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 b 20. ln(k) vs. 1/T for an endothermic chemical reaction e