Exam 4-solutions

Exam 4-solutions - Version 273 Exam 4 Sutclie(53770 This...

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Version 273 – Exam 4 – Sutcliffe – (53770) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Consider the equation NH 4 Br(s) NH 3 (g) + HBr(g) , and Δ H = +188.3 kJ. Which response de- scribes the thermodynamic spontaneity of the reaction? 1. We cannot tell from the information given. 2. The reaction is spontaneous only at rela- tively high temperatures. correct 3. The reaction is not spontaneous at any temperatures. 4. The reaction is spontaneous at all tem- peratures. 5. The reaction is spontaneous only at rela- tively low temperatures. Explanation: Entropy ( S ) is high for systems with high degrees of freedom, disorder or randomness and low for systems with low degrees of free- dom, disorder or randomness. S(g) > S ( ) > S (s) . A reaction is spontaneous only when Δ G is negative. Δ H is positive for 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 of T . 002 10.0 points Predict which of ethanol (CH 3 CH 2 OH) or dimethyl ether (CH 3 OCH 3 ) has the greater viscosity. Assume that they are both at the same temperature and in their liquid form. 1. Equal viscosities 2. ethanol correct 3. dimethyl ether Explanation: Ethanol should have a greater viscosity than dimethyl ether because ethanol can hy- drogen bond while dimethyl ether cannot. 003 10.0 points Calculate the standard entropy of fusion of ethanol at its melting point 159 K. The stan- dard molar enthalpy of fusion of ethanol at its melting point is 5.02 kJ · mol 1 . 1. +31 . 6 J · K 1 · mol 1 correct 2. 44 . 0 J · K 1 · mol 1 3. 5 . 02 kJ · K 1 · mol 1 4. 31 . 6 J · K 1 · mol 1 5. +5 . 02 kJ · K 1 · mol 1 Explanation: Δ H fus = 5020 J · mol 1 T BP = 159 K Δ S cond = q T = Δ H fus T MP = 5020 J · mol 1 · K 1 159 K = +31 . 5723 J · mol 1 · K 1 . 004 10.0 points For liquid mercury in a capillary tube, 1. the meniscus is convex and the cohesive forces are stronger than the adhesive forces. correct 2. None of these

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Version 273 – Exam 4 – Sutcliffe – (53770) 2 3. the meniscus is concave and the adhesive forces are stronger than the cohesive forces. 4. the meniscus is concave and the cohesive forces are stronger than the adhesive forces. 5. the meniscus is convex and the adhesive forces are stronger than the cohesive forces. Explanation: Forces between molecules within a liquid are called cohesive forces. Forces of attrac- tion between a liquid and a surface are called adhesive forces. In liquid mercury in a test tube cohesive forces are stronger than adhe- sive forces; thus a convex meniscus forms. 005 10.0 points Which of the compounds I) N 2 H 4 ( ); II) CuO(s); III) C 6 H 12 ( ) (cyclohexane); IV) PCl 3 (g); is/are stable and will not decompose into their elements under standard conditions at 25 C?
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