exam 4 - Version 131 Exam 4 mcdevitt (52365) This print-out...

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Version 131 – Exam 4 – mcdevitt – (52365) 1 This print-out should have 27 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. You are responsible For answering 25 ques- tions correctly. There are two extra credit questions. 001 10.0 points A reaction For which Δ S is (negative, posi- tive) and Δ H is (negative, positive) can only be spontaneous at su±ciently high tempera- tures. 1. negative; positive 2. negative; negative 3. positive; negative 4. positive; positive correct Explanation: ²or spontaneous process, Δ G must be neg- ative. IF Δ S = (+) and Δ H = (+), then Δ G = Δ H - T Δ S = (+) - T (+) = (+) - T, and Δ G will be negative only at high values oF T . 002 10.0 points Heat ³ow is considered positive when heat ³ows (into, out oF) a system; work is consid- ered positive when work is done (by, on) a system. 1. into; on correct 2. out oF; on 3. into; by 4. out oF; by Explanation: Heat ³ow q is considered positive when heat ³ows into a system. Work w is considered positive when work is done on a system. 003 10.0 points Consider the equation NH 4 Br(s) NH 3 (g) + HBr(g) careFully, and think about the sign oF Δ S For the reaction it describes. Δ H = +188.3 kJ. Which response describes the thermodynamic spontaneity oF the reaction? 1. The reaction is spontaneous only at rela- tively low temperatures. 2. All responses are correct. 3. The reaction is spontaneous at all tem- peratures. 4. The reaction is not spontaneous at any temperatures. 5. The reaction is spontaneous only at rela- tively high temperatures. correct 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 Δ G is negative. Δ H is positive For this reaction and Δ S is positive. Δ G = Δ H - T Δ S = (+) - T (+) = (+) - T Δ G will be negative (spontaneous reactions) only at high values oF T . 004 10.0 points Consider the reaction 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) Which statement is true For this reaction? 1. Δ S > 0
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Version 131 – Exam 4 – mcdevitt – (52365) 2 2. Δ S = 0 3. Δ S < 0 correct 4. S m = 0 for O 2 (g) Explanation: For the reaction 3 mol gas 2 mol gas. The fewer the moles of gas, the lower the disorder, so Δ S is negative. 005 10.0 points Consider the following compounds and their standard free energies of formation: cpd energy 1 C 6 H 12 ( ) 6.4 kJ · mol 1 cyclohexane 2 CH 3 OH( ) - 166 kJ · mol 1 methanol 3 N 2 H 4 ( ) 149 kJ · mol 1 hydrazine 4 H 2 O 2 ( ) - 120 kJ · mol 1 hydrogen peroxide 5 CS 2 ( ) 65.3 kJ · mol 1 carbon disul±de Which of these liquids is/are thermody- namically stable? 1.
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This note was uploaded on 05/07/2009 for the course CH 52365 taught by Professor Mcdevitt during the Spring '09 term at University of Texas at Austin.

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exam 4 - Version 131 Exam 4 mcdevitt (52365) This print-out...

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