Question

Which of the following set of parameters indicate spontaneity of reaction at all temperatures?

• A. ΔH = +ve, ΔS = −ve, ΔG = +ve
• B. ΔH = +ve, ΔS = +ve, ΔG = −ve
• C. ΔH = −ve, ΔS = −ve, ΔG = +ve
• D. ΔH = −ve, ΔS = +ve, ΔG = −ve

Hint:

For a reaction to be spontaneous at all temperatures, the enthalpy change must be negative and the entropy change must be positive.

Answer 1

The Correct Option is D

Step 1: Understanding spontaneity of reactions.
For a reaction to be spontaneous at all temperatures, the Gibbs free energy (ΔG) must be negative. This can be derived from the equation: \[ \Delta G = \Delta H - T\Delta S \] For spontaneity at all temperatures, the enthalpy change (ΔH) must be negative and the entropy change (ΔS) must be positive. This ensures that ΔG will always be negative.
Step 2: Analyzing the options.
(A) ΔH = +ve, ΔS = −ve, ΔG = +ve: Incorrect. Positive ΔH and negative ΔS will always give a positive ΔG, making the reaction non-spontaneous.
(B) ΔH = +ve, ΔS = +ve, ΔG = −ve: Incorrect. While ΔS is positive, the positive ΔH term makes ΔG positive at high temperatures.
(C) ΔH = −ve, ΔS = −ve, ΔG = +ve: Incorrect. Negative ΔH and negative ΔS lead to a positive ΔG at higher temperatures.
(D) ΔH = −ve, ΔS = +ve, ΔG = −ve: Correct — Negative ΔH and positive ΔS guarantee that ΔG will be negative at all temperatures, indicating spontaneity.
Step 3: Conclusion.
The correct answer is (D) ΔH = −ve, ΔS = +ve, ΔG = −ve, which indicates that the reaction is spontaneous at all temperatures.