Education is not the filling of a pail, but the lighting of a fire.

Transcription

1 Education is not the filling of a pail, but the lighting of a fire. William Butler Yeats I. Some reactions release energy to the surroundings when they occur, and others absorb energy in order to occur. A. Exothermic B. Endothermic II. If the heat absorbed or released is included in the balanced equation, it is called a thermochemical equation. It can be shown in two ways. A. As if it were a substance B. Or by way of a new concept 1. Enthalpy 2. Reaction pathways A. Overall look

2 B. A more detailed look 3. So 4. Example: Parafilm, C25H52, from a candle reacts with oxygen to produce carbon dioxide, water, and J. Write the thermochemical equation using each of the possible formats. 5. Example: Given: 2 CH3OH + 3 O2 2 CO2 + 4 H2O; H = kj/mol. Write the equation using the other format. III. Thermostoich A. We bridge between a chemical and the heat term as opposed to building a bridge between two chemicals. B. Example: For the reaction 3 CO + Fe2O3 2 Fe + 3 CO kj What amount of heat would be released if 56 g of CO were allowed to react?

3 Homework (Use pencil!) To decompose one mole of CaCO 3 to CaO and CO 2, 176 kj must be supplied. Write the balanced thermochemical equation using each of the formats Methyl alcohol, CH 3OH, is used as a fuel in racing cars. It burns as shown below. 2 CH 3OH + 3 O 2 2 CO H 2O kj What amount of heat is given off when 1.00 g of alcohol burns? [27.3 kj] The thermite reaction is shown below. 2 Al + Fe 2O 3 Al 2O Fe H for this reaction if 850 kj. What mass of aluminum is needed to produce 35.8 kj of heat? [2.3 g] 1. Are the following exothermic or endothermic? a. a burning match b. dry ice becoming a gas c. a solution is made whereby the container it is in feels warm to the touch Reading at Home pages ; section 8-9 Websites Lab: So you want to be a blonde? We will use calorimetry to determine the heat of reaction for the decomposition of hydrogen peroxide that s the chemical that is used to strip hair of its natural pigments before dying it a new color. Pre-lab/chemistry 1. Hydrogen peroxide, H 2O 2, as purchased in the drugstore, is an aqueous solution. On its own, it decomposes slowly into water and oxygen. Write and balance the equation. Include phase symbols. 2. A catalyst is a substance that speeds up certain reactions, but is not ultimately consumed (it s still there at the end). A catalyst for our reaction is a solution of iron (III) nitrate. A catalyst is a special condition for a reaction. You were taught how to include this information in the balanced equation (this was learned in your reactions unit, A Balancing Act ). Include it now. Use the formula of the catalyst, not its name.

4 3. The reaction is exothermic, so the temperature of the surroundings will increase. The surroundings will consist of the solution itself and the Styrofoam cup that holds the solution. Since the thermometer cannot respond instantaneously to temperature changes, it would be inaccurate to use it to record the temperatures present at the start and end of the rapid decomposition. So we will record time and temperature data both before and after the decomposition (it occurs at the five minute mark) and extrapolate to find the exact temperature. A sketch is shown of what you will see. (The vertical and almost horizontal lines are best-fit, hand-drawn, not computer generated.) Tf 4. Draw the raw data table. You will note from the graph shown that we will record the temperature of the solution in the cup every minute for four minutes. On the fifth minute, we add the catalyst but NOT attempt to measure a temperature, only record observations. So, at minute 5 in your data table, just put a dashed line for the temperature. At the 6 th minute, we will re-commence recording temperatures every minute and continue until the 20 th minute. Include a final column for observations. 5. Write the axis labels and title for the graph (see lab techs for title format of graphs) Ti 6. We will assume that the heat given off by the reaction is absorbed by the solution and by the cup. We will further assume that the solution is sufficiently aqueous that it has the specific heat of water, 4.18 J/g o C, and the density of water, 1.00 g/ml. We will know the temperature change that the solution undergoes from the graph, and there will be 60.0 ml of solution present at the time. Show the sample calculation for obtaining the heat that just the solution will absorb. First, you ll need the specific heat formula, rearrange it, then put in values and boxes (with units outside of boxes). 7. The cup will also absorb some heat. The specific heat of a typical Styrofoam cup is 5.0 J/ o C. Notice there is no gram in the unit that s because the cup has a constant mass that has already been incorporated in the specific heat. We will assume that the temperature change the cup undergoes is the same as that of the solution. (Think about why that might be a good or bad assumption.) Show the sample calculation for obtaining the heat the cup will absorb. Again, you ll need the specific heat formula, rearrange it, etc. 8. The total heat absorbed is the sum of the two values from #6 and #7. Be sure to write a formula first (just put in symbols for heat and show what you must do with them) then use the empty boxes for a sample calculation. 9. The heat absorbed was given off by the original sample of hydrogen peroxide. The solution was 3.0 g of H 2O 2 per 1OŌ g of total solution, and we will have measured 50.0 ml of the solution into the cup. The density of the solution is 1.00 g/ml. Show the sample calculation for obtaining moles of pure hydrogen peroxide (NOT solution) that will have been present. HINTS: Begin with the concentration of H 2O 2 per total solution (given in the second sentence of this step be careful, include all of the correct unit; failure to do this could cause you to get a wrong answer) no boxes because all values are known at this time you will end with a sub-answer. 10. With your answers from #8 and #9, you will be able to find the heat per mole of hydrogen peroxide otherwise known as H for the decomposition reaction! Show the sample calculation for obtaining H it s not the one we learned in class (not: H = H 2 H 1). Instead, it is a short domino that can be gleaned from the underlined words of this step.

