CH
Exam 4-solutions.pdf

# 2 the change in internal energy of a system is equal

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2. The change in internal energy of a system is equal to w at constant volume. 3. The change in internal energy of a system is equal to q at constant volume. correct 4. The change in internal energy of a system is constant at constant volume. 5. The change in internal energy of a system is equal to q at constant pressure. 6. The change in internal energy of a system is equal to w at constant pressure. Explanation: 015 5.0points Rank the compounds O 2 ( ) , O 2 (g) , O 3 (g) , O 2 (s) by molar entropy, from least to greatest. 1. O 2 (s) < O 2 ( ) < O 3 (g) < O 2 (g) 2. O 2 (s) < O 2 ( ) < O 2 (g) < O 3 (g) correct 3. O 2 (g) < O 2 ( ) < O 2 (s) < O 3 (g) 4. O 3 (g) < O 2 ( ) < O 2 (s) < O 2 (g) 5. O 3 (g) < O 2 (s) < O 2 ( ) < O 2 (g) Explanation: The three phases of O 2 can be easily ranked, since phase changes have very predictable ef- fects on molar entropy, typically increasing significantly going from solid to liquid and again from liquid to gas. Comparing O 2 (g) to O 3 (g), simply require comparing number of bonds, with O 3 (g) having greater molar

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tran (tmt2433) – Exam 4 – laude – (50105) 6 entropy because it has more bonds. 016 5.0points Copper and glass have heat capacities of 0.4 J/g C and 0.8 J/g C, respectively. If enough heat is added to a 20 gram block of copper to raise its temperature by 10 C, what would the temperature change be if the same amount of heat was added to a 10 gram piece of glass? 1. 5 C 2. 10 C correct 3. 40 C 4. 20 C Explanation: 017 5.0points What is the internal energy associated with the translation of 5 methane molecules? 1. 30 k T 2. 45 k T 3. 15 2 k T correct 4. 15 k T 5. 45 2 k T Explanation: Five molecules times 3 2 k T total transla- tional energy per molecule is 15 2 k T. 018 5.0points Given the enthalpy of vaporization (Δ H vap = 5 . 945 kJ · mol 1 ) and entropy of vaporization S vap = 94 . 06 J · mol 1 · K 1 ) of nitrogen, at what temperature does this phase transi- tion occur? 1. 15 . 8 K 2. - 63 . 2 C 3. 15 . 8 C 4. 36 . 9 K 5. 63 . 2 K correct Explanation: T vap = Δ H vap Δ S vap = 5 . 945 kJ · mol 1 0 . 09406 kJ · mol 1 · K 1 = 63 . 2 K 019 5.0points For the methanol combustion reaction 2 CH 3 OH( ) + 3 O 2 (g) -→ 2 CO 2 (g) + 4 H 2 O(g) estimate the amount of P Δ V work done and tell whether the work was done on or by the system. Assume a temperature of 27 C. 1. 2.5 kJ, work done by the system 2. 7.5 kJ, work done by the system correct 3. 2.5 kJ, work done on the system 4. No work is done in this reaction. 5. 7.5 kJ, work done on the system Explanation: T = 27 C + 273 = 300 K Considering only moles of gas, Δ n = n f - n i = (2 + 4) - 3 = 3 .
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• Fall '07
• Fakhreddine/Lyon

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