CHEMISTRY 202 Hour Exam II October 25, 2016 Dr. D. DeCoste Name Signature T.A. This exam contains 22 questions on 10 numbered pages. Check now to make sure you have a complete exam. You have two hours to complete the exam. Determine the best answer to the first 20 questions and enter these on the special answer sheet. Also, circle your responses in this exam booklet. Show all of your work and/or provide complete answers to questions 21 and 22. 1-20 (60 pts.) 21 (30 pts.) 22 (30 pts.) Total (120 pts.) Useful Information: Always assume ideal behavior for gases (unless explicitly told otherwise). 760 torr = 1.00 atm R = 0.08206 Latm/molK = 8.314 J/Kmol K = C + 273 N A = 6.022 x 10 23 E = q + w S = q rev /T H = E + PV G = H TS Here are some of the formulas we used/derived in studying thermodynamics. An individual formula may or may not apply to a specific problem. This is for you to decide! S = nrln(v 2 /V 1 ) S = H/T C v = (3/2)R C p = (5/2)R S = ncln(t 2 /T 1 ) G = G + RTln(Q) S surr = q/t w = P V q rev = nrtln(v 2 /V 1 ) q = nc T ln(k) = H R 1 + T S R K ln K 2 1 H = R 1 T2 1 T 1
Hour Exam II Page No. 1 1. The heat of combustion of acetylene, C 2 H 2 (g) at 25 C, is 1304 kj/mol. The products of this reaction are CO 2 (g) and H 2 O(l). At this temperature, the ΔH f values for CO 2 (g) and H 2 O(l) are 393 kj/mol and 286 kj/mol, respectively. Determine ΔH f for acetylene. a) 232 kj/mol b) 232 kj/mol c) 420. kj/mol d) 420. kj/mol e) 464 kj/mol 2. At 25 C, the following enthalpies of reaction are known: 2ClF(g) + O 2 (g) Cl 2 O(g) + F 2 O(g) 2ClF 3 (g) + 2O 2 (g) Cl 2 O(g) + 3F 2 O(g) 2F 2 (g) + O 2 (g) 2F 2 O(g) H = 167.4 kj H = 341.4 kj H = 43.4 kj Calculate ΔH for the following reaction: ClF(g) + F 2 (g) ClF 3 (g) a) 217.4 kj b) 217.4 kj c) 108.7 kj d) 108.7 kj e) 232.7 kj 3. You add 50.0 g of ice at 10.0 C to 75.0 g of water at 75.0 C in a perfectly insulated Styrofoam cup calorimeter. Given the following information, determine the final temperature of the water. Specific heat capacity of H 2 O(s) = 2.03 J/g C Specific heat capacity of H 2 O(l) = 4.18 J/g C ΔH fusion = 6.01 kj/mol a) Not all of the ice melts. b) 11.1 C c) 14.0 C d) 15.1 C e) 43.0 C 4. Consider the reaction between baking soda (sodium bicarbonate) and HCl as shown in the following chemical equation: NaHCO 3 (s) + HCl(aq) NaCl(aq) + CO 2 (g) + H 2 O(l) In a perfectly insulated Styrofoam cup calorimeter, you react 2.000 g of baking soda with 50.00 ml of 1.000 M HCl(aq). In doing so, you note that the temperature of the solution changes from 28.10 C to 24.80 C. Determine H for the reaction between baking soda and HCl. Assume the density of the solution and the specific heat capacity of the solution are the same as that of water (that is, 1.000 g/ml and 4.18 J/ g C, respectively). a) 14.35 kj/mol b) 14.35 kj/mol c) 30.14 kj/mol d) 30.14 kj/mol e) 1.159 kj/mol 5. For how many of the quantities ΔE, ΔH, ΔS and ΔS surr, ΔS univ is the value equal to zero for the vaporization of liquid water at 92 C and 1 atm? a) 0 b) 1 c) 2 d) 3 e) 4
Hour Exam II Page No. 2 6. One mole of an ideal gas (the system) is expanded isothermally under such conditions that no work is produced in the surroundings. How many of the following statements are correct? I. ΔS = 0 II. ΔS surr = 0 III. ΔS univ = 0 IV. ΔS = ΔS surr a) 1 b) 2 c) 3 d) 4 e) It is impossible for the gas to expand with no work. 7. Consider the following reactions/process as described below. Choose the one for which the value of ΔS is positive and has the largest magnitude. a) Heating 2.00 moles of an ideal monatomic gas from 25 C to 125 C at constant volume. b) Heating 2.00 moles of an ideal monatomic gas from 25 C to 125 C at constant pressure. c) An isothermal compression of 5.00 moles of an ideal monatomic gas from 1.00 atm to 10.00 atm. d) The vaporization of one mole of water (ΔH = 40.7 kj/mol) at the boiling point. e) The reaction N 2 (g) + 3H 2 (g) 2NH 3 (g) at 25 C and 1 atm. 8. You and your lab partner notice an old beaker covered in salt sitting on a lab bench and you say, Hmmm, there must have been a solution of salt water in this beaker that evaporated over time leaving the salt. Your lab partner says, But wait, evaporation is an endothermic process which makes ΔS surr negative and the solid salt coming out of solution means ΔS is negative. This means ΔS univ is negative, and the process shouldn t happen. Which statement below best applies to this situation? a) Your initial theory must be wrong a salt water solution cannot evaporate and leave salt behind or else it will violate the first law of thermodynamics. b) Your initial theory must be wrong a salt water solution cannot evaporate and leave salt behind or else it will violate the second law of thermodynamics. c) A salt water solution can do this. This is an example of an exception that violates the first law of thermodynamics. It, of course, does not violate the second law of thermodynamics. d) A salt water solution can do this. Your lab partner is mistaken about either the sign of ΔS surr for the evaporation of water or the sign of ΔS for solid salt coming out of solution (or both!). e) A salt water solution can do this. Your lab partner has neglected to consider all variables.
