Exam 1-solutions

# Exam 1-solutions - Version 349 – Exam 1 – Holcombe...

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Unformatted text preview: Version 349 – Exam 1 – Holcombe – (52460) 1 This print-out should have 35 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. Helpful constants and conversions 1 atm = 760 torr = 760 mm Hg 0 degC = 273.15 K R = 8.314 J/K*mol R = 0.082 L*atm/K*mol 001 10.0 points Consider the equation NH 4 Br(s) → NH 3 (g) + HBr(g) carefully, and think about the sign of Δ S for the reaction it describes. Δ H = +188.3 kJ. Which response describes the thermodynamic spontaneity of the reaction? 1. All responses are correct. 2. The reaction is spontaneous at all tem- peratures. 3. The reaction is spontaneous only at rela- tively high temperatures. correct 4. The reaction is spontaneous only at rela- tively low temperatures. 5. The reaction is not spontaneous at any 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 ( ℓ ) > 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 For the endothermic reaction H 2 (g) + I 2 (g) ⇀ ↽ 2 HI(g) which of I) increasing the pressure; II) decreasing the volume; III) increasing the temperature will shift the equilibrium toward the prod- ucts? 1. I and III only 2. II only 3. III only correct 4. I and II only 5. I, II and III 6. I only Explanation: Since the number of moles of gas are equal on both sides of the equation, then volume and pressure will have no affect on the equi- librium. Since the reaction is endothermic, lowering the temperature will favor the prod- ucts. 003 10.0 points Calculate the vapor pressure at 25 ◦ C of a mixture of benzene and toluene in which the mole fraction of benzene is 0.650. The vapor pressure at 25 ◦ C of benzene is 94.6 torr and that of toluene is 29.1 torr. 1. 84.4 torr 2. 124 torr 3. 51.3 torr 4. 71.7 torr correct 5. 61.5 torr Version 349 – Exam 1 – Holcombe – (52460) 2 Explanation: 004 10.0 points Suppose the reaction A ⇀ ↽ B has an equilibrium constant of 1.0 and the ini- tial concentrations of A and B are 0.5 M and 0.0 M, respectively. Which of the following is the correct value for the final concentration of A? 1. 1.00 M 2. 0.500 M 3. 0.250 M correct 4. 1.50 M 5. None of these is correct. Explanation: K = 1 . [A] ini = 0 . 5 M [B] ini = 0 M A ⇀ ↽ B ini, M 0.5 0.0 Δ, M- x x eq, M . 5- x x K = [B] [A] = 1 . x . 5- x = 1 . x = 0 . 25 M [A] = 0 . 5- x = 0 . 25 M 005 10.0 points Given that CO 2 (g) reacts with C(s) via the reaction C(s) + CO 2 (g) ⇀ ↽ 2 CO(g) and K p = 1.90 atm, what is the equilibrium partial pressure of CO 2 if 1.00 atm of CO 2 is placed in a vessel with PURE SOLID CAR- BON? (Note: There was no CO initially.) 1. 0.43 atm 2. 0.60 atm 3. 0.51 atm0....
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## This note was uploaded on 03/15/2010 for the course CH 52460 taught by Professor Holcombe during the Spring '10 term at University of Texas.

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Exam 1-solutions - Version 349 – Exam 1 – Holcombe...

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