry beaker (7). 8. Subtract to get the weight of the water (9). 4. Estimation of Avogadro's number Briey, as a group with your TA, you will make an approximate (order of magnitude) estimate of Avogadro's number by determining the amount of stearic acid that it takes to form a single layer (called a monolayer) on the surface of water. By making simple assumptions about the way the stearic acid molecules pack together to form the monolayer, we can determine its thickness, and from that thickness we can estimate the size of a carbon atom. Knowing the size of a carbon atom, we can compute its volume; and if we know the volume occupied by a mole of carbon (in the form of a diamond), we can divide the volume of a mole of carbon by the volume of an atom of carbon to get an estimate of Avogadro's number. Procedure Special Supplies: 14 cm watch glass; cm ruler; polyethylene transfer pipets; 1-mL syringes; pure distilled water free of surface active materials; disposable rubber gloves (for cleaning own watch glasses in 0.1 M NaOH in 50:50 methanol/water): 13 X 100 mm test tubes with rubber stoppers to t. Chemicals: pure hexane, g/l stearic acid (puried grade) solution in hexane. 0.1 M NaOH in 50:50 methanol/water used for washing the watch glasses.

10 4 CHAPTER 1. INITIAL LAB: AVOGRADRO AND ALL THAT SAFETY PRECAUTIONS: Hexane is ammable! There must be no open ames in the laboratory while hexane is being used. WASTE COLLECTION: At the end of the experiment, unused hexane solvent and stearic acid in hexane solution should be placed in a waste container, marked "Waste hexane/stearic acid solution in hexane." Measurement of the volume of stearic acid solution required to cover the water surface Your TA will do this as a group demonstration: 1. Using a transfer pipette, obtain about 3-4 ml g/l stearic acid solution in hexane in a clean, dry 13 X 100 mm test tube. Keep the tube corked when not in use. 2. Fill the clean watch glass to brim with deionized water. One recommended way to do this is to set up your 25 ml burette on a ring stand. Wash and drain the burette with deionized water. (the deionized water comes from the white handled spouts at each sink) 3. In a freshly cleaned and rinsed beaker, obtain more distilled water and ll the burette. Place your watch glass directly under the burette (about 1 inch or less from the tip) and dispense the water until the entire watch glass is full. You may have to rell the burette 4 or 5 times to do this. With careful dispensing, the surface tension of the water should allow you to ll the entire watch glass with relative ease. 4. Carefully measure the diameter of the water surface with a centimeter ruler. It should be close to 14 cm, + or - a couple of millimeters. Next, rinse and ll your 1 ml syringe with stearic acid solution, taking care to eliminate bubbles in the solution inside the syringe. 5. Read and record the initial volume of the syringe (1 ml is always a good place to start.) 6. Then add the stearic acid solution drop by drop to the water surface. Initially, the solution will spread across the entire surface, and it will continue to do so until a complete monolayer of stearic acid has been formed. If your rst few drops do not spread and evaporate quickly, either your water or watch glass is still dirty. As this point is approached, the spreading will become slower and slower, until nally a drop will not spread out but will instead sit on the surface of the water (looking like a little contact lens). If this "lens" persists for at least 30 s, you can safely conclude that you have added 1 drop more than is required to form a complete monolayer. 7. Now, read and record the nal volume reading of the syringe.takes 10 min 8. Thoroughly clean the watch glass (or get another one), and repeat the experiment. Repeat until the results agree to within 2 or 3 drops (0.04 ml). When you have completed all of your measurements, rinse your syringe with pure hexane, and dispose of all the hexane-containing solutions in the waste collection bottle provided. Calculation Of Avogadro's Number The calculation proceeds in several steps. We calculate the volume of stearic acid solution in hexane required to deliver enough stearic acid to form a monolayer. All of the hexane evaporates, leaving only the thin monolayer lm of stearic acid, so we next calculate the actual mass of pure stearic acid in the monolayer. We calculate the thickness of the stearic acid monolayer, using the known density of stearic acid and the area of the monolayer. Assuming the stearic acid molecules are stacked on end and are tightly packed, and knowing that there are 18 carbon atoms linked together in the stearic acid molecule, calculate the diameter and volume of a carbon atom. Calculate the volume of a mole of carbon atoms in diamond; divide the molar volume of carbon (diamond) by the volume of a single carbon atom to obtain an estimate of Avogadro's number. Remember that the units of Avogadro's number are mol-1, so you can use unit analysis to check your answer. Initial Lab: Avogadro and All ThatReport 1

11 Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students' work (including your laboratory partner) or misrepresent your own data (see honor code). Name(Print then sign): Lab Day: Section: TA Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students' work (including your laboratory partner) or misrepresent your own data (see honor code). Demonstrations: Balloons: 1. Oxygen 5 1. Hydrogen 2. Mixture of Hydrogen and Oxygen with relevant equation: H 2 + O 2 Thermite: Include description and relevant equation: Fe 2 O 3 + Al Dry Ice and Magnesium: Include description and relevant equation: MgO + C 1. Identication of Apparatus 1. beaker 1. erlenmeyer ask 1. graduated (measuring) cylinder 1. pipette 1. burette 1. Bunsen burner 1. test tube 2. watch glass 2. Balance Use 1. Mass of weighing paper and solid, g 2. Mass of weighing paper, g

12 6 CHAPTER 1. INITIAL LAB: AVOGRADRO AND ALL THAT 3. Mass of solid, g 4. Mass of solid on tared weighing paper g 3. Measuring the volume of a liquid 1. Mass of 50 ml beaker and water, g 2. Mass of 50 ml beaker, g 3. Mass of water from graduated cylinder, g 4. Mass of 50 ml beaker and water, g 5. Mass of 50 ml beaker, g 6. Mass of water from pipette, g 7. Mass of 50 ml beaker and water, g 8. Mass of 50 ml beaker, g 9. Mass of water from burette, g From a consideration of the masses of water measured above, and given that the density of water is 1 g/ml, decide on an order of which is the most accurate method of volume measurement measuring cylinder, pipette, or burette with (1) being the most accurate? (1) (2) (3) How precisely could each of the apparatus used be read? (1) measuring cylinder (2) pipette (3) burette 4. Estimation of Avogadro's Number Measurement of the volume of stearic acid solution required to cover the water surface Record the diameter of the water surface Record the volume of stearic acid solution required to cover the surface Record the concentration of the stearic acid solution Trial 1 Trial 2 Table 1.1 Calculation Of Avogadro's Number a. Calculation of the thickness of a monolayer of stearic acid Trial 1 Trial 2 continued on next page

13 7 From your data, the volume of stearic acid solution required to form a monolayer was Calculate the mass of stearic acid contained in that volume of stearic acid solution (the concentration in grams per liter will be given to you) Calculate the volume, V, of pure stearic acid in the monolayer on the water surface. You will need the density of solid stearic acid, which is 0.85 g/ml (or g/cm 3 ). Calculate the area of the monolayer (A = πr 2, r is the radius of the water surface) Calculate the thickness of the monolayer (t = Volume/Area) Table 1.2 b. Estimation of the size and volume of a carbon atom A stearic acid molecule consists of 18 carbon atoms linked together. Assuming that the thickness, t, of a monolayer is equal to the length of the stearic acid molecule, calculate the size of a carbon atom, s = t/18 Assuming that a carbon atom is a little cube, calculate the volume of a carbon atom, volume = s 3 Trial 1 Trial 2 Table 1.3 c. Calculation of the volume of a mole of carbon atoms Trial 1 Trial 2 continued on next page

14 8 CHAPTER 1. INITIAL LAB: AVOGRADRO AND ALL THAT Calculate the molar volume of carbon (diamond) by ( using the density of diamond 3.51g/cm 3 ) and the atomic mass of a mole of carbon Is the volume of a mole of diamond the same as the actual volume of a mole of carbon atoms? Table 1.4 d. Calculation of the volume of a mole of carbon (diamond) volume of a single carbon atom (Avogadro's number) Calculate Avogado's number (NA) from the appropriate ratio of volumes Calculate the average value of NA from your results Express your results as a number Are you within a power of 10 of the accepted value of ? Trial 1 Trial 2 Table 1.5

15 Chapter 2 Stoichiometry: Laws to Moles to Molarity Experiment 2: Stoichiometry: Laws to Moles to Molarity Objective To determine the mass of a product of a chemical reaction To make a solution of assigned molarity your accuracy will be tested by your TA by titration! Grading Pre-lab (10%) Lab Report (80%) TA points (10%) 1 This content is available online at < 9

16 10 CHAPTER 2. STOICHIOMETRY: LAWS TO MOLES TO MOLARITY Before Coming to Lab.. Read the lab instructions Complete the pre-lab, due at the beginning of the lab Introduction The word stoichiometry derives from two Greek words: stoicheion (meaning "element") and metron (meaning "measure"). Stoichiometry deals with calculations about the masses (sometimes volumes) of reactants and products involved in a chemical reaction. Consequently, it is a very mathematical part of chemistry. In the rst part of this lab, sodium bicarbonate is reacted with an excess of hydrochloric acid. NaHCO 3 (s) + HCl (aq) NaCl (aq) + CO 2 (g) + H 2 O By measuring the mass of NaHCO 3 and balancing the equation (above), the mass of NaCl expected to be produced can be calculated and then checked experimentally. Then, the actual amount of NaCl produced can be compared to the predicted amount. This process includes molar ratios, molar masses, balancing and interpreting equations, and conversions between grams and moles and can be summarized as follows: 1. Check that the chemical equation is correctly balanced. 2. Using the molar mass of the given substance, convert the mass given in the problem to moles. 3. Construct a molar proportion (two molar ratios set equal to each other). Use it to convert to moles of the unknown. 4. Using the molar mass of the unknown substance, convert the moles just calculated to mass. In the second part of this lab, since a great deal of chemistry is done with solutions, a solution will be prepared of allocated molarity. Molarity, or more correctly molar concentration, is dened to be the number of moles of solute divided by the number of liters of solution: c M = n substance V solution with units of [mole/l]. However molar concentration depends on the temperature so a higher temperature would result in an increased volume with a consequential decrease in molar concentration. This can be a signicant source of error, of the same order as the error in the volume measurements of a burette, when the temperature increases more than 5 º C. Steps to preparing a solution of a certain concentration: 1. First need to know the formula for the solute, e.g. potassium chromate: K 2 CrO Need the molecular weight of the solute: by adding up the atomic weights of potassium, chromium and oxygen: 39.10, and in the correct ratios: , 52.0 and = 194.2g/mole. 4. Then the volume of solution, usually deionised water: e.g. for one liter of solution use a 1000 ml volumetric ask. So a 1M solution would require 194.2g of solid K 2 CrO 4 in 1 L, 0.1M 19.42g of solid K 2 CrO 4 and so on. 5. Remember to ensure that all the solute is dissolved before nally lling to the mark on the volumetric ask. If there is any undissolved solute present in the solution, the water level will go down slightly below the mark, since the volume occupied by the solute diers from the actual volume it contributes to the solution once it is dissolved. Your teaching assistant will check the accuracy of the solution that you have made by titration, which is a method of quantitatively determining the concentration of a solution. A standard solution (known concentration) is slowly added from a burette to a solution of the analyte (unknown concentration your solution) until the reaction between them is judged to be complete equivalence point). In colorimetric titration, some indicator must be used to locate the equivalence point. One example is the addition of acid to base using phenolphthalein (indicator) to turn a pink solution colorless in order to determine the

17 concentration of unknown acids and bases. Record your TAs value of the molarity of your solution on your report form along with your percent error. 11 Figure 2.1 Figure 1: Reading the Burette When an acid is neutralized by a base, since there is stoichiometrically equal amounts of acid and base and the ph = 7, it is possible to accurately determine the concentration of either the acid or base solution. Since: Moles of a substance = Concentration of solution (moles/l) x Volume (L) We can calculate the concentration of the acid or base in the solution using: Balance Base [U+E09E] Bb [U+E09F] Moles of Acid = Moles of Base Balance Acid [U+E09E] Ba [U+E09F] B b C a V a = Ba C b V b Titration Calculations: Step 1:Balance the neutralization equation. Determine Balance of Acid and Base. Step 2:Determine what information is given. Step 3:Determine what information is required. Step 4:Solve using the equation below. B b C a V a = Ba C b V b Example: Calculate the concentration of a nitric acid solution HNO 3 if a 20 ml sample of the acid required an average volume of 55 ml of a mol/l solution of Ba[U+E09E]OH[U+E09F] 2 to reach the endpoint of the titration.