Hour Exam II Page No. 3 9, 10. Consider the following reactions I. XeF 4 (s) Xe(g) + 2F 2 (g) II. 2H 2 O 2 (l) 2H 2 O(l) + O 2 (g) III. NH 3 (g) + HCl(g) NH 4 Cl(s) IV. 2HF(g) H 2 (g) + F 2 (g) 9. Rank the reactions from most positive value of ΔS to most negative value of ΔS. a) I, IV, II, III b) I, II, IV, III c) III, IV, II, I d) III, II, IV, I e) IV, II, III, I 10. Which reaction is predicted to have a value of ΔS closest to zero? a) I b) II c) III d) IV e) More information is needed. --------------------------------------------------------------------------------------------------------------------- 11. Calcium carbonate is the main constituent in marble, chalk, and egg shells. When decomposed it forms calcium oxide and carbon dioxide gas. Which of the following statements is true concerning the decomposition of calcium carbonate? Note: do not use any background knowledge about the reaction (that is, the question is asking If the reaction is exothermic or If the reaction if endothermic ). a) If the reaction is endothermic it will never be spontaneous at any temperature. b) If the reaction is endothermic, it will be spontaneous at relatively low temperatures but not at higher temperatures. c) If the reaction is exothermic then it must be spontaneous at all temperatures. d) If the reaction is exothermic it will be spontaneous at relatively low temperatures but not at higher temperatures. e) It does not matter whether the reaction is exothermic or endothermic, this reaction would always be spontaneous at any temperature. --------------------------------------------------------------------------------------------------------------------- 12, 13. Consider the reaction 2NO 2 (g) N 2 O 4 (g). At 338.0K, the value of the equilibrium constant, K p = 0.4847, ΔH = 57.00 kj/mol of N 2 O 4 (g) produced, and S [N 2 O 4 (g)] = 319.0 J/Kmol. 12. Determine ΔG for the reaction at 338.0K. a) 50.82 kj/mol b) 164.8 kj/mol c) 164.8 kj/mol d) 2.035 kj/mol e) 2.035 kj/mol 13. Determine S for NO 2 (g) at 338.0K. a) 246.8 J/Kmol b) 246.8 J/Kmol c) 72.17 J/Kmol d) 72.17 J/Kmol e) 144.3 kj/mol
Hour Exam II Page No. 4 14. A machine is being used which uses the isothermal expansion of one mole of an ideal monatomic gas from 4.50 L to 15.00 L. At 25 C, the machine produces 1.50 kj of work. How efficient is this machine? That is, what percent of the maximum work is the machine producing? a) 12.7% b) 26.0% c) 50.3% d) 86.5% e) 98.3% -------------------------------------------------------------------------------------------------------------------------- 15, 16. Consider 1.00 mole of an ideal monatomic gas in a 20.0-L container fitted with a massless, frictionless piston. The gas is heated from 35 C to 125 C. 15. Determine ΔS for the gas due to the volume change. a) 2.13 J/Kmol b) 2.42 J/Kmol c) 2.91 J/Kmol d) 3.20 J/Kmol e) 5.33 J/Kmol 16. Determine ΔS for the gas due to the temperature change. a) 1.87 J/Kmol b) 2.13 J/Kmol c) 3.20 J/Kmol d) 3.46 J/Kmol e) 5.33 J/Kmol -------------------------------------------------------------------------------------------------------------------------- 17. Given the following data, determine the temperature range for which the reaction 3O 2 (g) 2O 3 (g) is spontaneous: O 2 (g) O 3 (g) H f (kj/mol) 0 143 S (J/mol K) 205 239 a) The reaction is spontaneous above temperatures of 2.088K b) The reaction is spontaneous below temperatures of 4205K. c) The reaction is spontaneous below temperatures of 2088K. d) The reaction is spontaneous at all temperatures. e) The reaction is never spontaneous. 18. You are doing a lab in which you produce liquid hydrogen peroxide liquid from hydrogen and oxygen gases at 25 C as shown in the following balanced equation: H 2 (g) + O 2 (g) H 2 O 2 (l) You collect data and determine the following results for H 2 O 2 (l): ΔG f = 188 kj/mol ΔH f = 125 kj/mol Do these data look trustworthy? a) No. Hydrogen peroxide is a very reactive substance so ΔH f should be positive. b) No. The data indicate that the reaction is not spontaneous. c) Yes. The data indicate that the reaction is endothermic, as we would expect. d) No. The quantity (ΔG f ΔH f ) should be positive for this reaction. e) Yes. While the absolute numbers may be incorrect, the relative magnitudes and signs look correct.
