AP Chemistry Question 536: Answer and Explanation
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4. A certain reaction is nonspontaneous under standard conditions, but becomes spontaneous at higher temperatures. What conclusions may be drawn under standard conditions?
- A. ΔH < 0, ΔS < 0 and ΔG = 0
- B. ΔH > 0, ΔS < 0 and ΔG > 0
- C. ΔH < 0, ΔS < 0 and ΔG > 0
- D. ΔH > 0, ΔS > 0 and ΔG > 0
Correct Answer: D
D-The relationship, given on the equations page of the exam is ΔG = ΔH - TΔS. Nonspontaneous under standard conditions means: ΔG > 0. To become spontaneous, ΔG must be less than zero. Increasing the temperature will change the TΔS term (entropy). If ΔH is greater than zero and ΔS is also greater than zero, the combination will be positive as long as the ΔH is greater than TΔS. As the temperature increases, TΔS will eventually become larger than ΔH making the process spontaneous.