18 12 CHAPTER 2. STOICHIOMETRY: LAWS TO MOLES TO MOLARITY Step 1: 2HNO 3 + Ba[U+E09E]OH[U+E09F] 2 Ba[U+E09E]NO 3 [U+E09F] 2 + 2H 2 OBalance Base = 1Balance Acid = 2 Step 2:Given informationvolume Acid = 20 mlvolume Base (average) = 55 ml Concentration of Base = mol/l Step 3: Required informationconcentration of AcidStep 4:Solve using the equation. B b C a V a = Ba C b V b 1 Ca 20ml = mol/1 55ml Ca = mol/l ( considering signicant gures 0.26 mol/l) Experimental Materials List sodium bicarbonate [U+E09E]NaHCO 3 [U+E09F] 3M hydrochloric acid (HCl) solution Procedure Part 1 1. Weigh an empty 150-mL beaker on the electronic balance. Record this value in your data table. 2. Remove the beaker from the balance and add one spoonful of sodium bicarbonate (approximately 5 g). Re-weigh and record this value. 3. Pour approximately 20 ml of 3M hydrochloric acid into a 100-mL beaker. Rest a Pasteur pipette in the beaker. 4. Add 3 drops of acid to the NaHCO 3 beaker, moving the pipette so that no drops land on each other. The key point is to spread out the adding of acid so as to hold all splatter within the walls of the beaker. 5. Continue to add acid slowly drop by drop. As liquid begins to build up, gently swirl the beaker. This is done to make sure any unreacted acid reaches any unreacted sodium bicarbonate. Do not add acid while swirling. 6. Stop adding the hydrochloric acid when all bubbling has ceased. So that the minimum amount of HCl has reacted with all of the sodium bicarbonate. Check when all the bubbling has ceased, by swirling the beaker and to ensure that there is no more bubbling. When all the bubbling has ceased, add one drop more of acid and swirl. 7. Weigh the beaker and contents, record. 8. Using a microwave oven, dry to constant weigh, initially for 1 min, when there is plenty of solution, and then 10 second intervals thereafter. Measure weight to the nearest milligram Materials List 100 mls volumetric ask 3M hydrochloric acid (HCl) solution sodium bicarbonate [U+E09E]NaHCO 3 [U+E09F] methyl orange indicator Part 2 1. Ask you TA for your assigned molarity it will range from 0.7 M to 1.2 M. 2. First need to know the formula for the solute. 3. Need the molecular weight of the solute in g/mole. 4. The volume of solution, 100 mls. 5. Remember to ensure that all the solute is dissolved before nally lling with deionised water to the mark on the volumetric ask.

19 6. Take your solution to your TA to check the molarity by titration, record value on your report form and your percent error. 13

20 14 CHAPTER 2. STOICHIOMETRY: LAWS TO MOLES TO MOLARITY 2.2 Pre-Lab 2: Stoichiometry (Total 10 points) Click here 2 to print the Pre-Lab Note: In preparing this Pre-Lab you are free to use references and consult with others. However, you may not copy from other students' work (including your laboratory partner) or misrepresent your own data (see honor code). Name(Print then sign): Lab Day: Section: TA Circle the correct answer: 1) Which one of the following is a correct expression for molarity? A) mol solute/l solvent B) mol solute/ml solvent C) mmol solute/ml solution D) mol solute/kg solvent E) µmol solute/l solution 2) What is the concentration (M) of KCl in a solution made by mixing 25.0 ml of M KCl with 50.0 ml of M KCl? A) B) C) D) E) 125 3) How many grams of CH 3 OH must be added to water to prepare 150 ml of a solution that is 2.0 M CH 3 OH? A) B) C) 2.4 D) 9.6 E) 4.3 4) The concentration of species in 500 ml of a M solution of sodium sulfate is M sodium ion and M sulfate ion. A) 2.104, B) 2.104, C) 2.104, D) 1.052, E) 4.208, ) Oxalic acid is a diprotic acid. Calculate the percent of oxalic acid H 2 C 2 O 4 in a solid given that a g sample of that solid required ml of M NaOH for neutralization. A) B) C) D) E) ) A 31.5 ml aliquot of H 2 SO 4 (aq) of unknown concentration was titrated with M NaOH (aq). It took 23.9 ml of the base to reach the endpoint of the titration. The concentration (M) of the acid was. A) B) C) D)

21 15 E) ) What are the respective concentrations (M) of Fe 3+ and I aorded by dissolving mol FeI 3 in water and diluting to 725 ml? A) and B) and C) and D) and E) and ) A 36.3 ml aliquot of M H 2 SO 4 (aq) is to be titrated with M NaOH (aq). What volume (ml) of base will it take to reach the equivalence point? A) 93.6 B) 46.8 C) 187 D) 1.92 E) ) A 13.8 ml aliquot of M H 3 PO 4 (aq) is to be titrated with M NaOH (aq). What volume (ml) of base will it take to reach the equivalence point? A) 7.29 B) 22.1 C) 199 D) 66.2 E) ) A solution is prepared by adding 1.60 g of solid NaCl to 50.0 ml of M CaCl 2. What is the molarity of chloride ion in the nal solution? Assume that the volume of the nal solution is 50.0 ml. A) B) C) D) E) Report 2: Stoichiometry (Total 80 points) Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students' work (including your laboratory partner) or misrepresent your own data (see honor code). This is only an advisory template of what needs to be include in your complete lab write-up. Name(Print then sign): Lab Day: Section: TA Part Data Table Mass empty 150-mL beaker NaHCO 3 in beaker Grams Mass of NaHCO 3 Table 2.1

22 16 CHAPTER 2. STOICHIOMETRY: LAWS TO MOLES TO MOLARITY Mass NaCl plus beaker rst weighing NaCl plus beaker second weighing NaCl plus beaker third weighing Grams Table 2.2 1) The grams of NaHCO 3 you had in your beaker was 2) Calculate how many moles of NaHCO 3 the mass is 3) Write the molar ratio for the NaHCO 3 / NaCl ratio 4) Write the number of moles of NaCl you predict were produced in your experiment. 5) Calculate the mass of NaCl you predict will be produced. 6) Determine, by subtraction, the actual mass of NaCl produced in your experiment. a) rst weighing b) second weighing c) third weighing 7) Calculate your percentage yield Discussion Questions 1. Compare the numerical value of the observed ratio for maximum yield to the best ratio Part 2 Record your TAs value of the molarity of your solution. Calculate your percent error from your assigned value. Complete the equation for the titration of NaHCO 3[U+E09E]aq[U+E09F] + HCl [U+E09E]aq[U+E09F]

23 Chapter 3 VSEPR: Molecular Shapes and Isomerism 1 Molecular Shapes & Isomerism Objectives Understand the 3-dimensional nature of molecules Learn about Molecular Symmetry Be able to identify the various isomers possible for one molecular formula Be able to identify enantiomers 3.1 Grading Quiz (10%). Lab Report Form (90%). Before Coming to Lab... Look over the following to make sure you have a basic understanding of the topics presented. Drawing Lewis Structures Determining the Shapes of Molecules from their Lewis Structures Some Basic Aspects of Bonding Model Kits Introduction The shape of a molecule is extremely important in determining its physical properties and reactivity. A multitude of shapes are possible, and in today's lab, you will be looking at several. In Part 1, you will be exploring the various symmetry elements that can be present in molecules. The symmetry elements you will be looking for are mirror planes, rotation axes, and inversion centers. Being able to determine which symmetry elements are present in a molecule help in understanding its chemistry. If there is a plane present in the molecule that has the exact same arrangement of atoms on either side of the plane, then the molecule has a mirror plane (σ). It is important to note that a molecule can have more than one mirror plane. Rotation axes are represented as Cn (n = 1, 2, 3...). The subscript indicates how many degrees of rotation (360 o /n) are needed in order to return to the same orientation of atoms with which you started. So if there is a C 2 axis, the rotation would be 180 o. An example of a molecule having a C 2 axis is H 2 O. 1 This content is available online at < 17

24 18 CHAPTER 3. VSEPR: MOLECULAR SHAPES AND ISOMERISM Figure 3.1 The third symmetry element is an inversion center (i). In such molecules, starting at any position and drawing a line through the center and an equal distance to the opposite side of the molecule, you will end up at a position with an identical environment to the one you started from. Figure 3.2 Part 2 of the lab introduces the concept of enantiomers. Enantiomers are molecules sharing the same molecular conguration, but they are non-superimposable images of each other. This concept should become clearer as you build the models for this part of the lab. Enantiomers share many of the same physical properties. The property which distinguishes them is the direction in which they rotate plane-polarized light. They will rotate the light in equal amounts but in dierent directions ( plane-polarized light is just light in which all wave vibrations have been ltered out except for those in one plane ). If both enantiomers are present in a 1:1 ratio, the eects of the rotation of light cancel and no net rotation is observed. Such a

25 mixture of isomers is known as a racemic mixture or as a racemate. Because these isomers rotate planepolarized light, they are also known as optical isomers. Compounds that form optical isomers are said to be chiral. The chemistry of enantiomers is of great importance in the eld of medicine. It has been discovered that with many drugs, one enantiomer will be biologically active while the other will be inactive or even produce undesired side eects. For this reason, it has become advantageous for pharmaceutical companies to try to synthesize the active enantiomer exclusively. The next part of the lab deals with isomers. Isomers are molecules having the same molecular formula, but the atoms are arranged in a dierent manner, while still obeying the rules of bonding. There are dierent classications for isomers. For example, structural isomers dier from one another in the order in which the atoms are bound to each other (connectivity is dierent). On the other hand, geometrical isomers have the same order of atoms, but the spatial arrangement of atoms is dierent (connectivity is the same). A common example of geometrical isomers is the cis and trans forms of double bonds: 19 Figure 3.3 ** NOTE: Remember that molecules having single carbon-carbon bonds cannot have cis/trans isomers because there is free rotation about single bonds. By building the models of various molecules during this lab, you will be able to better understand molecular symmetry and isomers. Building models is not dicult; however, the chemical principles involved are very important and you may nd some surprises in how atoms can be t together. Finally, in Part 4, you will be applying your knowledge of VSEPR (Valence Shell Electron Pair Repulsion) Theory in order to determine the geometry of several dierent molecules. VSEPR theory is useful in helping to determine how atoms will orient themselves in molecules. Basically, the idea is that the arrangement adopted by a molecule will be the one in which the repulsions among the various electron domains are minimized. The two kinds of electron domains are bonding (electron pair shared by two atoms) and nonbonding (electron density centralized on one atom) pairs of electrons. Experimental Procedure For Parts 1 & 2: You and your lab partner are to work with one other lab group in preparing these models (no more than 3-4 students). Your TA will assign each group a certain set of molecules to make and answer questions pertaining to those molecules. Each group will then present their answers to the class. These models will need to be completed and answers determined within 30 minutes so that we can continue to Parts 3 & 4 as soon as possible. For Parts 1-4, the work should be divided among the group members. Be sure to discuss the questions and answers among yourselves, but put your own conclusions on the Report Form.

26 20 CHAPTER 3. VSEPR: MOLECULAR SHAPES AND ISOMERISM 1. Symmetry Elements Using the Molecular Framework models, make models of the following compounds: a. CH 4 b. CH 3 Cl c. CH 2 Cl 2 d. CHCl 3 e. CH 2 ClF f. CHBrClF g. BF 3 h. BF 2 Cl i. PH 3 j. PH 2 Cl Choose a color to represent each atom. For example, make all C atoms black, all H atoms white, etc. Once the models are created, look for symmetry elements that may be present. Ask yourselves the following questions: Does the molecule contain a mirror plane (σ)? In other words, is there a plane within the molecule which results in one half being a mirror image of the other half? Does the molecule contain a two-fold rotation axis (C2)? Remember from the Introduction that the subscript indicates the degrees of rotation necessary to reach a conguration that is indistinguishable from the original one. In this case, the rotation will be 180 o. Does the molecule contain any higher-order rotation axes? C3 rotation by 120 o C4 rotation by 90 o C (innity rotation axis) rotation of any amount will result in an indistinguishable orientation Does the molecule have an inversion center (i)? Determine which of these symmetry elements are present in your assigned molecules. All of the columns of the table on the report form should be lled out. If you have any diculty determining whether such symmetry elements are present in the molecules you are assigned, your TA can provide examples of each symmetry element. Extra credit points can be earned by indicating in the table how many of each symmetry element are present for each molecule (i.e. How many mirror planes are present?). 2.Mirror Images Using the model kits, build models which are the mirror images of the models you were assigned to build (b, c, d, e, f, g, h, i and j) in Part 1. With the two mirror images in hand, try to place the models on top of one another, atom for atom. If you can do this, the model and its mirror image are superimposable mirror images of one another. If not, the molecule and its mirror image form nonsuperimposable mirror images. Nonsuperimposable mirror images are also known as enantiomers. For each compound, decide whether the mirror image is superimposable or nonsuperimposable. Can you make a generalization about when to expect molecules to have nonsuperimposable mirror images? 3.Isomers In this exercise you will build models of compounds which are structural and/or geometrical isomers of one another. Make the following models: A. Structural Isomers 1. Make a model(s) of C 2 H 5 Cl. How many dierent structural isomers are possible?

