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Unformatted text preview: Version 370 Exam 4 McCord (53745) 1 This print-out should have 26 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 1 cal = 4.184 J 1 L atm = 101.325 J R = 8.314 J/mol K 001 10.0 points At the normal boiling point of water, H vap = 40 kJ/mole. What is the entropy change for H 2 O( ) H 2 O(g) ? 1. 400 J/mol K 2.- 40 kJ/mol K 3.- 107 J/mol K 4. 40 kJ/mol K 5. 107 kJ/mol K 6. 107 J/mol K correct Explanation: 002 10.0 points For which reactions I) O 2 (g) + H 2 (g) H 2 O 2 ( ) II) C(s , diamond) + O 2 (g) CO 2 (g) III) N 2 ( ) + 3 F 2 (g) 2 NF 3 ( ) IV) C(s , graphite) + 3 2 O 2 (g) + H 2 (g) CO 2 (g) + H 2 O(g) V) 2 Fe(s) + 3 2 O 2 (g) Fe 2 O 3 (s) would H r = H f ? 1. I, III and V only 2. I, III, IV and V only 3. I, II, III, IV and V 4. I and V only correct 5. II and IV only 6. II, III and IV only 7. IV and V only 8. I and II only Explanation: 003 10.0 points Which of the statements below concerning thermodynamic sign convention is NOT true: 1. H is negative when heat is released to the surroundings. 2. G is negative when a reaction is spon- taneous. 3. Work is done on the system when V is negative. 4. w is positive when work is done by the system. correct 5. S is positive when there is increasing disorder. Explanation: w is positive only when work is done ON the system, not BY the system. When the system does work, volume increases or the number of moles increases ( V > 0, n > 0). w =- P V =- nRT and will therefore be negative when V or n is positive. 004 10.0 points The internal energy of a substance can be brought to zero by cooling the substance all the way down to absolute zero ( T = 0 K). 1. False correct 2. True Explanation: The very fact that you have any matter at all means you have some internal energy - even at absolute zero. Version 370 Exam 4 McCord (53745) 2 005 10.0 points For the reaction 2 C(s) + 2 H 2 (g) C 2 H 4 (g) H r = +52 . 3 kJ mol 1 and S r =- 53 . 07 J K 1 mol 1 at 298 K. This reac- tion will be spontaneous at 1. all temperatures. 2. temperatures below 985 K. 3. temperatures below 1015 K. 4. temperatures above 985 K. 5. no temperature. correct Explanation: G = H- T S is used to predict spon- taneity. ( G is negative for a spontaneous reaction.) T is always positive. We thus have G = (+)- T (- ). For G to be neg- ative, T would have to be negative, which is not physically possible, so the reaction is nonspontaneous at any temperature. 006 10.0 points A friend says that as a plant grows and takes an unordered set of molecules and puts them in a highly organized system the second law of thermodynamics is violated. Your best response is that 1. you must be thinking of the third law....
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This note was uploaded on 02/09/2010 for the course CH 301 taught by Professor Fakhreddine/lyon during the Spring '07 term at University of Texas at Austin.
- Spring '07