Exam 4 - Version 119 Exam 4 mccord (50970) This print-out...

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Version 119 – Exam 4 – mccord – (50970) 1 This print-out should have 26 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. 001 10.0 points Reactions with positive values oF Δ S r always become spontaneous at low temperatures. 1. True 2. ±alse correct Explanation: Δ G = Δ H - T Δ S is used to predict spon- taneity. (Δ G is negative For a spontaneous reaction.) T is always positive. When Δ S is positive and T is small, then in order For Δ G to be negative, Δ H must be negative, zero or have a very small positive value. But this is not always the case, so not ALL reactions with a positive Δ S are spontaneous at low temperatures. 002 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. 1. 0, +, + 2. - , - , - 3. - , +, + correct 4. - , 0, + 5. - , +, - Explanation: Δ G is negative For a spontaneous reaction. Sublimation requires energy to Facilitate the solid becoming a gas, so the process is en- dothermic (Δ H is positive). ±inally, the en- tropy oF a gas is more than that oF a solid, so disorder increases (Δ S is positive). 003 10.0 points A sample oF 1 . 55 grams oF compound Y is burned completely in a bomb calorimeter which contains 2500 g oF water. The temper- ature rises From 24 . 358 C to 24 . 762 C. What is Δ U rxn For the combustion oF compound Y? The hardware component oF the calorimeter has a heat capacity oF 3 . 18 kJ/ C. The spe- cifc heat oF water is 4.184 J/g · C, and the MW oF Y is 128 g/mol. 1. -256.5 2. -529.6 3. -477.3 4. -189.3 5. -419.8 6. -225.9 7. -350.1 8. -366.7 9. -455.1 10. -552.3 Correct answer: - 455 . 1 kJ / mol oF Y. Explanation: SH = 4.184 J/g · C HC = 3 . 18 kJ/ C m Y = 1 . 55 g MW Y = 128 g/mol m water = 2500 g Δ T = 24 . 762 C - 24 . 358 C = 0 . 404 C Heat = (SH)(m water )(Δ T ) + (HC)(Δ T ) , Then divide the heat by the number oF moles which is mass/MW. 004 10.0 points All systems undergoing spontaneous reaction at constant T and P 1. have Δ H > 0. 2. have Δ S > 0. 3. give o² heat. 4. have Δ H < 0. 5. are slow. 6. have Δ S < 0. 7. are Fast. 8. have Δ G > 0.
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Version 119 – Exam 4 – mccord – (50970) 2 9. have Δ G < 0. correct 10. absorb heat. Explanation: Δ G is always negative (Δ G < 0) for spon- taneous reactions. 005 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 released 2. 0.253 kJ absorbed 3. 3.95 kJ released 4. 3.95 kJ absorbed 5. 2.52 kJ absorbed 6. 2.52 kJ released correct Explanation: MW CH 3 OH = 32.042g ΔH(fusion) = - ΔH(freezing) q = 25 . 6 g × 1mol 32 . 042g × - 3 . 16 kJ mol = - 2 . 525 kJ 006 0.0 points This question starts out at zero points but could very well increase after the grading. Now, if more points are awarded (the curve)
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Exam 4 - Version 119 Exam 4 mccord (50970) This print-out...

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