27 21 2. Make a model(s) of C 3 H 7 Cl. How many dierent structural isomers are possible? 3. Make a model(s) of C 3 H 6 Cl 2. How many dierent structural isomers are possible? B. Geometrical Isomers 1. Make a model(s) of C 2 H 3 Cl. How many dierent structural and geometrical isomers are possible? 2. Make a model(s) of C 2 H 2 Cl 2. How many dierent structural and geometrical isomers are possible? 3. Make a model(s) of cyclobutane (C 4 H 8 ). HINT: cyclo = ring of C atoms 4. Now make dichlorocyclobutane (C 4 H 6 Cl 2 ) by replacing two H atoms on cyclopropane with Cl atoms. How many dierent structural and geometrical isomers are possible for dichlorocyclobutane? You may wish to make a couple of cyclobutane molecules so that you can compare the structures. Do any of the isomers have nonsuperimposable mirror images? C. Aromatic Ring Compounds 1. Make a model of benzene, C 6 H 6. Even though your model will contain alternating double and single bonds, remember that in the real molecules of benzene all the C-C bonds are equivalent. What symmetry elements does benzene possess? 2. Make a model(s) of chlorobenzene, C 6 H 5 Cl. How many dierent structural and geometrical isomers are possible? 3. Make a model(s) of dichlorobenzene, C 6 H 4 Cl 2. How many dierent structural and geometrical isomers are possible? 4. Make a model(s) of trichlorobenzene, C 6 H 3 Cl 3. How many dierent structural and geometrical isomers are possible? 4. Hypervalent Structures Hypervalent compounds are those that have more than an octet of electrons around them. Such compounds are formed commonly with the heavier main group elements such as Si, Ge, Sn, Pb, P, As, Sb, Bi, S, Se, Te, etc. but rarely with C, N or O. Refer to the rules for Electron Domain theory in order to assign Lewis structures to the following molecules. Describe possible isomeric forms and the bond angles between the atoms. How many lone pairs of electrons are present on the central atom of each molecule, if any? (** Your book will be very useful in aiding you with these structures **) a. PF 5 b. PF 3 Cl 2 c. SF 4 d. XeF 2 e. BrF 3

28 22 CHAPTER 3. VSEPR: MOLECULAR SHAPES AND ISOMERISM

29 Chapter 4 Beer's Law and Data Analysis1 4.1 Beer's Law and Data Analysis Objectives Learn or review typical data analysis procedures plotting data with excel, performing linear regression analysis, etc. Explore the concepts and applications of spectrophotometry Grading Pre-lab (10%) Lab Report Form including plot (80%) TA points + Pop Quiz (10%) Before coming to lab... Read the lab instructions Print out the lab instructions and report form. Complete the pre-lab, due at the beginning of the lab Introduction When describing chemical compounds, scientists rely on their chemical and physical properties. In lab, we might observe that a metal reacts violently with water, that a reactant is liquid at room temperature, or that a powder is yellow. Chemical and physical properties can be used qualitatively to identify a material or to predict its behavior, or quantitatively to determine how much of that material is present in a solution. In this lab, we will develop a scheme to determine the concentration of copper sulfate in aqueous solution using spectrophotometry. To start, we will consider light and its interaction with matter. Chemicals exhibit a diverse range of colors, especially when they contain transition metal ions. In order for a compound to have color, it must absorb visible light. Visible light consists of electromagnetic radiation with wavelengths ranging from 1 This content is available online at < 23

30 24 CHAPTER 4. BEER'S LAW AND DATA ANALYSIS approximately 400 nm to 700 nm, a small section of the electromagnetic radiation spectrum shown below. Light is characterized by its frequency ( ν), the number of times the crest of the wave passes some point in space per second, or by its wavelength ( λ), the distance between two successive crests. These two quantities are related by the speed of light, a fundamental constant: λν = c = m/s. Planck related the frequency of light to its energy (E) according to E = hν, where h is Planck's constant, h = J/s. A compound will absorb light when the radiation posesses the energy needed to move an electron from its lowest energy (ground) state to some excited state. The particular energies of radiation that a substance absorbs dictate the colors that it exhibits. Conversely the color of a compound can help us to determine its electronic conguration. White light contains all wavelengths in this visible region. When a transparent sample (like most aqueous solutions) absorbs visible light, the color we perceive is the sum of the remaining colors that are transmitted by the object and strike our eyes.

31 25 If an object absorbs all wavelengths of visible light, none reaches our eyes, and it appears black. If it absorbs no visible light, it will look white or colorless. If it absorbs all but orange, the material will appear orange. We also perceive an orange color when visible light of all colors except blue strikes our eyes. Orange and blue are complementary colors; the removal of blue from white light makes the light look orange, and vice versa. Thus, an object has a particular color for one of two reasons: It transmits light of only that color or it absorbs light of the complementary color.

32 26 CHAPTER 4. BEER'S LAW AND DATA ANALYSIS Figure 4.1 Complementary colors can be determined using an artist's color wheel. The wheel shows the colors of the visible spectrum, from red to violet. Complementary colors, such as orange and blue, appear as wedges opposite each other on the wheel. With our eye, we can make qualitative judgments about the color(s) of light a sample absorbs. However, given a red solution of [Ti (H 2 O) 6 ] 3+ we can not determine if it absorbs green light or if it absorbs all colors of light but red. To quantitatively determine the amount of light absorbed by a sample as a function of wavelength, we will measure its absorption spectrum using a UV-visible spectrophotometer. Typical absorption spectra of aqueous [Ti (H 2 O) 6 ] 3+ solutions are shown below.

33 27 Notice the absorption maximum is at 490 nm. Because the sample absorbs more strongly in the green and yellow regions of the visible spectrum, it appears red-violet. Measuring the absorption spectrum of a second, more dilute solution demonstrates that the spectrum changes as a function of the concentration of the solution. To understand how to use the absorption spectrum as a quantitative tool for chemical analysis, read on! Spectrophotmetric Basics The essential components of a spectrophotometer consist of a radiation source, a wavelength selector (monochromator), a photodetector and read-out device.

34 28 CHAPTER 4. BEER'S LAW AND DATA ANALYSIS Figure 4.2 The incident light from a tungsten (visible light source) or deuterium (UV light source) lamp is focused by a lens and passes through an entrance slit. By passing the beam through the monochromator (either a prism or a diraction grating) it is separated into monochromatic (i.e., one-color or single-wavelength) light. One particular wavelength of monochromatic light is selected and allowed to pass through the exit slit into the sample. Light transmitted through the sample is detected by a photodetector which converts the signal to an electrical current which is measured by a galvanometer and sent to a recording device, typically a computer. The measurement of transmittance (T) is made by determining the ratio of the intensity of incident ( I 0 ) and transmitted (I) light passing through pure solvent and sample solutions as a function of wavelength. [Note: The percent transmittance (%T) is obtained by multiplication of T by 100.] The logarithm of the reciprocal of the transmittance is called the absorbance (A), A = log (1 / T) Care must be taken when small values of transmittance are being measured as stray light from either the room or scattering within the instrument can cause large errors in your readings! Extracting Quantitative Information The Beer-Lambert law relates the amount of light being absorbed to the concentration of the substance absorbing the light and the pathlength through which the light passes: A = εbc. In this equation, the measured absorbance (A) is related to the molar absorptivity constant ( ɛ), the path length (b), and the molar concentration (c) of the absorbing. The concentration is directly proportional to absorbance. The single largest application of the spectrophotometer is for quantitative analysis. The prerequisite for such analysis is a known absorption spectrum of the compound under investigation. Of particular importance is the maximum absorption (at λ max ) [Why choose the maximum? Could the choice alter the precision of our experiment? the accuracy?], which can be easily obtained by plotting absorbance vs. wavelength at a xed concentration. Next, a series of solutions of known concentration are prepared and their absorbance is

35 measured at λ max. Plotting absorbance vs. concentration, a calibration curve can be determined and t using linear regression (least-squares t). An unknown concentration can be deduced by measuring absorbance at the absorption maximum and comparing it to the standard curve. Caution: The Beer-Lambert Law is only obeyed (the standard curve is linear) for reasonably dilute solutions. Only those points in the linear range of the standard curve may be used for accurate concentration determination. Typical results are shown for the absorbance of [Ti (H 2 O) 6 ] 3+ measured at 490 nm. 29 Concentration (mg/ml) %Transmittance Absorbance Table 4.1 Figure 4.3 Over the studied range the solutions obey Beer's Law. If a solution has a measured absorbance of 0.450, we can calculate its concentration to be 1.5 mg/ml. 4.2 Experimental Procedure In this experiment, each lab pair will measure the absorbance of CuSO 4 at six concentrations. You will create a calibration curve to correlate copper sulfate concentration to absorbance. This curve will be used next week to determine the concentration of an unknown copper sulfate solution and, in turn, the percent yield of a series of chemical reactions. Materials CuSO 4 5H 2 O distilled water pipette bulb 1cm cuvette 4-25 ml volumetric ask for your dilutions Note: You will be borrowing these and must collect them from your TA. Do not forget to return the ask at the end of the lab). All students will lose 3 points in that lab section if any go missing! 100 ml volumetric ask for the parent solution (in your drawer)10 ml volumetric pipette or 10 ml graduated cylinder

36 30 CHAPTER 4. BEER'S LAW AND DATA ANALYSIS 1. Measure out an appropriate mass of CuSO 4 5H 2 O to get 100ml of 0.1M solution and record the mass on your report form. Show your calculation to the TA before making the solution. This is your parent solution. Calculate the molarity using the actual mass measured and record it. 2. Do the following dilutions and calculate the concentrations for each. Dilution (ml parent : ml total) 0:25 (DI H 2 O) 5:25 10:25 15:25 20:25 25:25 (parent solution) Table Measure the absorbance of the 6 solutions you have prepared and the unknown given to you by your TA Analysis Plot the concentration as a function of absorbance for your six solutions. Perform a linear regression analysis and determine the equation of a best-t line.

37 PreLab: Spectrophotometry and Data Analysis Beer's Law Hopefully here 2 for the Pre-Lab Name(Print then sign): Lab Day: Section: TA This assignment must be completed individually and turned in to your TA at the beginning of lab. You will not be allowed to begin the lab until you have completed this assignment. In many of the experiments that you will do throughout the duration of this course you will be asked to analyze your data by making plots and calculating the best t line through your data. One program commonly used to analyze data in this fashion is Microsoft Excel. The following exercise will help you through the process used to obtain a plot and linear regression for a set of data. Suppose you go for a 5 mile run and you tabulate the after each mile as in the following table. Distance Traveled (miles) Time (sec) Table Questions: 1. Plot the distance traveled in miles vs. the time in seconds. 2. Use linear regression to obtain a trendline and give the equation obtained in terms of the variables distance traveled and time and the R-squared value. Comment on the meaning of the R-squared value and its signicance when doing data analysis. 3. Using the equation you obtained by doing linear regression, estimate how long the 6th mile will take you to run. 4. Assuming this linear trend persists, how far have you run if you nish in 2900 sec. EXCEL INSTRUCTIONS: In order to plot this data in excel, you should enter the data exactly as above in to column A (rows 1-6) and column B (rows 1-6). To plot the data you will need to go to Insert on the tool bar and then click Chart. A Chart Wizard will appear. Select XY(Scatter) as the Chart type and choose the sub-type that does not have any lines connecting the points, then click next. On Step 2, click on the series tab near the top of the screen and click Add. You do not need to name the series unless you have multiple plots on one graph, but you can type in a name if you wish. To insert the X data, click on the icon at the far left of the x series box. Select the x values by clicking on the rst one and while the left mouse button is down dragging the mouse down to the last value. When the values have been selected click the icon again and repeat for the y values. You may also manually enter the values by separating them with a comma. Don't forget you need to remember what units you are using when answering questions. 2

38 32 CHAPTER 4. BEER'S LAW AND DATA ANALYSIS Click next when you have x and y values entered correctly. The next step is just entering title information and changing the appearance of your plot; click next when nished. Choose your chart location and then nish. You now need to place a linear regression line on your plot. Right click on a data point and pick add trendline. Choose linear as the type and click the options tab. Check the boxes to get the equation and R-squared value and click ok.

39 Chapter 5 Hydrogen and Fuel Cells1 5.1 Hydrogen and Fuel Cells Experiment Objective Build a fuel cell in order to appreciate practically a range of important chemical and physical principles, such as galvanic cells, energy conversion, energy quality, combustion reactions, water electrolysis and bio-fuels. Crituique the design in order to improve the eciency of the fuel cell and to accomplish it practically Grading Pre-lab (10%) Lab Report (80%) TA points (10%) 1 This content is available online at < 33

40 34 CHAPTER 5. HYDROGEN AND FUEL CELLS Before Coming to Lab... Read the lab instructions Introduction As the world's reserves of fossil fuels are diminishing and our awareness of environmental protection is increasing, we strive to develop alternative ways of energy production. Thus in many countries research into construction of stable and ecient fuel cells has been given high priority. Indeed, President Bush in his January 28th 2003 State of the Union address, proposed a $1.2 billion fuel-cell research and development program. Fuel cells are used for direction conversion of the energy of combustion reactions to electrical energy. A possible fuel is hydrogen, which can be produced from water in electrolysis plants driven by solar cells or windmills. A future interesting fuel source for operation fuel cells might be bio-fuels i.e fuels produced from non-fossil organic material such as methane from biogas plants, alcohol produced by fermentation of sugar or hydrolyzed starch (or, in the not so distant future, perhaps also from enzymatically hydrolyzed cellulose). Conventional power plants turn approximately 40% of the fuel energy into electricity; we say that the eciency of the plant is 40%. (Although, in some modern plants surplus heat is reused for district heating thus increasing the actually eciency somewhat). However, with fuel cells the eciency of chemical-toelectric energy conversion is unsurpassed, namely about 70% (or even high in some experimental plants). U.S. energy dependence is higher today than it was during the oil shock of the 1970's, and oil imports are project to increase. Passenger vehicles alone consume 6 million barrels of oil every day, equivalent 85% of oil imports. If just 20% of cars used fuel cells, we could cut oil imports by 1.5 million barrels a day. If every new vehicle sold in the U.S. next year was equipped with a 60kW fuel cell, we would double the amount of the country's available electricity supply. 10,000 fuel cell vehicles running on non-petroleum feul would reduce oil consumption by 6.98 million gallons per year. Fuel cells could dramatically reduce urban air pollution, decrease oil imports, reduce the trade decit and produce American jobs. The U.S. Department of Energy projects that if a mere 10% of automobiles nationwide were powered by fuel cells, regulated air pollutants would be cut by one million tons per year and 60 million tons of the greenhouse gas carbon dioxide would be eliminated. DOE projects that the same number of feel cell cars would cut oil imports by 800,000 barrels a day about 13% of total imports. Since fuel cells run on hydrogen derived from a renewable source, the fuel cell emissions will be nothing but water vapor. 5.2 The Chemistry of a Fuel Cell A fuel cell is a galvanic cell in which electricity is generated by a combustion reaction. The fuel cell consists of two electrodes between which electrical contact is established by means of an electrolyte. Oxygen or just plain atmospheric air is fed continuously to the cathode and the fuel is fed continuously to the anode. The fuel could be any of a vast number of combustible materials, e.g. methane, ethane or ethanol (all organic fuels) hydrogen, hydrazine or sodium borohydride (inorganic fuels). With the hydrogen burning cell as an example we can describe the chemistry of the cell by the following reactions: Anode at which oxidation of the fuel takes place: H2 + 2OH- -> 2H2O + 2e- Cathode at which reduction of oxygen takes places ½ O2 + H2O + 2e- -> 2OH- The next reaction for the cell: H2 + ½ O2 -> H2O

