Exam 4-solutions

# Exam 4-solutions - Version 289 Exam 4 mccord (51600) 1 This...

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Unformatted text preview: Version 289 Exam 4 mccord (51600) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. mccord-ch301 10am class only - 51600 C s (water) = 4.184 J/g C waters normal boiling pt = 100 C Thermodynamic Data at 25 C H f G f C p ( kJ mol ) ( kJ mol ) ( J mol K ) H 2 O(l)-285.8-237.2 75.3 CCl 4 (l)-135.4-65.21 131.75 CHF 3 (g)-695.4 51.0 001 0.0 points If more points are awarded on this assign- ment, would you like them added to your score? 1. YES, I would like the points and the higher score. correct 2. NO, leave my score alone, I prefer the lower score Explanation: This should be a no-brainer. Most students want higher scores. If you picked yes, you got credit for the question and you got the extra points you asked for (if they were granted by your instructor). If you answered NO, you also got what you wanted... no points awarded. 002 4.0 points Two iron blocks of equal mass are brought into thermal contact. Each block has a heat capacity of 1000 J K 1 . One block is initially at a temperature of 25 C the other is at 75 C. After they come to thermal equilibrium what is the total change in entropy for the two blocks combined? 1. S = 0 . 00 J K 1 2. S = +6 . 00 J K 1 correct 3. S =- 2500 J K 1 4. S =- 3 . 00 J K 1 5. S = +2500 J K 1 Explanation: Because the blocks have the same heat ca- pacity, the final temperature will simply be the average of the temperature of the two block which is 50 C. The total change in en- tropy will be the change in entropy for both blocks added together. The equation for en- tropy change for both blocks is C ln 323 . 15 298 . 15 + C ln 323 . 15 348 . 15 = 6 . 00 J K 1 003 4.0 points In a system at equilibrium at constant tem- perature and pressure, H is necessarily equal to 1. G. 2. T S. correct 3. 0. 4. P V. 5. U. Explanation: G = 0 for a system at equilibrium, so G = H- T S 0 = H- T S H = T S 004 4.0 points How is S univ related to G sys ? Version 289 Exam 4 mccord (51600) 2 1. G sys =- S univ T 2. G sys =- T S univ correct 3. G sys = S univ 4. G sys =- S univ 5. G sys = T S univ 6. G sys = S univ T Explanation: 005 4.0 points It is usually reasonable to estimate that 1. H does not change much with tempera- ture. 2. H and S do not change much with temperature. correct 3. G does not change much with tempera- ture. 4. S does not change much with tempera- ture. 5. G , H , and S do not change much with temperature. Explanation: As G can be obtained from the equation G = H- T S and this equation includes a term for tem- perature, it is reasonable to assume that G is dependent on temperature, while H and S must not vary much with temperature....
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## This note was uploaded on 03/26/2012 for the course CHEM 301 taught by Professor Wandelt during the Spring '08 term at University of Texas at Austin.

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Exam 4-solutions - Version 289 Exam 4 mccord (51600) 1 This...

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