Exam 4 Quy - Version 028 Exam 4 quy (50970) 1 This...

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Unformatted text preview: Version 028 Exam 4 quy (50970) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points Which would you expect to be least viscous? 1. C 8 H 18 at 50 C 2. C 8 H 18 at 30 C 3. C 4 H 8 at 50 C correct 4. C 4 H 8 at 30 C Explanation: 002 10.0 points The energy change H associated with the reaction NBr 3 (g) + 3 H 2 O(g) 3 HOBr(g) + NH 3 (g) is +81 kJ/mol rxn. These bond energy values might be useful: O-H 459 kJ/mol; N-H 386 kJ/mol; O-Br 201 kJ/mol. The strength of the N Br bond is 1. 66 kJ/mol 2. 4 kJ/mol 3. 280 kJ/mol 4. 248 kJ/mol 5. 128 kJ/mol 6. 155 kJ/mol correct 7. 465 kJ/mol Explanation: H = 81 kJ Bonds broken: 3 N Br bonds at x kJ each 6 O H bonds at 459 kJ each Bonds made: 3 N H bonds at 386 kJ each 3 O H bonds at 459 kJ each 3 O Br bonds at 201 kJ each. H = (bonds broken)- (bonds made) 81 = [3 x + 6(459)]- [3(386) + 3(459) + 3(201)] 81 = 3 x + 2754- 3138 3 x = 465 x = 155 kJ 003 10.0 points The normal melting point for ice is 273 K. Ice melting at atmospheric pressure and 272 K is 1. exothermic and spontaneous. 2. endothermic and nonspontaneous. cor- rect 3. endothermic and spontaneous. 4. exothermic and nonspontaneous. Explanation: To melt ice, energy must be added, so the process is endothermic and H is positive. At phase change points, G = 0. Below 273 K G will be negative and this will be a nonspontaneous process. 004 10.0 points Consider the equation NH 4 Br(s) NH 3 (g) + HBr(g) carefully, and think about the sign of S for the reaction it describes. H = +188.3 kJ. Which response describes the thermodynamic spontaneity of the reaction? 1. We cannot tell from the information given. 2. The reaction is spontaneous only at rela- tively low temperatures. 3. The reaction is spontaneous only at rela- tively high temperatures. correct Version 028 Exam 4 quy (50970) 2 4. The reaction is not spontaneous at any temperatures. 5. The reaction is spontaneous at all tem- peratures. Explanation: Entropy ( S ) is high for systems with high degrees of freedom, disorder or randomness and low for systems with low degrees of free- dom, disorder or randomness. S(g) > S ( ) > S (s) . A reaction is spontaneous only when G is negative. H is positive for this reaction and S is positive. G = H- T S = (+)- T (+) = (+)- T G will be negative (spontaneous reactions) only at high values of T . 005 10.0 points From the following data (at 298 K), Substance H f S kcal/mol cal/mol K NO 22.0 50.0 H 2 O- 68 . 17.0 NH 3- 11 . 46.0 O 2 0.0 49.0 find G for the reaction 4 NO + 6 H 2 O 4 NH 3 + 5 O 2 ....
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This note was uploaded on 12/03/2011 for the course CH 301 taught by Professor Fakhreddine/lyon during the Fall '07 term at University of Texas at Austin.

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Exam 4 Quy - Version 028 Exam 4 quy (50970) 1 This...

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