41 35 With ethanol as the fuel the matter becomes somewhat more complicated, since ethanol is oxidized in steps to ethanal, ethanoic acid and carbon dioxide respectively. In an ideally working fuel cell we assume that ethanal and ethanoic acid are further oxidized so that the only carbon compound of the overall process is carbon dioxide. We have not succeed ( by simple chemical tests) to detect either ethanol or ethanoic acid (or rather ethanoate due to the strongly basic electrolyte solution) as intermediate products in our own cells. However we still suggest a three-step oxidation of ethanol(and at the same time admitting that the last step is dubious): Anode: Step 1: CH3CH2OH + 2 OH- -> CH3CHO + 2 H2O + 2e- Step 2: CH3HO + 2 OH- -> CH3COOH + H2O + 2 e- Step 3: CH3COOH +8 OH- -> 2 CO2 + 6 H2O + 8 e- Sum: CH3CH2OH + 12OH- -> 2 CO2 + 9 H2O + 12 e- Cathode: 3O2 + 6 H2O + 12 e- -> 12OH- Overall reaction: CH3CH2OH + 3O2 -> 2CO2 + 3H2O Sodium borohydride can power a cell in either a direct or indirect manner. Indirectly sodium boroydride will decompose in water to produce NaBO2 (borax) and hydrogen NaBH4 + 2H2O -> NaBO2 + 4H2 This hydrogen will then fuel the cell as shown above. However, sodium borohydride can directly power a cell with higher energy yields. Anode: NaBH4 + 8OH- -> NaBO2 + 6H2O + 8e- Cathode: 2O2 + 4H2O + 8e- -> 8OH- While sodium borohydride costs $50 per kilogram, it has projected that mass production and borax recycling could reduce that price to as low as $1 per kilogram. 5.3 Experimental 5.4 Caution!!! Plastic can burn. To get good results, very careful measurements are required. Be sure to wear suitable eye protection. Materials: 2X 50-60mL disposable hypodermic syringes without needles and pistons. 3X pieces of nickel net (2 cut to cover the anges of the syringe cylinders approximately 2cm X 10cm + 1 extra piece) The net should be a very ne mesh. 2X machine screws with nuts and waters (all brass) 2X 20cm pieces of insulated 1mm copper wire with 1.5cm insulation removed from each end Heating plate Aluminum plate 4-6mm thick with 7-8mm hole drilled through center Baking paper Screwdriver, drill, spanner, at bit, scissors, wooden board and small saw tape Lab stand with clamps 600mL beakers 1.5V electric motor Red LED digital mulitimeter balloons

42 36 CHAPTER 5. HYDROGEN AND FUEL CELLS electrical leads with alligator clips 1M sodium hydroxide solution 4M nitric acid ethanol methanol Palladium chloride solution (very expensive and should be recycled) NaBH4 Oxygen gas Hydrogen gas Building an electrode (each group should build 2) Cut a piece of nickel mesh to cover the ange of the syringe cylinder completely Place an aluminum plate on a heating plate. Place the baking paper on the Al plate and the nickel net on the paper. Heat the plate to a temperature that will melt the plastic but not burn it. 4. Place the ange of the syringe on the nickel net on the heating plate. Press down rmly so that the nickel net is melted onto the ange. Make sure that the net is sealed tight to the whole of the ange surface, but take care not to melt so much plastic that the cylinder hold itself is covered with molten plastic. Remove the syringe and net form the eating plate and allow to cool. At one of the sides of the ange drill a hole through the ange using the electric drill. Place a piece of wood beneath to prevent drilling into the lab bench. (see picture) Push the machine screw through the hole and fasten using a washer and nut. (see picture) Mount a piece of insulated copper wire around the machine screw by twisting an end into a loop with a at bit and fastening it with the nut. Tighten it so that good electrical contact is established between the wire and the nickel net. Use tape to attach the wire to the syringe cylinder. Cut o excess nickel net around the ange. Clean the nickel net by immersing the electrode in 4M nitric acid for at least ve minutes. Also clean the extra piece of nickel net in this manner. This much be carried out in the fume hood since poisonous umes may evolve. Rinse thoroughly with water. Place the nickel net of the electrode in a solution of palladium chloride for 30 minutes and then gently rinse with water. Be sure to put the extra piece of nickel net in the palladium chloride solution as well. The electrode is now ready. You should have something that resembles the picture.

43 37 Figure 5.1 Figure 1: Drilling holes in ange.

44 38 CHAPTER 5. HYDROGEN AND FUEL CELLS Figure 5.2 Figure 2: Wire connection assembly.

45 Figure

46 40 CHAPTER 5. HYDROGEN AND FUEL CELLS Figure 3: Final assembled cell Building the cell First cut top o of one of the syringes. This will be the electrode you introduce the liquid/solid fuel. Place your two electrodes into a 600mL beaker containing 1M NaOH solution. The nickel meshing should be completely submerged in solution. Fill a balloon with oxygen gas (from gas cylinder) and connect using rubber hosing to the syringe that was not cut. The oxygen may bubble slowly through the syringe. Roll up the extra piece of nickel mesh and place into the cut syringe. Add 20mg of NaBH4 to the syringe with the extra piece of nickel mesh. If time permits you may test other fuels later. Figure 5.4 Figure 4: Functional cell layout. Testing the cell Measure the voltage generated by your cell by taking a digital multimeter and setting it to DC voltage.

47 Connect one probe to each wire of the cell. The reading may continue to grow for a while and then stabilize. Record this stable voltage. It should read between 0.8V-1V. Measure the current your cell sources by keeping the probes connected and switching to current mode. This reading should be between 30mA-50mA. Powering an LED One fuel cell does not generate enough voltage to power anything of interest. Just like you would connect 2 or 4 AA batteries in series to power a portable CD player, it is necessary to connect multiple fuel cells to generate larger voltages. Pair up with another group and connect the positive terminal of one cell to the negative terminal of the other cell using the wires with alligator clips. Now connect the unwired positive terminal to the positive (longer lead) of the red LED. The unwired negative terminal should be connected to the negative (short lead) of the red LED. At this point the LED should be lit. If you do not see any light, you should use the multimeter to check the voltage generated by the two cells in series and verify that it is greater that 1.5V. If you do not measure any voltage verify that you have wired everything correctly. 41 Figure 5.5

48 42 CHAPTER 5. HYDROGEN AND FUEL CELLS Figure 5.6 Figure 5: Powering an LED circuit. Powering a small motor While two cells in series generate the proper voltage to operate the motor, they cannot source enough current to run a motor longer than a few seconds. By putting cells in parallel more current can be obtained. You will need two sets of two cells in series as described in the Powering an LED section (4 groups are needed for this part). Take the positive connection from each series cell and connect to one terminal of the electric motor. Take the negative connection from each series cell and connect to the other terminal on the electric motor. At this point the motor shaft should begin to turn. If not, check the wiring and verify that you are applying at least 1.5V. It is also possible that 2 parallel cells will not generate enough current. Additional cells can be added in parallel to generate more current.

49 43 Figure 5.7 Figure 6: Powering a motor circuit.

50 44 CHAPTER 5. HYDROGEN AND FUEL CELLS Pre-Lab: (Total 10 Points) Name(Print then sign): Lab Day: Section: TA This assignment must be completed individually and turned in to your TA at the beginning of lab. You will not be allowed to begin the lab until you have completed this assignment. 1.Fill in the blanks: Fuel cells are used for direction conversion of the energy of combustion reactions to. A fuel cell is a in which electricity is generated by a combustion reaction. A fuel cell provides a voltage that can be used to power motors, lights or any number of electrical appliances. 2.T or F At the anode, oxidation of the fuel takes place. 3.T or F The fuel cell emissions will be nothing but water vapor. 4.T or F The eciency of fuel cells, chemical-to-electric energy conversion, is approximately 40%. Review of series and parallel circuits: In a series circuit, the electrons in the current have to pass through all the components, which are arranged in a line. Consider a typical series circuit in which there are three resistors of value R1, R2, and R3. Figure 5.8 There are two key points about a series circuit: The current throughout the circuit is the same. The voltages add up to the battery voltage. Therefore: VT = V1 + V2 + V3 From Ohm's Law: VT = IRT; V1 = IR1; V2 = IR2; V3 = IR3 Þ IRT = IR1 + IR2 + IR3 Therefore: RTot = R1 + R2 + R3 5.In the circuit below, the current is 100 ma.

51 45 Figure 5.9 (a) What is the current in each resistor? (b) What is the voltage across each resistor? (c) What is the total resistance? (d) What is the battery voltage? Figure 5.10 Parallel circuits have their components in parallel branches so that an individual electron can go through one of the branches but not the others. The current splits into the available number of branches. In this case, the current will split into three. For a parallel circuit: The voltage across each branch is the same. The currents in each branch add up to the total current. From this: Itot = I1 + I2 + I3 From Ohm's Law: I T = V ; I1 = V; I2 = V; I3 = V RT R1 R2 R3 Þ V = V + V + V RT R1 R2 R3 Þ 1/RTot = 1/R1 + 1/R2 + 1/R3 6.This question refers to the circuit below.

52 46 CHAPTER 5. HYDROGEN AND FUEL CELLS Figure 5.11 (a) What is the total resistance of the circuit? (b) What is the current through each resistor? (c) What is the total current? Figure 5.12 For resistors in both series and parallel, follow these guidelines: Work out the total resistance of the parallel combination. Work out the total resistance of the circuit by adding your answer in the previous step to the values of the series resistors. 7.What is the single resistor equivalent of this circuit?

53 5.4.2 Report (80 points) Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students' work (including your laboratory partner) or misrepresent your own data (see honor code). The following tables and questions should be answered in your written report. Please put the information in the relevant section of your report (i.e. observations and results, discussion) What would happen if zinc screws were used instead of brass? What is the purpose of the palladium coating on the anode? What is the purpose of the palladium coating on the cathode? What fuel cell worked best? Explain, in detail, why you think that the best fuel cell worked better than the others? Debate fuel cells. 47

54 48 CHAPTER 5. HYDROGEN AND FUEL CELLS

55 Chapter 6 The Best Table in the World The Best Table in the World! Objective The goals of this experiment are: to observe the reactions of several metals with cold water, hot water, acids and then other metal ions. to prepare an activity series of the metals based on the observations from the above reactions Grading You will be assessed on: observations of the reactions of several metals with cold water, hot water, acids and then other metal ions. preparation of an activity series of the metals based on the observations from the above reactions. answers to the post-lab questions Background Information First, you are going to travel back to 1869 and marvel at how the rst periodic law and table were born when only 63 elements had been discovered at the time. A 35 year old professor of general chemistry, Dmitri Ivanovich Mendeleev at the University of St. Petersburg (now Lennigrad) in Russia was shuing his cards with the properties of each element on each card trying to organize his thoughts for his soon-to-be famous textbook on chemistry. When he realized that if the elements were arranged in the order of their atomic weights, there was a trend in properties that repeated itself several times! His paper was delivered by his graduate student, Nikolai Aleksandrivich Menchutkin before the Russian Chemical Society while Medeleev was busy visiting cheesemaking cooperatives at the time! In order to see and nd order among the elements, we must have some general acquaintance with them. Elements are made of matter, and matter is dened as anything that has mass and occupies space. This includes everything that you can see and a lot that you cannot. It follows that in order to distinguish between dierent types of matter (in other words dierent elements) we have to assess their properties. There are two types of properties: intensive and extensive. In the former case, intensive properties do not depend on the how much of an element is present but do include state (whether a substance is a solid, 1 This content is available online at < 49

56 50 CHAPTER 6. THE BEST TABLE IN THE WORLD liquid or gas), color and chemical reactivity. Extensive properties depend on the quantity of matter present - mass and volume are extensive properties. Properties can be further categorized as either chemical or physical. A chemical change describes how the substance may change composition, such as spontaneously by combustion or in combination with other substances. On the other hand, physical changes are those properties that can be measured without changing the composition of the matter. Condensation of steam to water is a physical change Introduction What is there to know about the periodic table? Why is it important? Why does it appear in nearly every science lecture room and labs? Is it just a portrait of an aspect of chemistry or does it serve a useful purpose? Why is the name periodic appropriate? Why is the table arranged in such a way? What are the important features of the table? Does it give order to the approximately 120 known elements? 6.2 Relative Reactivity of Metals and the Activity Series A supercial glance at the Periodic Table will reveal that all known elements are listed by their chemical symbols. An in depth glance at the Periodic Table yields information on the mass of an atom of the element in atomic mass units (amu) for the molar mass of a mole ( ) of atoms in grams below the chemical symbol for each element. Above the chemical symbol for each element, there is a second number listed, the atomic number, which gives the number of protons (positively charged particles in the nucleus), or the number of electrons (negatively charged outside the nucleus) for a neutral atom. Mendeleev arranged the elements in the Periodic Table in order of increasing atomic number in horizontal rows of such length that elements with similar properties recur periodically; that is to say, they fall directly beneath each other in the Table. The elements in a given vertical column are referred to as a family or group. The physical and chemical properties of the elements in a given family change gradually as one goes from one element in the column to the next. By observing the trends in properties, the elements can be arranged in the order in which they appear in the Periodic Table. 6.3 Procedure I. Activity Series Part 1. Reactions of Metals with Water CAUTION! Sodium reacts very rapidly with water to evolve hydrogen and heat. This is potentially dangerous because of the possibility of the violent explosive reaction of H 2 (g) with O 2 (g) present in the air. CAUTION! Sodium causes severe chemical burns when it comes into contact with the skin. Note: Metallic sodium must be stored below the surface of an inert liquid such as kerosene to prevent oxidation by air. 1. I will demonstrate the reaction of sodium and then potassium with water. Observe the rate of evolution of H 2 gas as I use tweezers to place a tiny pea-size piece of sodium then potassium into a 500-mL beaker full of deionised water. Record your observation on the Report Form and write a balanced equation for this reaction. 2. Place 5 ml H 2 O in each of four clean tubes and label them as follows: A. Mg B. Cu C. Zn D. Ca

