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OldExamsNew - Selected 1C problems from old exams(The...

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Selected 1C problems from old exams (The problems here are from several old exams. The Final exam will have one multiple choice type question and four additional problems similar to the problems below ) Problem 1 1. An ideal gas undergoes an isothermal expansion during which 500J of work are done by the system. What is the heat transfer q ? (a) q = 0 (b) q = 500J (c) q = 500J Answer = b. w = 500J. Δ E = 0 for isothermal process in ideal gas, thus q = Δ E w = 500J 2. For the reaction CH 4 (g)+2O 2 CO 2 + 2H 2 O at 298K (a) Δ H > Δ E (b) Δ H = Δ E (c) Δ H < Δ E Answer = c. Δ H = Δ E + Δ nRT. Δ n = 1 3 = 2, so Δ H < Δ E. 3. For the reaction NH 3 (g)+HCl(g) NH 4 Cl(s) Δ S 0 is (a) negative (b) zero (c) positive Answer = a . Solid are more ordered. 4. An exothermic reaction which has a positive Δ S o is (a) spontaneous at all temperatures (b) spontaneous at low temperatures (c) spontaneous at high temperatures Answer = a. Δ G = Δ H –T Δ S . Δ H < 0 , T Δ S >0, so Δ G is always negative. 5. The solution 0.1M Cu(NO 3 ) 2 has a higher boiling point than the solution 0.2M NaCl (a) true (b) false (c) can’t tell Answer = b . Total ionic concentration in Cu(NO 3 ) 2 is 0.1 x 3 = 0.3 is less than in NaCl (0.4) 6. A substance whose triple point pressure is greater than 1atm does not go through a solid-liquid transition at a normal pressure. (a) true (b) false (c) can’t tell Answer = a . At pressure below the triple point pressure, a solid undergoes sublimation. 7. The cell potential of a spontaneous redox reaction is Answer = b . This corresponds to negative Δ G . 8. The cell potential is zero when the reaction is Answer = c . 9. The oxidation number of Cr in C r 2 O 7 " 2 is Answer = b . The sum of oxidation number must equal the real charge of 2. 10. The vapor pressure of water at 90 o C is 530 Torr. What is the boiling point of water on top of a mountain where the atmospheric pressure is 530 Torr? (a) 90 o C (b) 100 o C (c) slightly over 100 o C Answer = a . The boiling point is the temperature at which the vapor pressure is equal to the outside pressure.
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Problem 2 For the reaction N 2 O 4 (g) 2NO 2 (g), " H 298 0 = 57.2kJ/mol, " S 298 0 = 176J/(mol K), Determine the equilibrium constant for this reaction at 325K. Solution: Assuming that " H 298 0 and " S 298 0 are temperature-independent, " G 325 0 # " H 298 0 $ T " S 298 0 = (57.2 kJ/mol) $ (325 K)(176 J/mol K)(10 $ 3 kJ/J) = 0 kJ/mol Thus, using " G 0 = # RT ln K eq , we find that lnK eq = 0, and K eq =1.
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Problem 3 For the reaction: 2NO(g) + O 2 (g)
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