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Question:

Which of the following statements is correct for the spontaneous adsorption of a gas?

ΔS is negative and therefore, ΔH should be highly negative.

ΔS is positive and therefore, ΔH should be highly negative.

ΔS is positive and therefore, ΔH should also be highly positive.

ΔS is negative and therefore, ΔH should be highly positive.

Solution:

The relationship between the Gibbs free energy change, enthalpy change and entropy change is ΔG = ΔH − TΔS. ΔS = -ve for adsorption because there is a decrease in entropy as there is a decrease in randomness due to association between adsorbate and adsorbent. As it is spontaneous, ΔG is negative and therefore, ΔH should be highly negative.