57 51 Table Use sandpaper or steel wool to remove the oxide from the surfaces of Mg, Cu, and Zn. 2. Place several small pieces of Mg, Cu, and Zn in the correctly labeled test tube prepared above. Place two or three (not more!) pieces of Ca turnings in the test tube labeled "Ca". 3. Watch for evidence of reaction by noting evolution of gas bubbles and any change in the color or size of the metal. Record your observations and write net ionic equations for each reaction. Note: Trapped air bubbles on the metal surfaces are not indicative of a reaction. CAUTION: H 2 is FLAMMABLE! CAUTION: Residual calcium should be discarded in a special container designated by your instructor. Note: Net ionic equations must balance in mass (atoms) and in total charge on each side of the equation Part 2. Reactions of Metals with HCl CAUTION: The reaction of Ca with HCl is not studied. Residual calcium should be discarded in a special container designated by your instructor. 1. Decant the water from each test tube used in the procedure above and leave the pieces of metal that remain unreacted in each test tube. 2. Place the test tubes in a test tube rack/holder. 3. Add 2 ml of 3 M HCl solution to each test tube. CAUTION: Some of the test tubes may become very hot. Leave them in the rack/holder while you are making observations. 1. Observe relative rate of H 2 gas evolution for up to 10 minutes and record your observations on your report form. 2. Based on the observations in the previous steps, list the elements that react in 3M HCl in order of increasing strength as reducing agents and write net ionic equations for all reactions Part 3. Reactions of Metals with Other Metal Ions 1. Place a clean 1 inch-square of metal foil (sheet) of each of these metals Cu, Zn and Pb on a at surface. 2. Clean the metal surfaces by sanding them with ne sandpaper or steel wool. 3. Place one or two drops in spots of each of these solutions in a clockwise order on the metal surfaces: A. 0.5 M Ag + B. 0.5 M Cu 2+ C. 0.5 M Zn 2+ D. 0.5 M Pb 2+ Table NOTE: Do not test a cation of a metal on a square of the same metal such as Cu 2+ ion and Cu metal. 2. Watch for color changes in each spot as evidence of reaction. If you are not sure whether the reaction has occurred, rinse the plate with water. A distinct spot of a dierent color on the surface is good evidence for the reaction. 3. Write net ionic equations for each reaction. Arrange Ag, Cu, Pb and Zn in order of their increasing strength as reducing agents. If a metal A reacts with a cation of another metal B, metal A is a stronger reducing agent, more reactive than metal B. 4. Rinse and dry each square of metal and return it to the correct beaker on the reagent shelf for other students to use.

58 52 CHAPTER 6. THE BEST TABLE IN THE WORLD Part 4. Flame Tests One station set up that all sections will rotate through Clean a spatula wire by dipping it into dilute hydrochloric acid (3M) and then holding it in a hot Bunsen ame. Repeat this until the spatula doesn't produce any color in the ame. When the spatula is clean, moisten it again with some of the acid and then dip it into a small amount of the solid you are testing so that some sticks to the spatula. Place the spatula back in the ame again. If the ame color is weak, it is often worthwhile to dip the spatula back in the acid again and put it back into the ame as if you were cleaning it. You often get a very short but intense ash of color by doing that. Chemicals/Materials: 1. Chloride salts of Li, Na, K, Rb, Cs, Ca, Ba, Cu, Pb, Fe (II) and Fe(III) Sr (nitrate salt). 2. Glass rods with loops of Pt wire. 3. Bunsen burner/clicker. 4. Concentrated nitric acid or hydrochloric acid. Record your observations on your report form. It should be noted that sodium is present as an impurity in many if not most metal salts. Because sodium imparts an especially intense color to a ame, ashes of the sodium may be observed in nearly all solutions tested.

59 Pre-Lab 5: The Best Table in the World! Hopefully here 2 for the Pre-Lab Name(Print then sign): Lab Day: Section: TA This assignment must be completed individually and turned in to your TA at the beginning of lab. You will not be allowed to begin the lab until you have completed this assignment. 1. The mass of an atom of the element in atomic mass units (amu) for the molar mass of a mole ( ) of atoms in grams above or below the chemical symbol for each element? Circle the correct one. 2. The second symbol listed for each element is the, symbol? Fill in the blanks. 3. The number in question 2 gives the number of or the number of for a neutral atom. Fill in the blanks 4. The elements in a given vertical column are referred to as a or. Fill in the blanks. 5. The horizontal rows are called? Fill in the blank 6. The block of elements between groups II and III are called? Fill in the blanks. 7. Elements 58 to 71 are known as or? Fill in the blanks. 8. Elements 90 to 103 are known as? Fill in the blanks. 9. Do elements with larger atomic numbers than 92 occur naturally? True or false? Circle the correct one. 6.5 Report 5: The Best Table in the World! Hopefully here 3 for the Report Form Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students' work (including your laboratory partner) or misrepresent your own data (see honor code). Name(Print then sign): Lab Day: Section: TA

60 54 CHAPTER 6. THE BEST TABLE IN THE WORLD I. Activity Series Part 1. Reactions of Metals with Water Metal Observations Net Ionic Equations (If NoReaction Occurs, write N.R) Na K Mg Cu Zn Ca Table Part 2. Reactions with HCl Metal Observations Net Ionic Equations (If No Reaction Occurs, Write N.R.) Mg Cu Zn Table Based on your experimental results place Mg, Cu, Zn and Ca in order of increasing strength as reducing agents Part 3. Reactions with Other Metal Ions 1. Write in the appropriate box either REACTION or NO REACTION. Ag + Cu 2+ Zn 2+ Pb 2+ Zn Cu Pb Do not test Do not test Do not test Table Write balanced equations to represent the results tabulated above. 3. Based on your experimental results, arrange Ag, Cu, Zn and Pb in order of increasing strength as reducing agents. 4. Arrange Ag +, Cu 2+, Zn 2+ and Pb 2+ in order of increasing strength as oxidizing agents. 5. Combine the results from Part 2 and Part 3. Arrange Mg, Cu, Zn, Ca, Ag and Pb in order or increasing strength as reducing agents. 6. Place Ni in this row, if it is found that Ni will deposit on Zn foil, but not on Pb foil when a drop of NiSO 4 is placed on both.

61 Part 4. Flame Tests Element Li Na K Rb Cs Ca Sr Ba Cu Pb Color in ame Table 6.6 What are the limitations of this test?

62 56 CHAPTER 6. THE BEST TABLE IN THE WORLD

63 Chapter 7 Bonding This content is available online at < 57

64 58 CHAPTER 7. BONDING Lab 5: Bonding Objective To test various compounds and determine their conductivity and bonding. To understand how electronegativity can predict bond type. To learn the relationship between bonding and conductivity. 7.3 Grading Pre-Lab (10%) Lab Report Form (80%) TA Points (10%) 7.4 Background Information A chemical bond is a link between atoms that results from the mutual attraction of their nuclei for electrons. Bonding occurs in order to lower the total potential energy of each atom or ion. Throughout nature, changes that decrease potential energy are favored. The main types of bonds that we will be covering are ionic bonds, covalent bonds, and metallic bonds. An ionic bond is the chemical bond that results from the electrostatic attraction between positive (cations) and negative (anions) ions. The ionic relationship is a give and take relationship. One ion donates or gives electrons, while the other ion receives or takes electrons. A covalent bond is a chemical bond resulting from the sharing of electrons between two atoms. There are two main types of covalent bonds. The rst being non-polar covalent bonds. These are bonds in which the bonding electrons are shared equally by the united atoms-with a balanced electrical charge. Polar covalent bonds are covalent bonds in which the united atoms have an unequal attraction for the shared electrons.

65 59 Figure 7.1 The role of electrons in bonding has been well-studied. The ability of an atom or element to attract electrons to itself is known as the element's electronegativity. A scale was rst calculated by the Nobel laureate Linus Pauling and is commonly called the Pauling electronegativity scale. The actual electronegativity values aren't as important as how they compare to a dierent element. In Part I of today's experiment, you will compare electronegativity values to predict the type of bond that will exist between two elements. In the solution state, ionic compounds dissociate to give a separation of charge. The separation of charge allows for the ow of electrons through solution. The ow of electrons is classied as conductivity. A strong electrolyte is a compound that when dissolved in water will completely ionize or dissociate into ions. That is, the compound exists in water only as individual ions, and there are no intact molecules at all. This solution conducts electricity well. A weak electrolyte is a compound that when dissolved in water only partially ionizes or dissociates into ions. That is, the compound exists in water as a mixture of individual ions and intact molecules. This solution conducts electricity weakly. A nonelectrolyte is a compound that when dissolved in water does not ionize or dissociate into ions at all. In water, this compound exists entirely as intact molecules. The solution does not conduct electricity at all. By measuring the conductivity of a dissolved compound, we can classify it as a nonelectrolyte, weak electrolyte, or strong electrolyte and determine its ability to dissociate into ions. There are four common compounds that you will encounter in today's lab. ACIDS are molecular compounds which ionize (turn into ions) in water. The cation that is formed is always H +. Therefore, in the formulas for simple acids, H is always the rst element listed. Some acids are strong electrolytes and some acids are weak electrolytes. There are no acids which are nonelectrolytes because by denition an acid is a H + donor. BASES can be molecular compounds or ionic compounds. Some bases are soluble and some are not. The

66 60 CHAPTER 7. BONDING 07 soluble bases ionize or dissociate into ions in water, and the anion formed is always OH. The ionic bases have hydroxide ( OH ) as the anion. If they are soluble, the ions simply separate (dissociate) in the water. All of the ionic bases which are soluble are strong electrolytes. SALTS are ionic compounds which are not acids or bases. In other words, the cation is not hydrogen and the anion is not hydroxide. Some salts are soluble in water and some are not. All of the salts which are soluble are relatively strong electrolytes. NONELECTROLYTES are compounds which dissolve in water but do not ionize or dissociate into ions. These would be molecular compounds other than the acids or bases already discussed. 7.5 Experimental Procedure Caution:Acids and bases are corrosive and can cause burns Part I. Predicting bond type through electronegativity dierences. Using the electronegativity table provided in the lab manual, predict the type of bond that each of the following compounds will have by the following process: Find the electronegativity for each element or ion in compound using electronegativity table provided. Subtract the electronegativites (using absolute value). If values are between: Ionic bond % ionic Polar Covalent bond-5-50% ionic Non-Polar Covalent-0-5% ionic Determine the type of bonding in the following compounds: KCl, CO, CaBr 2, SiH 4, MgS.

67 61 Figure Part II. Weak and strong electrolytes Chemicals tap water 0.1 M hydrochloric acid, HCl 0.1 M acetic acid, HC 2 H 3 O M sulfuric acid, H 2 SO M sodium hydroxide, NaOH 0.1 M ammonia, NH M sodium acetate, NaC 2 H 3 O M sodium chloride, NaCl 0.1 M ammonium acetate, NH 4 C 2 H 3 O M ammonium chloride, NH 4 Cl methanol, CH 3 OH ethanol, C 2 H 5 OH

68 62 CHAPTER 7. BONDING 07 sucrose solution, C 12 H 22 O 11 In today's lab, you will be using a MicroLab conductivity probe to determine how well electrons ow through a given solution. First, you will need to calibrate the probe with a non-electrolyte (distilled water) and a very strong electrolyte. To quantify how well a solution conducts, we will assign numerical values to the conductance probe. A non-conducting solution will have a conductance value of 0, a poor conducting solution will have a reading of 0 to 1,000, and good conductors will have readings of 3,000 up.

69 Instructions for MicroLab Conductivity Experiment Open the MicroLab Program by clicking on the Shortcut to MicroLab.exe tab on the desktop. On the Choose an Experiment Type Tab, enter a name for the experiment, and then double click on the MicroLab Experiment icon Click Add Sensor, Choose sensor = Conductivity Probe Choose an input, click on the red box that corresponds to the port that your conductivity sensor is connected to. Choose 20,000 microseconds Choose a Sensor, click radial button that says Conductivity Probe. Click next. Click Perform New Calibration Click Add Calibration Point place the conductivity probe in the non-conductive standard solution, while swirling wait until the value is constant and then enter 0.0 into the Actual Value box in MicroLab and hit ok. Again, Click Add Calibration Point place the conductivity probe in the conducting standard solution, while swirling wait until the value is constant and then enter 1020 into the Actual Value box in MicroLab and hit ok. Repeat for 3860 as the Actual Value. Under Curve Fit Choices, click on First order (linear) and then Accept and Save this Calibration, when prompted to Enter the units for this calibration, leave as is and click ok, save as your name-experiment-date. Click nish. In the sensor area, left click on the conductivity icon and drag it to the Y-axis over data source two, also click and drag to column B on the spreadsheet and also click and drag to the digital display window. When ready to obtain data, click start. This is very important: Be sure to thoroughly since the probe with DI water between every use. Beginning with the tap water, measure the conductance of each of the following solutions. Using the information provided in the lab manual, classify each solution as a non-, weak, or strong electrolyte. For those solutions that are electrolytes, record the ions present in solution Part III. Electrolyte strength and reaction rate Chemicals calcium carbonate powder - shake once 1 M HCl - stopper it 1 M HC 2 H 3 O M H 2 SO 4 Test tube gas collection apparatus - end at 20mL Measure 2 g of powdered calcium carbonate ( CaCO 3 ) onto a piece of weigh paper. Obtain 30 ml of 1 M HCl in a graduated cylinder. Pour the acid into the test tube apparatus. Add the calcium carbonate to the acid and QUICKLY stopper the tube to begin collecting gas. Record the time it takes to collect 20 ml of gas. The acid may react very fast with the CaCO 3 generating the gas very rapidly. Clean out the test tube apparatus and repeat the experiment using 1 M HC 2 H 3 O 2 and 0.5 M H 2 SO Part IV. Chemical reactions Chemicals 0.01 M calcium hydroxide, Ca (OH) 2 Plastic straws Obtain 20 ml of saturated calcium hydroxide solution. Make sure it is clear and colorless. Place the conductivity probe in the solution and begin monitoring it conductivity. With your straw, slowly exhale into the solution. Note any observations in the solution and the conductivity.