Hour Exam II Page No. 5 19, 20. Choose the most appropriate plot for each of the following. A plot may be used once, more than once, or not at all. a) b) c) d) e) 19. w vs. number of steps in the isothermal expansion of an ideal gas against constant pressure 20. ln(k) vs. 1/T for an endothermic chemical reaction
Hour Exam II Page No. 6 21. In lecture we spent a good amount of time on the isothermal expansion of a gas. That is, we discussed an ideal, monatomic gas trapped in a container fitted with a massless, frictionless piston. When the external pressure is suddenly lowered, we expect the volume of the gas to increase. a. Using the principles you have learned in thermodynamics, explain why the expansion of the gas is spontaneous. In your explanation you should: [8 pts.] State and explain the signs (+,, or 0) for ΔS, ΔS surr, and ΔS univ. Explain which factor(s) favor the expansion. Explain which factors(s) do not favor the expansion, if any. b. Specifically, we discussed 1.000 mole of an ideal, monatomic gas in a 2.000-L container fitted with a frictionless, massless piston. The initial external pressure was 10.00 atm and this changed suddenly to 5.000 atm, resulting in a final volume of 4.000 L. Provide quantitative proof that the expansion is spontaneous. Explain your answer and show all work. [10 pts.]
Hour Exam II Page No. 7 21. c. Explain why further spontaneous expansion will not occur as long as the external pressure remains at 5.000 atm and 1.000 mole of gas remains in the container. In your explanation you should: [6 pts.] Explain why 4.000 L is the limit to the spontaneous expansion. Explain what would have to happen so that the gas did spontaneously expand further. Note: we are not saying this will happen, but what would have to happen in order for this gas sample to expand further against a constant pressure of 5.00 atm? Explain which factor(s) would favor the further expansion, if any. Explain which factor(s) would not favor the further expansion. d. Provide quantitative proof that the 1.000 mole gas sample will not spontaneously expand from 4.000 L to 4.100 L against an external pressure of 5.000 atm. Explain your answer and show all work. [6 pts.]
Hour Exam II Page No. 8 22. This question consists of two different scenarios and you are to use the principles of thermodynamics that you have learned to investigate them. a. Suppose you heat 50.0 g of iron metal (specific heat capacity = 0.450 J/g C) in a hot water bath (100 C) and quickly transfer it to 100.0 g of water (specific heat capacity = 4.18 J/g C) at 25 C in a Styrofoam calorimeter. i. Assuming there is no heat loss to the surroundings in the time it takes to reach thermal equilibrium (same temperature of metal and water) and that the specific heat capacities are independent of temperature, provide quantitative proof that reaching thermal equilibrium is spontaneous. Show all work and explain your answer. [8 pts.] ii. Even with the fancy Styrofoam cup calorimeters, eventually both the metal and water will reach room temperature (25 C). Provide quantitative proof that this will occur spontaneously. Again, assume specific heat capacities are independent of temperature. Show all work and explain your answer. [7 pts.]
Hour Exam II Page No. 9 22. b. Water boils at 1 atm and 100 C. That is, the process H 2 O(l) H 2 O(g) is spontaneous at 1.00 atm at temperatures greater than 100 C. But we also know that water evaporates at temperatures lower than this. How can this be? i. First, use the data below to determine ΔG for the vaporization of 1.00 mole of H 2 O(l) at 25 C. Show all work and discuss how your value shows this process is not spontaneous at standard conditions. [4 pts.] H 2 O(l) H 2 O(g) H f (kj/mol) 285.83 241.82 S (J/mol K) 69.90 188.84 ii. Although this process is not spontaneous, it does not mean it will not occur to some extent. Determine the equilibrium constant, K p, for the vaporization of 1.00 mole of H 2 O(l) at 25 C. Show all work. Is it greater or less than 1? Why does this make sense? [3 pts.]
Hour Exam II Page No. 10 22. b. (con t) iii. As we discussed when considering equilibrium, when water is placed in a sealed container it will evaporate until the vapor reaches the equilibrium vapor pressure. If we consult a handbook (or even the internet) we see that the vapor pressure of H 2 O(l) at 25 C is 23.9 torr. Use your answer to part ii on page 9 to verify that this number is correct. Show all work. [3 pts.] iv. Given your answers to the previous parts of this question, explain why the evaporation of water is spontaneous at 25 C. [5 pts.]