Exam 4-solutions

# Exam 4-solutions - Version 240 Exam 4 mccord(50970 This...

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Version 240 – Exam 4 – mccord – (50970) 1 This print-out should have 26 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points 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. 7.5 kJ, work done on the system 2. No work is done in this reaction. 3. 2.5 kJ, work done on the system 4. 7.5 kJ, work done by the system correct 5. 2.5 kJ, work done by the system Explanation: T = 27 C + 273 = 300 K Considering only moles of gas, Δ n = n f - n i = (2 + 4) - 3 = 3 . w = - Δ nRT = - (3 mol) (8 . 314 J / mol · K) (300 K) = - 7500 J = - 7 . 5 kJ The system expands because Δ n is positive, so the system does the work on the surround- ings. Also, when w is negative, work is done by the system. 002 10.0 points The energy change Δ H associated with the reaction NBr 3 (g) + 3 H 2 O(g) 3 HOBr(g) + NH 3 (g) is +81 kJ/mol rxn. What is the expected energy change for the reaction of nine moles of HOBr and two moles of NH 3 ? 1. +81 kJ/mol 2. -243 kJ/mol 3. +162 kJ/mol 4. -365 kJ/mol 5. -162 kJ/mol correct 6. +243 kJ/mol 7. +365 kJ/mol 8. -81 kJ/mol Explanation: The reverse reaction is happening when HOBr and NH 3 are mixed. However, they are not mixed in stoichiometric amounts. The 2 moles of NH 3 would require only 6 moles of HOBr. Therefore the HOBr is in excess and the NH 3 is the limiting reactant. Now you only need to double the Δ H for the reaction and change the sign to negative. This gives you -162 kJ/mol. 003 10.0 points The enthalpy of fusion of methanol (CH 3 OH) is 3.16 kJ/mol. How much heat would be absorbed or released upon freezing 25.6 grams of methanol? 1. 0.253 kJ absorbed 2. 2.52 kJ released correct 3. 3.95 kJ absorbed 4. 2.52 kJ absorbed 5. 3.95 kJ released 6. 0.253 kJ released Explanation: MW CH 3 OH = 32.042g ΔH(fusion) = - ΔH(freezing)

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Version 240 – Exam 4 – mccord – (50970) 2 q = 25 . 6 g × 1mol 32 . 042g × - 3 . 16 kJ mol = - 2 . 525 kJ 004 10.0 points Which of the following reactions is an en- thalpy of formation reaction? 1. CH 4 (g) + 2O 2 (g) ←→ CO 2 (g) + 2H 2 O 2. 2Fe(s) + 3 / 2O 2 (g) ←→ Fe 2 O 3 (s) cor- rect 3. Hg(s) + 1 / 2O 2 (g) ←→ HgO(s) 4. C diamond (s) + 2H 2 (g) ←→ CH 4 (g) Explanation: Formation reactions describe production of exactly one mole of one product from stoichio- metric quantities of elements in their standard states. 005 10.0 points A can of spray paint uses isobutane (C 4 H 10 ) as a propellant. The brand new can contains 15 g of propellant but after several uses the can now contains 13 . 5 g of propellant. What amount of work has been done by the propel- lant gas as it expands out of the can? Assume the can is stored and operated at 24 C. 1. 0.629 J 2. 5.11 J 3. 63.76 J correct 4. 3705.8 J Explanation: m 1 = 15 g m 2 = 13 . 5 g MW = 58.12 g/mol R = 8 . 314 J / mol · K T = 24 C + 273 . 15 = 297 . 15 K Δ n = (13 . 5 g - 15 g) 1 mol 58 . 12 g = - 0 . 0258087 mol , so w = - P Δ V = - nRT = - ( - 0 . 0258087 mol) (8 . 314 J / mol · K) × (297 . 15 K) = 63 . 7605 J 006 10.0 points The sublimation of solid carbon dioxide is a spontaneous process. Predict the sign (+, - , or 0) of Δ G r , Δ H r , and Δ S r , respectively.
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• Fall '07
• Fakhreddine/Lyon

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