70 64 CHAPTER 7. BONDING Pre-Lab 5: Bonding (Total 10 Points) Hopefully here 2 for the Pre-Lab Name(Print then sign): Lab Day: Section: TA This assignment must be completed individually and turned in to your TA at the beginning of lab. You will not be allowed to begin the lab until you have completed this assignment Part I. Bonding of chemicals in solution 1. Write out the formulas of the following acids: phosphoric perchloric nitric sulfuric hydrochloric acetic 1. Write out the formulas of the following bases: calcium hydroxide potassium hydroxide sodium hydroxide ammonia 1. Write out the formulas of the following salts: potassium chromate potassium sulfate copper(ii) nitrate calcium carbonate potassium iodide 2

71 Report 5: Bonding 07 Hopefully here 3 for the Report Form Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students' work (including your laboratory partner) or misrepresent your own data (see honor code). Name(Print then sign): Lab Day: Section: TA Part I. Predicting bond type through electronegativity dierences. Chemical Formula Electroneg (1) Electroneg (2) Di Electroneg Type of bond KCl CO CaBr 2 SiH 4 MgS Table Part II. Weak and strong electrolytes Solution Tested Numerical Output Electrolyte Strength Ions Present 0.1 M HCl 0.1 M HC 2 H 3 O M H 2 SO M NaOH 0.1 M NH M NaC 2 H 3 O M NaCl 0.1 M NH 4 C 2 H 3 O M NH 4 Cl CH 3 OH C 2 H 5 OH Sucrose Tap water Table Why do we use deionized water instead of tap water when making solutions for conductivity measurements? 3

72 66 CHAPTER 7. BONDING Part III. Electrolyte strength and reaction rate 2. Time to collect 20 ml of gas using 1 M HCl. Write the reaction of HCl with CaCO Time to collect 20 ml of gas using 1 M HC 2 H 3 O 2. Write the reaction of HC 2 H 3 O 2 with CaCO Time to collect 20 ml of gas using 0.5 M H 2 SO 4.Write the reaction of H 2 SO 4 with CaCO Why does it take dierent lengths of time to collect 20 ml of gas? 6. Based on the time it took to collect 20 ml of gas, rank the acids in the order of increasing strength. 7. Why did we use 0.5 M H 2 SO 4 instead of 1.0 M H 2 SO 4? Part IV. Chemical reactions 8. Write the chemical reaction for calcium hydroxide with your exhaled breath. 9. Write your observations for the reaction that took place (i.e. appearance, conductivity, etc.) 10. When in separate solutions, aqueous ammonia, NH 3 (aq) and acetic acid HC 2 H 3 O 2 conduct electricity equally well. However, when the two solutions are mixed a substantial increase in electrical conductivity is observed. Explain. 11. Separately, ammonium sulfate and barium hydroxide solutions are very good conductors. When the two solutions are mixed a substantial decrease in conductivity is observed. Rationalize this.

73 Chapter 8 Solid State and Superconductors Solid State Structures and Superconductors Objectives Build examples of: simple cubic, body centered cubic and face centered cubic cells. Understand and familiarize with three-dimensionality of solid state structures. Understand how binary ionic compounds (compounds made up of two dierent types of ions) pack in a crystal lattice. Observe the special electromagnetic characteristics of superconducting materials using 1,2,3- superconductor YBa 2 Cu 3 O 8, discovered in 1986 by Dr. Paul Chu at the University of Houston Grading Your grade will be determined according to the following Pre-lab (10%) Lab report form. (80%) TA points (10%) Before coming to lab: Read introduction and model kits section Complete prelab exercise Introduction From the three states of matter, the solid state is the one in which matter is highly condensed. In the solid state, when atoms, molecules or ions pack in a regular arrangement which can be repeated "innitely" in three dimensions, a crystal is formed. A crystalline solid, therefore, possesses long-range order; its atoms, molecules, or ions occupy regular positions which repeat in three dimensions. On the other hand an amorphous solid does not possess any long-range order. Glass is an example of an amorphous solid. And even though amorphous solids have very interesting properties in their own right that dier from those of crystalline materials, we will not consider their structures in this laboratory exercise. 1 This content is available online at < 67

74 68 CHAPTER 8. SOLID STATE AND SUPERCONDUCTORS The simplest example of a crystal is table salt, or as we chemists know it, sodium chloride (NaCl). A crystal of sodium chloride is composed of sodium cations ( Na + ) and chlorine anions ( Cl ) that are arranged in a specic order and extend in three dimensions. The ions pack in a way that maximizes space and provides the right coordination for each atom (ion). Crystals are three dimensional, and in theory, the perfect crystal would be innite. Therefore instead of having a molecular formula, crystals have an empirical formula based on stoichiometry. Crystalline structures are dened by a unit cell which is the smallest unit that contains the stoichiometry and the spatial arrangement of the whole crystal. Therefore a unit cell can be seen as the building block of a crystal. The crystal lattice In a crystal, the network of atoms, molecules, or ions is known as a crystal lattice or simply as a lattice. In reality, no crystal extends innitely in three dimensions and the structure (and also properties) of the solid will vary at the surface (boundaries) of the crystal. However, the number of atoms located at the surface of a crystal is very small compared to the number of atoms in the interior of the crystal, and so, to a rst approximation, we can ignore the variations at the surface for much of our discussion of crystals. Any location in a crystal lattice is known as a lattice point. Since the crystal lattice repeats in three dimensions, there will be an entire set of lattice points which are identical. That means that if you were able to make yourself small enough and stand at any such lattice point in the crystal lattice, you would not be able to tell which lattice point of the set you were at the environment of a lattice point is identical to each correspondent lattice point throughout the crystal. Of course, you could move to a dierent site (a noncorrespondent lattice point) which would look dierent. This would constitute a dierent lattice point. For example, when we examine the sodium chloride lattice later, you will notice that the environment of each sodium ion is identical. If you were to stand at any sodium ion and look around, you would see the same thing. If you stood at a chloride ion, you would see a dierent environment but that environment would be the same at every chloride ion. Thus, the sodium ion locations form one set of lattice points and the chloride ion locations form another set. However, lattice points not only exist in atom positions. We could easily dene a set of lattice points at the midpoints between the sodium and chloride ions in the crystal lattice of sodium chloride. The unit cell Since the crystal lattice is made up of a regular arrangement which repeats in three dimensions, we can save ourselves a great deal of work by considering the simple repeating unit rather than the entire crystal lattice. The basic repeating unit is known as the unit cell. Crystalline solids often have at, well-dened faces that make denite angles with their neighbors and break cleanly when struck. These faces lie along well-dened directions in the unit cell. The unit cell is the smallest, most symmetrical repeating unit that, when translated in three dimensions, will generate the entire crystal lattice. It is possible to have a number of dierent choices for the unit cell. By convention, the unit cell that reects the highest symmetry of the lattice is the one that is chosen. A unit cell may be thought of as being like a brick which is used to build a building (a crystal). Many bricks are stacked together to create the entire structure. Because the unit cell must translate in three dimensions, there are certain geometrical constraints placed upon its shape. The main criterion is that the opposite faces of the unit cell must be parallel. Because of this restriction there are only six parameters that we need to dene in order to dene the shape of the unit cell. These include three edge lengths a, b, and c and three angles α, β[u+f02c][u+f020]and γ. Once these are dened all other distances and angles in the unit cell are set. As a result of symmetry, some of

75 these angles and edge lengths may be the same. There are only seven dierent shapes for unit cells possible. These are given in the chart below. 69 Unit Cell Type Triclinic Monoclinic Orthorhombic Restrictions on Unit Cell Parameters a is not equal to b is not equal to c; α[u+f020]is not equal to β[u+f020]is not equal to γ. a is not equal to b is not equal to c[u+f03b][u+f020][u+f020] α[u+f020]= γ[u+f020][u+f03d][u+f020] 90 [U+F020] β[u+f020]is not equal to 90. a is not equal to b is not equal to c[u+f03b][u+f020][u+f020] α[u+f020]= β[u+f020]= γ[u+f020][u+f03d][u+f020] 90 Tetragonal a =b is not equal to c [U+F03B][U+F020][U+F020] α[u+f020]= β[u+f020]= γ[u+f020][u+f03d] 90 Cubic a =b =c [U+F03B][U+F020][U+F020] α[u+f020]= β[u+f020]= γ[u+f020][u+f03d][u+f020] 90 [U+F020] Hexagonal, Trigonal a =b is not equal to c [U+F03B][U+F020][U+F020] α[u+f020]= β[u+f020]= 90 [U+F02C][U+F020][U+F020] γ[u+f03d][u+f020] 120 Rhombohedral* a =b =c [U+F03B][U+F020][U+F020] α[u+f020]= β[u+f020]= γ[u+f020]is not equal to 90 Highest Type of Symmetry Element Required no symmetry is required, an inversioncenter may be present highest symmetry element allowed is ac2 axis or a mirror plane has three mutually perpendicularmirror planes and/or C2 axes has one C4 axis has C3 and C4 axes C6 axis (hexagonal); (trigonal) C3 axis (trigonal) C3 axis Table 8.1 *There is some discussion about whether the rhombohedral unit cell is a dierent group or is really a subset of the trigonal/hexagonal types of unit cell Stoichiometry You will be asked to count the number of atoms in each unit cell in order to determine the stoichiometry (atom-to-atom ratio) or empirical formula of the compound. However, it is important to remember that solid state structures are extended, that is, they extend out in all directions such that the atoms that lie on the corners, faces, or edges of a unit cell will be shared with other unit cells, and therefore will only contribute a

76 70 CHAPTER 8. SOLID STATE AND SUPERCONDUCTORS fraction of that boundary atom. As you build crystal lattices in these exercises you will note that eight unit cells come together at a corner. Thus, an atom which lies exactly at the corner of a unit cell will be shared by eight unit cells which means that only 1/8 of the atom contributes to the stoichiometry of any particular unit cell. Likewise, if an atom is on an edge, only ¼ of the atom will be in a unit cell because four unit cells share an edge. An atom on a face will only contribute ½ to each unit cell since the face is shared between two unit cells. It is very important to understand that the stoichiometry of the atoms within the unit cell must reect the composition of the bulk material Binding forces in a crystal The forces which stabilize the crystal may be ionic (electrostatic) forces, covalent bonds, metallic bonds, van der Waals forces, hydrogen bonds, or combination of these. The properties of the crystal will change depending upon what types of bonding is involved in holding the atoms, molecules, or ions in the lattice. The fundamental types of crystals based upon the types of forces that hold them together are: metallic in which metal cations held together by a sea of electrons, ionic in which cations and anions held together by predominantly electrostatic attractions, and network in which atoms bonded together covalently throughout the solid (also known as covalent crystal or covalent network) Close-packing Close-packing of spheres is one example of an arrangement of objects that forms an extended structure. Extended close-packing of spheres results in 74% of the available space being occupied by spheres (or atoms), with the remainder attributed to the empty space between the spheres. This is the highest space-lling eciency of any sphere-packing arrangement. The nature of extended structures as well as close-packing, which occurs in two forms called hexagonal close packing (hcp) and cubic close packing (ccp), will be explored in this lab activity. Sixty-eight of the ninety naturally occurring elements are metallic elements. Forty of these metals have three-dimensional submicroscopic structures that can be described in terms of close-packing of spheres. Another sixteen of the sixty-eight naturally occurring metallic elements can be described in terms of a dierent type of extended structure that is not as ecient at space-lling. This structure occupies only 68% of the available space in the unit cell. This second largest subgroup exhibits a sphere packing arrangement called body-centered cubic (bcc). You should be able to calculate the % of void space using simple geometry Packing of more than one type of ion (binary compounds) in a crystal lattice A very useful way to describe the extended structure of many substances, particularly ionic compounds, is to assume that ions, which may be of dierent sizes, are spherical. The structure then is based on some type of sphere packing scheme exhibited by the larger ion, with the smaller ion occupying the unused space (interstitial sites). In structures of this type, coordination number refers to the number of nearest neighbors of opposite charge. Salts exhibiting these packing arrangements will be explored in this lab activity Coordination number and interstitial sites When spherical objects of equal size are packed in some type of arrangement, the number of nearest neighbors to any given sphere is dependent upon the eciency of space lling. The number of nearest neighbors is called the coordination number and abbreviated as CN. The sphere packing schemes with the