5 11. We will share ANSWERS in step #10 (not data) for a total of three trials. Draw a relative error table comparing our ANSWERS. Pre-lab/science 12. What is the major independent variable in the lab? What is the major dependent variable in the lab? Be careful these are not the variables on the axes of the graph. 13. Why is calorimetry a good choice of method? Hint: Look at the root word in calorimetry and ask with what do you associate that word? 14. Make a 3-column list of the necessary equipment, materials and safety (all in the first column). Include sizes (second column) and quantities (third column). Reading the procedure will help with this. 15. Would you expect a positive, negative, or no relationship between the variables you stated in # Have your pre-lab graded. NOTE: READ YOUR LAB (ESPECIALLY ABOUT THE GRAPH) BEFORE YOU COME TO DO THE LAB) Lab Safety: goggles, aprons, closed toe shoes NO GUM, FOOD, DRINK 17. A note about recording values over a period time. Begin by recording a beginning value. Only record new values when they change. For example, if at time = 1 minute, the temperature is 18 o C and it remains that temperature until minute 12, then record the 18 o C temperature at minute 1 only; do not record anything for minutes The same type of recording system is true for observations. 18. Set out a cup. Place another cup in it for good insulation. Set up a ring stand with a thermometer clamp secured to it. Place the thermometer in the clamp just a little, perhaps 2-3 mm, from the bottom. This will allow you to swirl the cup around the thermometer instead of using the thermometer as a stir rod it s safer that way. 19. Measure 50.0 ml of the hydrogen peroxide solution into a graduated cylinder. Start your stopwatch as you pour the peroxide solution into the cup. Record the temperature starting at the 1-minute mark. Measure 10.0 ml of the iron (III) nitrate solution into a separate graduated cylinder. (It s a good idea to begin the next step while the experiment is progressing.) At the five minute mark, pour the iron (III) nitrate solution into the cup, but do not record the temperature. DO record the observation, however. At the 6-minute mark, resume temperature measurements as well as visual observations. When completed, pour the cup contents down the drain. Rinse and dry the cup and thermometer with deionized water.

6 20. Using your lab techs, graph the data (all of it, even temperature readings that repeat themselves). However, no connecting lines, no regression line; no equation of the line; do include the 5 minute time (leave the temperature reading blank) do have grid lines. Remember, make sure that NO points fall on the edges of the graph. Also, make the upper limit for the y-axis around 50 o C AND have the intervals in the ones place, i.e. the temperature axis should begin with 1, then 2, 3, 4, 5 up to 50). Here s how to do that: a. right click on the y-axis values, i.e. the numbers b. click on format axis c. under axis options, bounds change the maximum value to 50 and under units, change the major axis to 1 d. click anywhere on the graph (notice, the y-axis values look all bunched up; that will work itself out IF you print a full page graph as indicated below Print a full page graph. Do this by pasting the graph into a Word document then stretching it out to a full page. Once you have the graph, USE A RULER AND THE PICTURE FROM STEP # 3 TO HAND DRAW THE APPROPRIATE THREE LINES ON THE GRAPH. Label T f and T i on the graph. Use these to determine T for the sample calculation. 21. Complete your calculations and placed shared answers with two other groups in your RE table. 22. Answer the following questions (on the back of your handout): a. Write a good title for this lab. Remember, use the lab report format (see lab techs). b. What is the type of lab and the general method of analysis used in the lab? (see lab techs) c. What was the purpose of this lab? Use a past tense sentence d. Write a good, past tense (no pronoun) summary of the procedure. No numbers or values. Use 4-5 sentences. e. Write the result in a good, past tense sentence. Use need to use values here. f. List one reasonable limitation of this lab. Does this limitation cause the final result to be higher or lower than expected? g. Give one reasonable area of further study. In other words, how could you extend the lab to answer another question? Don t just say that you would use another chemical in place of hydrogen peroxide. 23. Take the lab quiz. Going a Little Above and Beyond: Alternative Energy Sources- Global Warming This assignment is due on or before You are to write a one page, 8 ½ X 11, paper about one current energy source that would play a role in helping to curb global warming. For example, you may write about wind power, solar power, etc.-do not make up something that does not exist. You may use regular paper, card stock, etc. ***more information about this Above and Beyond on the next page***

7 The front side of the paper should: have an original (your own design) picture that is hand drawn; NO computer drawn or copied pictures the picture must be neat, colorful, creative, and fill the entire front side of the paper (no margins) the picture must include the name of the alternative energy you may use pencils, crayons, colored markers (not water soluble), etc. keep extra decorations to those that are relatively flat, e.g. glitter, pipe cleaners, etc. The back side of the paper should: have your name in the upper right corner (in margin area---next to edges of paper) with the class period have the title (with the name of the alternative energy) on the top line be about only one type of energy be one page (back side only) be neatly written (ink or pencil) in normal sized hand-writing; no writing in the left or right margins of the paper OR typed (double spaced, size 10 font, 1 inch margins all around; Times New Roman if typed) have an introductory paragraph stating the type of energy and how it works. have a second paragraph about o how this energy alternative would help to curb global warming o indicate where (city, state, country) this energy is currently being used o provide an example of a current product or invention that could use this energy source; this must be realistic; e. g. power a light bulb, fuel a car, etc. o advantages (other than curbing global warming) and disadvantages be grammatically correct with no spelling errors; be careful, in formal papers the use of pronouns and contractions is prohibited be succinct and not rambling On an index card that is turned in separately, you must have your name, period, and the name of your project a complete bibliography (MLA citation see me if you don t know what this looks like) two different sources (only one from the internet/computer)