Chemistry 122 [Tyvoll]
Quiz 9 Key
April 25, 2008
Multiple Choice (2 points each)
1. Which one of the following processes is not spontaneous and endothermic (at 1 atm)?
Answer 4. condensation of water at 99°C (spontaneous, but not endothermic!)
2. Which of the following is not a spontaneous process at room temperature?
Answer 1. N2 (g) + O2 (g) NO (g) (not spontaneous – these are main atmospheric gases)
3. For which one of the following would you expect the entropy change to be closest to zero?
Answer 2. N2 (g) + O2 (g) 2 NO (g) (2 mol gas to 2 mol gas, ΔS ≅ 0)
4. For which of the following processes would the entropy change be positive?
Answer 4. I2 (s) I2 (aq) ((aq) more disordered than (s), so ΔS = +)
5. Given the following standard molar entropies, calculate ΔS° (in J/K) for the reaction:
S° J/mol•K
C (s)
5.74
+ O2 (g)
205.0
CO2 (g)
213.6
Answer 4. +2.86 J/K (ΔS° = 213.6 − (5.74 + 205.0) = +2.86 J/K)
6. Which process is accompanied by a negative entropy change?
Answer 1. stacking firewood (stacked firewood is more ordered than unstacked firewood)
7. Which is necessarily true for a spontaneous process?
Answer 1. ΔSuniverse > 0 (defined – note that ΔSsystem can be < 0 for spontaneous processes)
8. Which one of the following statements is incorrect? (definition)
Answer 3. The greater the degree of disorder in a system, the more negative is its entropy.
9. Which of the following reactions involves an increase in entropy?
Answer 4. H2O (l) → H2O (g) (gas is more disordered than liquid)
10. What conditions would apply to a reaction that is not spontaneous at low temperature but
becomes spontaneous at higher temperatures?
Answer 3. increase in enthalpy and increase in entropy (at high T, −TΔS >ΔH)