For a chemical reaction, the standard enthalpy change is Delta H° = -120 kJ and the standard entropy change is Delta S° = -200 J/K. At what temperature T does the reaction switch from being spontaneous (Delta G° < 0) to non-spontaneous (Delta G° > 0)?

Answer and explanation

Correct answer: C. 600 K

  1. Step 1: Apply the Gibbs Free Energy equation: Delta G° = Delta H° - T * Delta S°.
  2. Step 2: The transition temperature between spontaneity and non-spontaneity occurs at equilibrium where Delta G° = 0.
  3. Step 3: 0 = Delta H° - T * Delta S° => T = Delta H° / Delta S°.
  4. Step 4: Convert units: Delta H° = -120 kJ = -120,000 J; Delta S° = -200 J/K.
  5. Step 5: Calculate: T = (-120,000 J) / (-200 J/K) = 600 K. The reaction is spontaneous at T < 600 K.
Answer
600 K.
Why not the others: '300 K' divides by 400 J/K; '240 K' computes -120 * 2; '1200 K' doubles the equilibrium temperature.

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