Exam 3 key - CHEM 1B NAME(LAST(First Show all calculations...

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CHEM 1B 4/12/2006 NAME__________________________ (LAST) (First) Show all calculations, and include units where applicable. (Total points = 100) 1. 2 points. Which of the following processes are spontaneous? (a) a house is built (b) a satellite is launched into orbit (c) a satellite falls back to earth (d) the kitchen gets cluttered Answer: c and d. Thermodynamics says nothing about how fast a spontaneous process will occur; that is the subject of Kinetics. It only says that the initial and final states differ in free energy, the change being negative. 2. 3 points. Will the following change be spontaneous at constant T and P ? Explain in three or four words. û + = -10. kJ, û 6 = +5.0 J/K, T = 298 K Answer: One measure of spontaneity is an overall decrease in free energy, û *±²DFFRPSDQ\LQJ²WKH² process. û *² ² û - ²7² û 6²³²²7KHUH²DUH²WZR²FRQWULEXWLRQV²WR²WKH²IUHH²HQHUJ\²FKDQJH³² The û +²RI²WKH² system, (which, by the way, impacts the entropy of the surroundings) ±²DQG²WKH² û 6²RI²WKH²V\VWHP³² In this problem, both û +²DQG² û 6²DUH²IDYRUDEOH²VLQFH²WKH\²HDFK²FRQWULEXWH²WR²DQ²LQFUHDVH²LQ²WKH²HQWURS\²RI² the universe. Alternately, they both contribute to a decrease in û *²PDNLQJ²WKH²SURFHVV²VSRQWDQHR us at all tempertures (T in Kelvin is always positive). 3. (a) 2 points. Which has the higher positional entropy, N 2 gas at 1 atm or N 2 gas at 1.0 × 10 -2 atm? Answer: Because of the inverse relationship between volume and pressure at constant temperature for a given number of moles, a larger volume is available to molecules at lower pressure. This allows for many more ways of arranging them. 4. 4 points. Calculate û 6 o for the reduction of aluminum oxide by hydrogen gas: Al 2 O 3 (s) + 3 H 2 (g) 2 Al (s) + 3 H 2 O (g) Al 2 O 3 (s) S o = 51, H 2 (g) S o = 131, Al(s) S o = 28, H 2 O (g) S o = 189 (J/K·mol) Answer: ( 3×189 + 2×28 ) - (51 + 3×131 ) J/K = 179 J/K This value is for the change from Initial state to Final state. The Initial state consists of 1 mole pure Al 2 O 3 (s) in its most stable form at 298 K, and 3 moles H 2 (g) gas at 1 atm and 298 K. The Final state consists of 2 moles Al (s) in its most stable form at 298 K and 3 moles H 2 O (g) at 1 atm and 298 K. If the number of moles is doubled for all the components in the balanced equation, so is the value of û 6³² Note that superscript zero (nought) designates the standard state of 1 atm and the pure elements/compounds in their most stable form at, usually, 298 K.
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5. 4 points. A chemical engineer wants to determine the feasibility of making ethanol by reacting water with ethylene according to the equation C 2 H 4 (g) + H 2 O (l) C 2 H 5 OH (l) Is this reaction spontaneous under standard conditions? Show your calculation. û * o f values are: C 2 H 5 OH (l) = -175 kJ/mol, H 2 O (l) = -237 kJ/mol, C 2 H 4 (g) = 68 kJ/mol Answer: û * o = û * o f (ethanol liquid) - ( û * o f (liquid water) + û * o f (ethane gas) ) = -6 kJ per mol rxn.
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