How bad is that snack anyway?

Transcription

1 Physical Sciences 11 Experiment 1 How bad is that snack anyway? Monday, 2/10 Wednesday, 2/12 Science Center Room 117 Please read this entire document and complete the attached prelab before your lab. This page and all subsequent pages should be stapled together and submitted to your Lab TF before you leave the laboratory. All students must submit their own lab reports. TF Name: Student Name: Lab Partner(s): Grading: Prelab: / 10 TF check Lab Report: / 20 Safety: / 3 Cleanup: / 2 Total: / 35 PS Experiment 1 1

2 Introduction Most of the calories in a snack such as a cheese ball are concentrated in carbohydrates (80%) and fats (15%). Your body metabolizes these classes of compounds to create energy that helps run cellular processes. In the first half of this lab you will attempt to determine the caloric content of a snack using simple yet effective laboratory procedures. Food scientists use high quality, sensitive devices called calorimeters to measure the energy content of foods in units of Calories (Cal; 1 Cal = 1,000 cal). In general, calorimetric measurements involve a complete burning of food in a vessel immersed in water, and the rise in temperature of the entire assembly is monitored. While we will use a much simpler setup to measure the caloric content of a snack in this experiment, the basic principles are similar. Before you measure the calories in a snack, you will first determine the efficiency of your calorimeter by measuring the energy density (i.e. energy content per unit mass) of a candle and comparing it to the known value. Your calorimeter will allow you to determine the temperature rise of water that is in contact with the object giving off energy (the candle). Using the temperature information and the relationship below, you will calculate the energy output of the candle. Also, we can safely assume all of the energy is in the form of heat because <1% of a candle s output goes into the generated light. q rxn = q calor = m C s ΔT = m C s (T f T i ) where q rxn = the heat of reaction q calor = the thermal energy received by the calorimeter m = mass of water C s = specific heat capacity of water Tf = the final temperature of water Ti = the initial temperature of water For more information on Heat and Heat Capacity, please refer to our textbook page You will then determine the calories in a snack by combusting it in a similar manner and taking into account the efficiency of your calorimeter. In the second half of this lab, you will use a coffee cup calorimeter to determine the enthalpy change of the following two chemical reactions. (1)The neutralization of citric acid with sodium hydroxide. (2) Dissolving ammonium chloride in water. Please read about Enthalpy in our textbook page , including the text in the blue box titled Reactions in the Liquid Phase: An Important Example of Reactions at Constant Pressure and Check Yourself 5 Measuring H rxn in a Coffee-Cup Container. PS Experiment 1 2

3 Procedure Safety Precautions Safety glasses and nitrile gloves and lab coats must be worn at all times. You will be working with fire. Long hair must be tied back. Keep all flammable materials (notebooks, lab reports, etc.) off of your lab bench during the experiment. Please see Laboratory Safety Manual for PS for detailed safety precautions. Part Ia. Determining the efficiency of the calorimeter 1) Measure the mass of a weighing dish and thumbtack using an analytical balance. Secure a candle to the weighing dish with the thumbtack. Measure the mass of the candle using the same balance. 2) Fill an empty soda can with 60 g of water (approximately 1/3 of a can). Determine the mass of the water in the can. Clamp the filled can one inch above the candle. 3) Clamp a digital thermometer in the can, making sure the tip of the thermometer is submerged in the water but not touching the bottom of the can. Record the initial temperature of the water. To do this, record the water temperature at 10s intervals for 40s. Is the temperature constant? If no, please wait for one more minute before re-measuring the temperature. Repeat this step until the temperature stabilizes before proceeding to step 4. 4) Light the candle and gently slide it under the can. Take temperature readings every 30 seconds till the temperature reaches around 80 o C. Do NOT stir with the thermometer. 5) Measure the mass of the extinguished candle (subtract the mass of the weigh boat and thumbtack). Part Ib. Determining the calories in a snack 1) Repeat the procedure with new water using a cheese ball secured onto a cork and wire in a weighing dish, but take temperature readings every 15 seconds until the cheese ball is completely burned. In addition, take temperature readings for an additional 30 seconds after the snack has extinguished. 2) Be careful when you carry the burned residue over to an analytical balance (you may want to carry it in a weighing dish). PS Experiment 1 3

