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HW09-solutions-QUY

# HW09-solutions-QUY - arriaga(yga77 HW09 quy(50970 This...

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arriaga (yga77) – HW09 – quy – (50970) 1 This print-out should have 19 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0points A system had 150 kJ of work done on it and its internal energy increased by 60 kJ. How much energy did the system gain or lose as heat? 1. The system lost 90 kJ of energy as heat. correct 2. The system gained 90 kJ of energy as heat. 3. The system lost 210 kJ of energy as heat. 4. The system gained 210 kJ of energy as heat. 5. The system gained 60 kJ of energy as heat. Explanation: Δ U = q + w q = Δ U w = 60 kJ (+150 kJ) = 90 kJ (Negative means the system lost energy as heat.) 002 10.0points When 2.00 kJ of energy is transferred as heat to nitrogen in a cylinder fitted with a piston at an external pressure of 2.00 atm, the nitro- gen gas expands from 2.00 to 5.00 L against this constant pressure. What is Δ U for the process? 1. +1 . 39 kJ correct 2. 0 . 608 kJ 3. 0 4. 2 . 61 kJ 5. +2 . 61 kJ Explanation: q = 2 kJ P = 2 atm V 0 = 2 L V f = 5 L For expansion against a constant external pressure, w = P ext Δ V = ( 2 atm)(5 L 2 L) × (101 . 325 J · L 1 · atm 1 ) = 607 . 95 J = 0 . 60795 kJ . Δ U = q + w = 2 kJ + ( 0 . 60795 kJ) = +1 . 39205 kJ . 003 10.0points A 1.00 g sample of n -hexane (C 6 H 14 ) under- goes complete combustion with excess O 2 in a bomb calorimeter. The temperature of the 1502 g of water surrounding the bomb rises from 22.64 C to 29.30 C. The heat capacity of the hardware component of the calorimeter (everything that is not water) is 4042 J/ C. What is Δ U for the combustion of n -C 6 H 14 ? One mole of n -C 6 H 14 is 86.1 g. The specific heat of water is 4.184 J / g · C. 1. 4 . 52 × 10 3 kJ/mol 2. 1 . 15 × 10 4 kJ/mol 3. 9 . 96 × 10 3 kJ/mol 4. 5 . 92 × 10 3 kJ/mol correct 5. 7 . 40 × 10 4 kJ/mol Explanation: m C 6 H 8 = 1.00 g m water = 1502 g SH = 4.184 J/g · C HC = 4042 J/ C Δ T = 29 . 30 C 22 . 64 C = 6 . 66 C The increase in the water temperature is 29.30 C 22.64 C = 6.66 C. The amount of heat responsible for this increase in tempera- ture for 1502 g of water is q = (6 . 66 C) parenleftbigg 4 . 184 J g · C parenrightbigg (1502 g) = 41854 J = 41 . 85 kJ

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arriaga (yga77) – HW09 – quy – (50970) 2 The amount of heat responsible for the warm- ing of the calorimeter is q = (6 . 66 C)(4042 J / C) = 26920 J = 26 . 92 kJ The amount of heat released on the reaction is thus 41.85 kJ + 26.92 kJ = 68.77 kJ per g of n -hexane.
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