Unit 9 – Thermodynamics and Kinetics (Ch. 17)
Overview: In this lab, you will get a chance to try to measure the DH values for some reactions that occur in aqueous solution. You will use a styrofoam cup calorimeter and various reactants according to the instructions below.
1. Obtain a styrofoam calorimeter and a piece of white paper. Fold the paper into quarters and poke a hole through the middle so that you have a makeshift lid for the cup. The hole is so that you can put a thermometer through.
2. Obtain a thermometer.
3. Measure out 40 ml of distilled water into the styrofoam cup. Measure the temperature of the water.
4. Weigh out about 2 grams of solid NaOH. Record the exact weight. Caution: sodium hydroxide is a strong base and will turn your skin to soap. Avoid contact!!!
5. Dump the solid NaOH into the styrofoam cup and swirl to dissolve the salt. Keep the lid on the calorimeter as much as possible and record the increase in temperature. You will have to judge when the dissolution reaction is complete.
6. Calculate the number of calories that were released in this exothermic process.
7. Try to come up with a value of kcal/mole for this reaction:
NaOH (s) à NaOH (aq) (“ideal” DH = -44.51 kJ/mol)
8. Convert kcal to kJ & compare to
the "ideal" value
given above.
Ponder reasons for differences between your number and the ideal number.
In this phase of the lab, you will try to measure the heat of reaction of various types of reactions. For each reaction that you do, your goal is to find try to come up with a value of kJ/mole.
Your group is to perform one of the following reactions: equations are unbalanced!
A) Zn (s) + HCl (aq) à ZnCl2 (aq) + H2 (g)
B) CuCl2 (aq) + Al (s) à AlCl3 (aq) + Cu (s)
C) Ca (s) + H2O (l) à Ca(OH)2 (s) + H2 (g)
D) HCl (aq) + Mg (s) à MgCl2 (aq) + H2 (g)
Note: Each reaction will be run using 50 ml of liquid. Concentrations and amounts of solids will vary according to guidelines below. For the purpose of counting calories, assume that the liquids are pure water.
Rxn A: Use up to 2.0 grams mossy zinc (small pieces better than large) & 6 M HCl. Avoid breathing fumes.
Rxn B: Use 0.5 M CuCl2 and up to 0.4 g aluminum foil (shredded to utilize max surface area)
Rxn C: Use up to 0.8 g calcium metal.
Rxn D: Use 2 M HCl and up to 0.8 grams Mg turnings. Do rxn in hood.
Your write-up consists of your recorded data and observations, as well as your work on the sheet for Part A. You will then answer the questions on the question page, and submit these sheets in a week.
1. Try to give three reasons why your number for part A (kJ/mol) is different from the ideal value. Do not resort to "human error"--try to be more specific and more insightful.
2. Show your method for calculating the kJ/mole for the reaction you ran in part B. Explain each step as if you were tutoring someone who was absent this week.
3. What is similar about all the reactions listed in part B?
4. Why is it important that these reactions be run in a styrofoam cup (as opposed to a beaker)?
5. The DH value for HCl (aq) is not listed on your thermodynamic data sheet. However, there is a listing for H+ (aq) and a listing for Cl- (aq). Do you think it would be valid to say that the DH value for HCl (aq) is the sum of the numbers for H+ (aq) and Cl- (aq)? Explain.
6. Is it possible to measure the temperature of a solution without changing the temperature? That is, does the presence of the measuring device (thermometer) affect the reading?
7. All the reactions done in this lab are exothermic. Do you think it would be possible to use a coffee cup and a thermometer to measure the DH value of an endothermic reaction? Explain.
8. Calculate how many moles of liquid reactant you had in your coffee-cup for part B. Compare this to the number of moles of solid you added. Note: you should find that you had excess liquid reactant.