77 71 highest space-lling eciency will have the highest CN. Coordination number will be explored in this lab activity. A useful way to describe extended structures, is by using the unit cell which as discussed above is the repeating three-dimensional pattern for extended structures. A unit cell has a pattern for the objects as well as for the void spaces. The remaining unoccupied space in any sphere packing scheme is found as void space. This void space occurs between the spheres and gives rise to so-called interstitial sites Synthesis of solid state materials There exist many synthetic methods to make crystalline solids. Traditional solid state chemical reactions are often slow and require high temperatures and long periods of time for reactants to form the desire compound. They also require that reactants are mixed in the solid state by grinding two solids together. In this manner the mixture formed is heterogeneous (i.e. not in the same phase), and high temperatures are required to increase the mobility of the ions that are being formed into the new solid binary phase. Another approach to get solid state binary structures is using a precursor material such as a metal carbonate, that upon decomposition at high temperatures loses gaseous CO 2 resulting in very ne particles of the corresponding metal oxide (e.g., BaCO 3(s) BaO (s) + CO 2(g) ) X-ray crystallography To determine the atomic or molecular structure of a crystal diraction of X-rays is used. It was observed that visible light can be diracted by the use of optical grids, because these are arranged in a regular manner. Energy sources such as X-rays have such small wavelengths that only grids the size of atoms will be able to diract X-rays. As mentioned before a crystal has regular molecular array, and therefore it is possible, to use X-ray diraction to determine the location of the atoms in crystal lattice. When such an experiment is carried out we say that we have determined the crystal structure of the substance. The study of crystal structures is known as crystallography and it is one of the most powerful techniques used today to characterize new compounds. You will discuss the principles behind X-ray diraction in the lecture part of this course Superconductors A superconductor is an element, or compound that will conduct electricity without resistance when it is below a certain temperature. Without resistance the electrical current will ow continuously in a closed loop as long as the material is kept below an specic temperature. Since the electrical resistance is zero, supercurrents are generated in the material to exclude the magnetic elds from a magnet brought near it. The currents which cancel the external eld produce magnetic poles opposite to the poles of the permanent magnet, repelling them to provide the lift to levitate the magnet 2. In some countries (including USA) this magnet levitation has been used for transporation. Specically trains can take advantage of this levitation to virtually eliminate friction between the vehicle and the tracks. A train levitated over a superconductor can attain speeds over 300 mph! 8.2 Solid State Model Kits In this experiment we will use the Institute for Chemical Education (ICE) Solid-State Model Kits which are designed for creating a variety of common and important solid state structures. Please be careful with these 2

78 72 CHAPTER 8. SOLID STATE AND SUPERCONDUCTORS materials as they are quite expensive. There is a list of kit components on the inside of the lid of each box. Please make sure that you have all the listed pieces and that these are in their proper locations when you nish using the kit. The TAs will deduct points from your lab grade if the kits are not returned with all pieces present and properly organized Use of the Solid State Model Kit: The following instructions are abbreviated. Please consult the instruction manual found in the kits for more details if you need assistance in building any of the structures given. Note that some of the model kits are older than others and the manuals' and page numbers may not correspond. There are four major part types in each model kit: *2 o-white, thick plastic template bases with holes (one with a circle, the other a semicircle); *cardboard templates (about 20 labeled A-T); *metal rods (to be inserted in the holes to support the plastic spheres) *plastic spheres in 4 sizes and colors. The spheres can represent atoms, ions, or even molecules depending upon the kind of solid it is. You will be given directions for the use of a specic base, template, placement of the rods, selection of spheres, and arrangement of the spheres as you progress. The ICE model kits make use of Z-diagrams to represent how the structure will be built up. Each type of sphere will be numbered with the z layer in which it belongs. As we build each structure in three-dimensional space, we will be drawing gures to represent the unit cell structures. Each level or layer of atoms, ions, or molecules in a unit cell can be represented by a two-dimensional base, that is, a square, hexagon, parallelogram, etc. To draw the Z-diagrams the bottom layer is referred to as z=0. We then proceed layer by layer up the unit cell until we reach a layer which is identical to the z=0 layer. This is z=1. Since z=0 and z=1 are identical by denition, we do not have to draw z=1, although you might want to do so as you are learning how to work with solid state gures. The layers between top and bottom are given z designations according to their positions in the crystal. So, for example, a unit cell with 4 layers (including z=0 and z=1) would also have z=0.33 (1/3) and z=0.67 (2/3). Each solid-state kit has two types of bases (one using rectangular coordinates, the other using polar coordinates) indicated by a full circle or semicircle, or by color (yellow and green.) You will rst build structures that involve only one type of atom, as you would nd in crystalline solids of the elements, especially that of the metals. Then you will examine ionic compounds which are known as binary solids. Binary solids are those composed of only two types of atoms, such as sodium chloride or calcium uoride. If time permits there is an extra credit exercise you can do. You may not do this extra credit exercise until the report form has been completed nor may you receive credit for the extra credit assignment unless you fully complete the report form Working groups and teams You and your lab partner will constitute a group. Each group will receive one model kit and two groups will work together as a team. Your TA will assign you the structures you have to do, and at the end each team will discuss the structures assigned on front of the class. The number of teams and the assignments the TA will give you will be decided based on the number of students in a particular laboratory session. The laboratory is divided for six teams (A-F)

79 Experimental Procedure Every part of the experimental procedure has correspondent questions on the Report Form. Do not proceed until ALL questions accompanying each section have been answered and recorded. 1. Demonstration of the 1,2,3-superconductor YBa 2 Cu 3 O 8 A pellet of the 1,2,3-superconductor YBa 2 Cu 3 O 8 is placed on the top of an inverted paper cup. The pellet is cooled down by carefully pouring liquid nitrogen over it until the bottom of the cup is lled up. After approximately 10 seconds (when the bubbling stops) the pellet should reach the liquid nitrogen temperature. Your TA will then place a very strong magnet over the pellet. What happens to the magnet? What happens as the superconductor warms up? What is the Meissner eect? (Write observations and answer these questions on your report form) Warning- LIQUID NITROGEN CAN CAUSE FROST BITE! Do not directly touch anything that has come into contact with the liquid nitrogen until it is warmed up to room temperature. NOTE TO TA: to remove a levitating magnet, simply wait until the liquid nitrogen fully evaporates or use another magnet to "grab" the oating magnet. Be careful not to lose or break these very tiny, yet expensive, magnets!!!! Cubic Cells There are many types of fundamental unit cells, one of which is the cubic cell. In turn, there are three subclasses of the cubic cell: a. simple or primitive cubic (P) b. body-centered cubic (bcc, I*) c. face-centered cubic (fcc, F) *The I designation for body-centered cubic comes from the German word innenzentriert. We do not have time to build models of all of the unit cells possible, so we will focus on the cubic structure and its variations. Our investigation will include several aspects of each cell type: the number of atoms per unit cell the eciency of the packing of atoms in the volume of each unit cell the number of nearest neighbors (coordination number) for each type of atom the stoichiometry (atom-to-atom ratio) of the compound A. Simple Cubic Unit Cells or Primitive Cubic Unit Cells (P) Team A Group 1. Single Unit Cell Construct a simple cubic cell using template A and its matching base. Insert rods in the 4 circled holes in the shaded region of the template. Build the rst layer (z = 0) by placing a colorless sphere on each rod in the shaded region. Draw a picture of this layer as previously described. Complete the unit cell by placing 4 colorless spheres on top of the rst layer. This is the z=1 layer Group 2. Extended Structure Construct an extended cubic cell using template A. Insert rods in the circled holes of template A in the area enclosed by the dotted lines. Construct a set of unit cells as described for making a single unit cell.

80 74 CHAPTER 8. SOLID STATE AND SUPERCONDUCTORS Look closely at the structures generated by both groups. They are called simple (or primitive) cubic. Considering all of the cells around it, answer the corresponding questions on the report form. B. Body-Centered Cubic Structure (BCC) Team B Group 1. Single Unit Cell Construct a body-centered cubic (bcc) structure using template F. Insert the rods in all 5 of the holes in the shaded region. Use the guide at left and place four colorless spheres in the rst layer (1) at the corners for z=0. Place one colorless sphere in the second layer (2) on the center rod for z=0.5 Construct the z=1 layer. Group 2.Extended Structure Using template F, construct an extended body-centered cubic structure. Insert rods in every hole of the template/block. Using the guide which follows, place colorless spheres for z=0 on every rod labeled 1. For z=0.5 place colorless spheres on each rod labeled 2. Complete the z=1 layer and then place another two layers on top. 1. Face-Centered Cubic (FCC) Structure Team C Group 1. Single Unit Cell Construct a single face-centered cubic cell using template C, colorless spheres and the layering as illustrated. Only put rods and spheres on one of the squares dened by the internal lines. Figure 8.1 Group 2. Extended Structure Construct an extended face-centered cubic structure using template C (You can nd instructions on how to do it in the manual that comes with the kit.)

81 75 3. Close-Packing: Sphere Packing & Metallic Elements Team D Group 1. Construct the hexagonal close-packing unit cell (use the one requiring the C6 template) Group 2. Construct the cubic close-packing unit cell (use the one requiring the C6 template) Team E Group 1. Add a 2' layer on top of the existing structure. Group 2. Add a 2' layer on top of the existing structure. Team F Using only the shaded portion on the template, construct the face-centered cubic unit cell which uses the C4 template. Compare the structures of the face-centered cubic unit cell made on the C4 template to that made on the C6 template Interstitial sites and coordination number (CN) Team A Group 1 - Construct CN 8, CN 6 and CN 4 (using the C4 template). Group 2 - Construct CN 6, CN 4 (body diagonal) (using the C6 template) Ionic Compounds Now we will look at some real ionic compounds which crystallize in dierent cubic unit cells. We will use the models to determine the stoichiometry ( atom-to-atom ratios) for a formula unit. Team B Cesium Chloride Construct a model of cesium chloride on template A. This time use colorless spheres as layers 1 and 1' and the green spheres for layer 2. Start with the shaded area and then work your way outward to an extended structure. Consider both simple and extended structures when answering the questions which follow Team C Fluorite: Calcium uoride Construct a model of uorite, which is calcium uoride, on template E. Green spheres will be used for layers 1, 3, and 1' while colorless spheres go on layers 2 and 4. Finish with a 1' layer on top. Build the structure by placing rods in all 13 holes in the area enclosed by the internal line.

82 76 CHAPTER 8. SOLID STATE AND SUPERCONDUCTORS Team D Lithium Nitride Use the L template and insert 6 rods in the parallelogram portion of the dotted lines Construct the pattern shown below. Be sure to include a z=1 layer. 1 is a green sphere while 1 and 2 are blue spheres. The 0 indicates a 4.0 mm spacer tube; the 2 is an 18.6 mm spacer. Figure Teams E and F Zinc Blende and Wurtzite: Zinc Sulde Team E. Zinc Blende: Use template D to construct the crystal pattern illustrated below. Numbers 2 and 4 are blue spheres while 1 and 3 are colorless spheres and 4 is a 16.1 mm spacer. Team F. Wurtzite: Use template L to construct the Wurtzite lattice. Numbers 1, 3 and 1' are colorless spheres and Numbers 2 and 4 are pink spheres.

83 77 Figure 8.3

84 78 CHAPTER 8. SOLID STATE AND SUPERCONDUCTORS 8.3 Pre-Lab: Solid State and Superconductors 8.4 (Total 10 Points) Hopefully here 3 for the Pre-Lab Name(Print then sign): Lab Day: Section: TA This assignment must be completed individually and turned in to your TA at the beginning of lab. You will not be allowed to begin the lab until you have completed this assignment. 1. List the existing crystal systems (unit cell types): 2. Which of these unit cells will we study in this laboratory exercise? 3. Which are the three subclasses of this type of unit cell? 4. Dene coordination number: 5. What is the volume of a sphere? Of a cube? 8.5 Report: Solid State and Superconductors Hopefully here 4 for the Report Form Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students' work (including your laboratory partner) or misrepresent your own data (see honor code). Name(Print then sign): Lab Day: Section: TA Part I Demonstration and Unit cell theory A. TA Demo of the superconductor Describe and explain your observations (What happens with the magnet? Briey describe the Meissner eect?) B. The unit cell 1. A cube (see below) has corners, edges & faces. Figure

85 2. Structure A below shows how a unit cell may be drawn where one choice of unit cell is shown in bold lines. In Structures B, C and D below, draw the outline(s) of the simplest 2-D unit cells (two-dimensional repeating patterns depicted by a parallelogram that encloses a portion of the structure). If the unit cell is moved in the X,Y-plane in directions parallel to its sides and in distance increments equal to the length of its sides, it has the property of duplicating the original structural pattern of circles as well as spaces between circles. Can a structure have more than one type of unit cell? 79 Figure 8.5 Structure A Structure B Structure C Structure D Table If the circle segments enclosed inside each of the bold-faced parallelograms shown below were cut out and taped together, how many whole circles could be constructed for each one of the patterns: Figure 8.6 Table Shown below is a 3-D unit cell for a structure of packed spheres. The center of each of 8 spheres is at a corner of the cube, and the part of each that lies in the interior of the cube is shown. If all of the sphere segments enclosed inside the unit cell could be glued together, how many whole spheres could be constructed?

86 80 CHAPTER 8. SOLID STATE AND SUPERCONDUCTORS number of whole spheres: 5. For each of the gures shown below, determine the number of corners and faces. Identify and name each as one of the regular geometric solids. Figure 8.7 AB Number of corners Number of faces Name of the shape of this object A B Table 8.4 Part II Experimental 1. Cubic Cells A. Simple Cubic Unit Cells or Primitive Cubic Unit Cells (P) a. How would you designate the simple cube stacking - aa, ab, abc, or some other? b. If the radius of each atom in this cell is r, what is the equation that describes the volume of the cube generated in terms of r? (Note that the face of the cube is dened by the position of the rods and does not include the whole sphere.)

