Homework 10 - Su, Yung Homework 10 Due: Dec 5 2007, 11:00...

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Su, Yung – Homework 10 – Due: Dec 5 2007, 11:00 pm – Inst: FAKHREDDINE 1 This print-out should have 26 questions. Multiple-choice questions may continue on the next column or page – fnd all choices be±ore answering. The due time is Central time. Each question is worth one point. 001 (part 1 o± 1) 2 points Consider the equation NH 4 Br(s) NH 3 (g) + HBr(g) care±ully, and think about the sign o± Δ S ±or the reaction it describes. Δ H = +188.3 kJ. Which response describes the thermodynamic spontaneity o± the reaction? 1. We cannot tell ±rom the in±ormation given. 2. The reaction is spontaneous at all tem- peratures. 3. The reaction is spontaneous only at rela- tively high temperatures. correct 4. The reaction is spontaneous only at rela- tively low temperatures. 5. The reaction is not spontaneous at any temperatures. Explanation: Entropy ( S ) is high ±or systems with high degrees o± ±reedom, disorder or randomness and low ±or systems with low degrees o± ±ree- dom, disorder or randomness. S(g) > S ( ) > S (s) . A reaction is spontaneous only when Δ G is negative. Δ H is positive ±or 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 o± T . 002 (part 1 o± 1) 2 points For which o± the ±ollowing processes does the entropy o± the universe decrease? 1. None o± these is correct. correct 2. ±reezing one mole o± water to ice at - 10 C 3. ±reezing one mole o± water to ice at 0 C and then cooling it to - 10 C 4. ±reezing one mole o± water to ice at 0 C 5. melting one mole o± ice to water at 0 C Explanation: For spontaneous changes, the entropy o± the universe increases. 003 (part 1 o± 1) 2 points The law states that a substance that is per- ±ectly crystaline at 0 K has an entropy o± zero. This law is called 1. None o± these 2. the third law o± thermodynamics. cor- rect 3. the second law o± thermodynamics. 4. the frst law o± thermodynamics. Explanation: The Third Law o± Thermodynamics states that the entropy o± a pure crystalline sub- stance is 0 at absolute 0 K. 004 (part 1 o± 1) 2 points Calculate the standard entropy o± condensa- tion o± chloro±orm at its boiling point (335 K). The standard molar enthalpy o± vaporiza- tion o± chloro±orm at its boiling point is 31.4 kJ · mol - 1 . 1. - 31.3 kJ · K - 1 · mol - 1 2. +506 J · K - 1 · mol - 1
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Su, Yung – Homework 10 – Due: Dec 5 2007, 11:00 pm – Inst: FAKHREDDINE 2 3. +31.4 kJ · K - 1 · mol - 1 4. +93.7 J · K - 1 · mol - 1 5. - 93.7 J · K - 1 · mol - 1 correct Explanation: 005 (part 1 of 1) 2 points For the four chemical reactions I) 3O 2 (g) 2O 3 (g) II) 2H 2 O(g) 2H 2 (g) + O 2 (g) III) H 2 O(g) H 2 O( ) IV) 2H 2 O( ) + O 2 (g) 2H 2 O 2 ( ) which one(s) is/are likely to exhibit a positive Δ S ? 1.
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This note was uploaded on 04/15/2009 for the course CH 54740 taught by Professor Fakhreddine during the Spring '09 term at University of Texas.

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Homework 10 - Su, Yung Homework 10 Due: Dec 5 2007, 11:00...

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