4 Part IIa. Determining the ΔH rxn Enthalpy of Reaction for Acid/Base Neutralization 1) In this part of the experiment, you will measure the enthalpy of reaction between citric acid (H 3 Cit) and sodium hydroxide (NaOH) using a procedure of your own design. 2) You will be given 0.50 M solutions of each, a digital thermometer, and Styrofoam coffee cups. Be sure to check the bottom of your cups for any holes. 3) As part of your prelab report, you wrote a detailed procedure for this part of the experiment based on a stoichiometric ideal; however, for the actual experiment you will collect a random volume of the solutions. The total quantity of NaOH and citric acid must be 30 ml and you should you not use less than 10 ml of each solution. 4) After you collect your data, you will share it with other groups in your lab section. Part IIb. Determining the ΔH rxn Enthalpy of Reaction for the Dissolution of Ammonium Chloride (NH 4 Cl) 1) In this part of the experiment, you will measure the enthalpy of reaction between ammonium chloride and water using a procedure of your own design. 2) You will be given solid ammonium chloride, distilled water, a digital thermometer, and Styrofoam coffee cups. Be sure to check the bottom of your cups for any holes. 3) As part of your prelab report, you wrote a detailed procedure for this part of the experiment. The quantity of ammonium chloride should be between 6 and 9 grams and the water should be roughly 30 grams. 4) After you collect your data, you will share it with other groups in your lab section. Waste Disposal and Clean-up Pour water from the soda cans down the drain and leave the empty can at your station. Snacks, ashes, used matches, and burnt candles go into the trash. Before disposing of used candles and used matches, run them under tap water in the sink to make sure they are extinguished before you dispose of them in the trash. Pour the liquid waste from Part II in the liquid waste container in the back. Leave corks, wires, and empty weigh boats back at your station. Wipe down your lab bench. PS Experiment 1 4

5 Lab Report Part Ia. Determining the efficiency of the calorimeter Mass of water Initial mass of candle Final mass of candle Initial Temperature recordings: 0s 10s 20s 30s 40s Temperature recorded while burning Time (s) Temperature ( C) Observations PS Experiment 1 5

6 1. Use Excel program on the lab computer, input the temperature v.s. time data, make a plot and find the initial slope of the curve. Send the Excel file to your TF s box before the end of the lab. Determine the ratio of the change in mass of the candle ( m) to the change in temperature of water ( T). You may assume that the burn rate is constant. 2. Calculate the apparent energy density of the candle in J/kg. 3. Given that the actual energy density of the candle is approximately J/kg, determine the efficiency of the calorimeter, i.e. the fraction of thermal energy released by the candle that gets converted into heat in the water. Efficiency = PS Experiment 1 6

7 Part Ib: Determining the calories of a cheese ball Mass of water Initial mass of snack Final mass of snack Time (s) Temperature ( C) Observations PS Experiment 1 7

8 4. Calculate the energy density of the cheese ball (in J/kg and Cal/g), taking into account the efficiency of the calorimeter that you calculated above. Energy Density = 5. State all the assumptions that you made in determining the caloric content of your snack. 6. What are some sources of error in the procedure or assumptions made? PS Experiment 1 8

9 Part IIa. Determining the ΔH rxn for Acid/Base Neutralization 7. Please consider what data you would like to collect in order to find the ΔH rxn for the reaction as written below, then create a data table. After you perform the experiment, please calculate ΔH rxn in terms of kj per mole of reaction. Is the reaction exothermic or endothermic? (Please use J/ g C for the specific heat of water) H 3 Cit (aq) + 3 NaOH (aq) Na 3 Cit (aq) + 3 H 2 O (l) Data Table Calculations Part IIb. Determining the ΔH rxn for the Dissolution of Ammonium Chloride 8. Please consider what data you would like to collect in order to find the ΔH rxn for the reaction as written below, then create a data table. After you perform the experiment, please calculate ΔH rxn in terms of kj per mole of reaction. Is the reaction exothermic or endothermic? (Please use J/ g C for the specific heat of water) NH 4 Cl (s) + H 2 O (l)!"# NH 4 + (aq) + Cl - (aq) + H 2 O (l) Data Table Calculations PS Experiment 1 9

10 Prelab To be completed prior to lab and checked off by your TF as you enter the lab. 1. Given the following data: Initial mass of snack food: g Final mass of residue: g Mass of water in can: g Initial temperature: 24.8 C Final temperature: 33.2 C Calculate the heat released, in calories per gram of oil, for this snack food. 2. The addition of hydrochloric acid to a silver nitrate solution precipitates silver chloride according to the following reaction:agno 3 (aq) + HCl (aq) AgCl (s) + HNO 3 (aq). When 50.0 ml of M AgNO 3 is combined with 50.0 ml of M HCl in a coffee-cup calorimeter, the temperature changes from C to C. Calculate ΔH rxn (kj per mole of reaction) for the reaction as written. Use 1.00 g/ml as the density of the solution and C = 4.18 J/g C as the specific heat capacity. (Hint: this is the practice question on page 3.24 of our textbook. Reading through pages should be helpful.) PS Experiment 1 10

11 3. Write out a detailed procedure for determining the change in enthalpy for the reaction between citric acid (H 3 Cit) and sodium hydroxide (NaOH). H 3 Cit + 3 NaOH Na 3 Cit + 3 H 2 O. Be sure to calculate what volume of each 0.50 M solution should be mixed to give a total volume of 30.0 ml and the reactants in the correct stoichiometry. Also, write a data table in the space given on page Please write out a data table for the dissolving of ammonium chloride on page You and your lab partners will use different amounts of solution in Part IIa and different amounts of ammonium chloride in Part IIb. Theoretically, should you find the same result for ΔH rxn (kj per mole of reaction) for both reactions? Please explain your answer. PS Experiment 1 11