87 81 c. Draw the z-diagram for the unit cell layers. d. To how many dierent cells does a corner atom belong? What is the fractional contribution of a single corner atom to a particular unit cell? e. How many corner spheres does a single unit cell possess? f. What is the net number of atoms in a unit cell? (Number of atoms multiplied by the fraction they contribute) g. Pick an interior sphere in the extended array. What is the coordination number (CN) of that atom? In other words, how many spheres are touching it?. h. What is the formula for the volume of a sphere with radius r? i. Calculate the packing eciency of a simple cubic unit cell (the % volume or space occupied by atomic material in the unit cell). Hint: Consider the net number of atoms per simple cubic unit cell (question g) the volume of one sphere (question i), and the volume of the cube (question b) B. Body-Centered Cubic (BCC) Structure a. Draw the z diagrams for the layers. b. Fill out the table below for a BCC unit cell Atom type Number Fractional Contribution Total Contribution Coordination Number Corner Body Table 8.5 c. What is the total number of atoms in the unit cell? d. Look at the stacking of the layers. How are they arranged if we call the layers a, b, c, etc.? e. If the radius of each atom in this cell is r, what is the formula for the volume of the cube generated in terms of the radius of the atom? (See diagrams below.) f. Calculate the packing eciency of the bcc cell: Find the volume occupied by the net number of spheres per unit cell if the radius of each sphere is r; then calculate the volume of the cube using r of the sphere and the Pythagoras theorem ( a 2 + b 2 = c 2 ) to nd the diagonal of the cube.

88 82 CHAPTER 8. SOLID STATE AND SUPERCONDUCTORS C. The Face Centered Cubic (FCC) Unit Cell a. Fill out the following table for a FCC unit cell. Atom type Number Fractional Contribution Total Contribution Coordination Number Corner Face Table 8.6 b. What is the total number of atoms in the unit cell? c. Using a similar procedure to that applied in Part B above; calculate the packing eciency of the face-centered cubic unit cell. 1. Close-Packing a. Compare the hexagonal and cubic close-packed structures. b. Locate the interior sphere in the layer of seven next to the new top layer. For this interior sphere, determine the following: Number of touching spheres: hexagonal close-packed (hcp) cubic close-packed (ccp) on layer below on the same layer on layer above TOTAL CN of the interior sphere Table 8.7 c. Sphere packing that has this number (write below) of adjacent and touching nearest neighbors is referred to as close-packed. Non-close-packed structures will have lower coordination numbers. d. Are the two unit cells the identical? e. If they are the same, how are they related? If they are dierent, what makes them dierent? f. Is the face-centered cubic unit cell aba or abc layering? Draw a z-diagram.

89 83 III.Interstitial sites and coordination number (CN) a. If the spheres are assumed to be ions, which of the spheres is most likely to be the anion and which the cation, the colorless spheres or the colored spheres? Why? b. Consider interstitial sites created by spheres of the same size. Rank the interstitial sites, as identied by their coordination numbers, in order of increasing size (for example, which is biggest, the site with coordination number 4, 6 or 8?). c. Using basic principles of geometry and assuming that the colorless spheres are the same anion with radius r A in all three cases, calculate in terms of ra the maximum radius, rc, of the cation that will t inside a hole of CN 4, CN 6 and CN 8. Do this by calculating the ratio of the radius of to cation to the radius of the anion: r C /r A. d. What terms are used to describe the shapes (coordination) of the interstitial sites of CN 4, CN 6 and CN 8? CN 4: CN 6: CN 8: IV.Ionic Solids A. Cesium Chloride 1. Fill the table below for Cesium Chloride Type of cubic structure Atom represented Colorless spheres Green spheres Table Using the simplest unit cell described by the colorless spheres, how many net colorless and net green spheres are contained within that unit cell? Do the same for a unit cell bounded by green spheres as you did for the colorless spheres in question 4. What is the ion-to-ion ratio of cesium to chloride in the simplest unit cell which contains both? (Does it make sense? Do the charges agree?) B. Calcium Fluoride 1. Draw the z diagrams for the layers (include both colorless and green spheres). 2. Fill the table below for Calcium Fluoride

90 84 CHAPTER 8. SOLID STATE AND SUPERCONDUCTORS Type of cubic structure Atom represented Colorless spheres Green spheres Table What is the formula for uorite (calcium uoride)? C. Lithium Nitride 1. Draw the z diagrams for the atom layers which you have constructed. 2. What is the stoichiometric ratio of green to blue spheres? 3. Now consider that one sphere represents lithium and the other nitrogen. What is the formula? 4. How does this formula correspond to what might be predicted by the Periodic Table? D. Zinc Blende and Wurtzite Fill in the table below: Stoichiometric ratio of colorless to pink spheres Formula unit (one sphere represents and the other the sulde ion) Compare to predicted from periodic table Type of unit cell Zinc Blende Wurtzite Table 8.10

91 Chapter 9 Organic Reactions1 9.1 Organic Reactions Objectives Synthesis of some important esters. Oxidation of a primary alcohol rst to an aldehyde and then a carboxylic acid. To saponify a typical vegetable oil Grading You will be assessed on detailed answers required in the lab report. the correctness and thoroughness of your observations Introduction Esters are an important class of organic compounds commonly prepared from the esterication reaction of an organic acid with an alcohol in the presence of a strong mineral acid (usually H 2 SO 4 ). They are chiey responsible for the pleasant aromas associated with various fruits, and as such are used in perfumes and avorings. Some esters also have useful physiological eects. The best known example is the analgesic ("pain killing") and anti-pyretic ("fever reducing") drug acetylsalicylic acid, otherwise known by its trade name aspirin. Liniments used for topical relief of sore muscles contain the ester methyl salicylate ("oil of wintergreen"), which is prepared from the reaction of methyl alcohol with the acid group of salicylic acid. Methyl salicylate acts as an analgesic and is absorbed through the skin; however, methyl salicylate is also a skin irritant (like many organic substances), which in this instance provides the benecial side eect of the sensation of warming in the area of the skin where the liniment is applied. Oxidation of a primary alcohol may yield either an aldehyde or a carboxylic acid, depending on the reaction conditions. For example, mild oxidation of ethanol produces acetaldehyde, which under more vigorous conditions may be further oxidised to acetic acid. The oxidation of ethanol to acetic acid is responsible for causing wine to turn sour, producing vinegar. A number of oxidising agents may be used. Acidied sodium dichromate (VI) solution at room temperature will oxidise primary alcohols to aldehydes and secondary alcohols to ketones. At higher temperatures primary alcohols are oxides further to acids. 1 This content is available online at < 85

92 86 CHAPTER 9. ORGANIC REACTIONS Figure 9.1 The dichromate solution turns from the orange color of the Cr 2 O7 2 (aq) to the blue color of the Cr 3+ (aq). This color change is the basis for the "breathalyser test". The police can ask a motorist to exhale through a tube containing some orange crystals. If the crystals turn blue, it shows that the breath contains a considerable amount of ethanol vapor. Soaps are produced by the reaction of metallic hydroxides with animal fats and vegetable oils. The major components of these fats and oils are triglycerides. Triglycerides are esters of the trihydroxy alcohol called glycerol and various long-chain fatty acids. Tristearin is a typical triglyceride. Upon reaction with sodium hydroxide, the ester bonds of tristearin are broken. The products of the reaction are the soap, sodium stearate, and glycerol. This type of reaction is called saponication (Greek: sapon, soap) and it is depicted below. Figure 9.2 Soap is made commercially by heating beef tallow in large kettles with an excess of sodium hydroxide. When sodium chloride is added to this mixture (called the "saponied" mixture), the sodium salts of the fatty acids separate as a thick curd of crude soap. Glycerol is an important by-product of the reaction. It is

93 recovered by evaporating the water layer. The crude soap is puried, and coloring agents and perfumes are added to meet market demands EXPERIMENTAL PROCEDURE CAUTION WEAR EYE PROTECTION! CAUTION - Concentrated sulfuric acid will burn and stain the skin as well as damage clothing. In case of skin or clothing contact, wash the area immediately with large amounts of water Synthesis of esters 1. Place approximately 2 g (or 2 ml if the substance is a liquid) of the organic acid and 2 ml of the alcohol in a large test tube. 2. Add 5-7 drops of concentrated (18 M) sulfuric acid, mix the solution well with a glass stirring rod and then place the test tube in a hot water bath (largest beaker in your drawer) ( 80 C) for 5-10 minutes. 3. Remove the test tube from the hot water bath and cautiously pour the mixture into about 15 ml of saturated sodium bicarbonate contained in a small beaker. The sodium bicarbonate will destroy any unreacted acid. 4. Observe the aroma produced from each of the following esterication reactions. Write the structure of the esters produced, and the balanced equations for the esterication and the acid/sodium bicarbonate reactions: 87 Complete the following reactions using the procedure above and record your observations. (1) C 7 H 6 O 3 + CH 3 OH salicylic acid + methyl alcohol (2) CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OH + CH 3 COOH 1 - octanol + glacial acetic acid (3) CH 3 CH 2 CH 2 CH 2 CH 2 OH + CH 3 COOH amyl alcohol + glacial acetic acid (4) C 2 H 5 OH + CH 3 COOH ethanol + acetic acid Oxidation of an alcohol with acidied potassium dichromate(vi) solution Add 10 drops of dilute sulfuric acid (6M) and 5 drops of potassium dichromate(vi) solution (0.01M) to 5 drops of ethanol. The oxidising agent is added slowly to the alcohol so that the temperature is kept below that of the alcohol and above that of the carbonyl compound. (Carbonyl compounds are more volatile than the corresponding alcohols). Usually the alcohol is in excess of the oxidant and the aldehyde is distilled o to avoid further oxidation. Note the color and smell cautiously (Royal Wave). Warm the mixture and smell cautiously (Royal Wave). Repeat the experiment using rst methanol and then propan-2-ol in place of ethanol. Describe what happens and explain the color changes. What conditions and techniques would favour the oxidation of ethanol to a. ethanal rather than ethanoic acid. b. ethanoic acid rather than ethanal?

94 88 CHAPTER 9. ORGANIC REACTIONS Oxidation of an alcohol with acidied potassium permanganate (VII) solution Add 10 drops of dilute sulfuric acid and 5 drops of potassium permanganate (VII) solution (0.01M) to 5 drops of ethanol. Note the color and smell cautiously. Warm the mixture and smell cautiously (Royal Wave). Repeat the experiment using rst methanol and then propan-2-ol in place of ethanol. Take the ph of your nal mixture using Universal indicator paper Describe what happens and explain the color changes. What is your nal product? Saponication of a vegetable oil CAUTION - Sodium hydroxide is a very caustic material that can cause severe skin burns. Eye burns caused by sodium hydroxide are progressive: what at rst appears to be a minor irritation can develop into a severe injury unless the chemical is completely ushed from the eye. If sodium hydroxide comes in contact with the eye, ush the eye with running water continuously for at least 20 minutes. Notify your TA immediately. If sodium hydroxide is spilled on some other parts of the body, ush the aected area with running water continuously for at least 2-3 minutes. Notify your TA immediately. Never handle sodium hydroxide pellets with your ngers. Use weighing paper and a scoopula. Solid sodium hydroxide will absorb water from the atmosphere. It is hygroscopic. Do not leave the container of sodium hydroxide open. Keep ethanol and ethanol-water mixtures away from open ames. Aqueous iron chloride will stain clothes permanently and is irritating to the skin. Avoid contact with this material. In this experiment, you will saponify a vegetable oil 1. Pour 5 ml (5.0 g) of vegetable oil into a 250-mL beaker. 2. Slowly dissolve 2.5 g of NaOH pellets in 15 ml of the 50% ethanol/water mixture in a 50-mL beaker. 3. Add 2-3 ml of the NaOH solution to the beaker containing the oil. Heat the mixture over a hot plate with stirring. CAUTION: Keep your face away from the beaker and work at arm's length. Stirring is required to prevent spattering. Every few minutes, for the next 20 minutes, add portions of the ethanol/water mixture while continuing to stir to prevent spattering. After about 10 more minutes of heating and stirring, the oil should be dissolved and a homogenous solution should be obtained. 4. Add 25 ml of water to the hot solution. Using the hot grips, pour this solution into a 250 ml beaker containing 150 ml of saturated NaCl solution. Stir this mixture gently and permit it to cool for a few minutes. 5. Skim the soap layer o the top of the solution and place it in a 50-mL beaker. 6. Into a test tube, place a pea-sized lump of your soap. Place a similar amount of laundry detergent in a second tube and a similar amount of laundry detergent in a second tube and a similar amount of hand soap in a third tube. Add 10 ml of water to each tube. Stopper each tube and shake thoroughly. 7. Estimate the ph of the solution using wide-range indicator solution or wide-range test paper. Record the results. Pour the contents of the test tubes into the sink and rinse the tubes with water. Figure 9.3

95 9.2 Pre-Lab: Introductory Organic Reactions 9.3 (Total 25 Points) Hopefully here 2 for the Pre-Lab Name(Print then sign): Lab Day: Section: TA This assignment must be completed individually and turned in to your TA at the beginning of lab. You will not be allowed to begin the lab until you have completed this assignment. For questions 1-4, draw the structural formulae of: 1) 2,2 - dimethylbutane 89 2) 3-ethyl-2,4-dimethylpentane 3) 2,3,4-trimethylhexane 4) 3-ethyl-2-methylheptane For questions 5-8, give the names of 5) CH 3 CH 2 CH 2 CH 2 CH = CH 2 6) CH 3 CH = C = CH 2 7) CH 3 CH = CHCH 3 8) (CH 3 ) 2 C = CHCH 3 For questions 9-11, give the structural formulae for: 9) hex-3-ene 10) 3-methylhex-1-ene 11) 2,5- dimethylhex-2-ene For questions 12-14, give the names of: 12) 2

96 90 CHAPTER 9. ORGANIC REACTIONS Figure ) Figure ) Figure 9.6 For questions 15-19, give the stuctural formulae of: 15) trans-1,2-dibromoethene 16) trans-1-chloroprop-1-ene 17) cis- hex-2-ene 18) pent-1-yne 19) 3-methylbut-1-yne For questions 20-25, name the following compounds: 20) Figure )

97 91 Figure ) Figure ) Figure ) Figure ) Figure 9.12