chem exam3laude - Warsi Kusum Exam 3 Due Dec 7 2006 4:00 pm...

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Warsi, Kusum – Exam 3 – Due: Dec 7 2006, 4:00 pm – Inst: David Laude 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. The due time is Central time. Please remember to write down your ver- sion number. GOOD LUCK! 001 (part 1 of 1) 6 points The combustion of a certain amount of methane produces 100 kJ of energy. Dur- ing this process the amount of energy in the universe 1. decreases by 100 kJ. 2. does not increase nor decrease. correct 3. The answer cannot be determined with- out knowing the chemical equation for this reaction. 4. increases by 100 kJ. Explanation: According to the First Law of Thermody- namics, the total amount of energy in the universe is constant. 002 (part 1 of 1) 6 points Calculate Δ S surr at 298 K for the reaction 6 C(s) + 3 H 2 (g) C 6 H 6 ( ) Δ H r = +49.0 kJ · mol - 1 and Δ S r = - 253 J · K - 1 · mol - 1 . 1. - 253 J · K - 1 · mol - 1 2. +164 J · K - 1 · mol - 1 3. +253 J · K - 1 · mol - 1 4. - 417 J · K - 1 · mol - 1 5. - 164 J · K - 1 · mol - 1 correct Explanation: 003 (part 1 of 1) 6 points A 50 gram sample of delicious pork rinds is placed in an open, constant pressure calorime- ter that contains 3000 grams of water. The temperature of the water increases by 47 C when the pork rinds are combusted. The heat capacity of water is 4.184 J/g C. Assume the heat lost to the calorimeter itself or to the air is negligible. Which of the following is correct for the SYSTEM? 1. Δ H = +590 kJ 2. Δ U = - 11 . 8 kJ 3. Δ U = +11 . 8 kJ 4. Δ H = - 11 . 8 kJ 5. Δ H = - 590 kJ correct 6. Δ U = +590 kJ 7. Δ H = +11 . 8 kJ 8. Δ U = - 590 kJ Explanation: At constant pressure q p = Δ H . q cal = m H 2 O c p Δ T = (3000 g) (4 . 184 J / g C) (47 C) = 589944 J = 589 . 944 kJ Thus q sys = - q cal = - 589 . 944 kJ. 004 (part 1 of 1) 6 points For a certain reaction Δ H is 0.00159114 kJ and Δ S is 1.1 J/K. This reaction can become spontaneous only above some temperature. What is this temperature? 1. 1.445 C 2. - 0.00445 C 3. 0.00145 C 4. - 271.55 C correct
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Warsi, Kusum – Exam 3 – Due: Dec 7 2006, 4:00 pm – Inst: David Laude 2 5. - 418.82 C 6. 691.82 C 7. 271.55 C 8. 418.82 C 9. - 691.82 C 10. - 1.445 C Explanation: 005 (part 1 of 1) 6 points Consider the reaction CH 4 (g) + I 2 (g) CH 3 I(g) + HI(g) Bond energy tables give the following values: C H : 416 kJ/mol I I : 151 kJ/mol H I : 299 kJ/mol C I : 213 kJ/mol This reaction is 1. endothermic by about 55 kJ/mol. cor- rect 2. endothermic, but insufficient information given to say by how much. 3. endothermic by about 206 kJ/mol. 4. exothermic by about 206 kJ/mol. 5. exothermic by about 55 kJ/mol. Explanation: C H = 416 kJ/mol I I = 151 kJ/mol H I = 299 kJ/mol C I = 213 kJ/mol Δ H 0 rxn = X BE rct - X BE prod BE rct : 4 (C H) : 4 (416 kJ / mol) = 1664 kJ / mol 1 (I I) : 1 (151 kJ / mol) = 151 kJ / mol X BE rct = 1815 kJ / mol BE prod : 3 (C H) : 3 (416 kJ / mol) = 1248 kJ / mol 1 (C I) : 1 (213 kJ / mol) = 213 kJ / mol 1 (H I) : 1 (299 kJ / mol) = 299 kJ / mol X BE prod = 1760 kJ / mol Δ H 0 rxn = X BE rct - X BE prod = (1815 - 1760) kJ / mol = +55 kJ / mol Therefore the reaction is endothermic by about 55 kJ/mol since the sign of Δ H rxn is positive.
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