KCET · Chemistry · Thermodynamics (C)

During the adsorption of a gas on the surface of a solid, which of the following is true?

A \(\Delta \mathrm{G}>0, \Delta \mathrm{H} < 0, \Delta \mathrm{S} < 0\)
B \(\Delta \mathrm{G} < 0, \Delta \mathrm{H} < 0, \Delta \mathrm{S} < 0\)
C \(\Delta \mathrm{G} < 0, \Delta \mathrm{H} < 0, \Delta \mathrm{S}>0\)
D \(\Delta \mathrm{G} < 0, \Delta \mathrm{H}>0, \Delta \mathrm{S} < 0\)

Verified Solution

Answer & Solution

Correct Answer

(B) \(\Delta \mathrm{G} < 0, \Delta \mathrm{H} < 0, \Delta \mathrm{S} < 0\)

Step-by-step Solution

Detailed explanation

During the adsorption of a gas on the surface of a solid there is decrease in surface energy, i.e., it is an exothermic process. Therefore, \(\Delta \mathrm{H} < 0\). Again when a gas is adsorbed, the freedom of movement of its molecules becomes restricted. This leads to decrease in the entropy of the gas after adsorption. Therefore, \(\Delta \mathrm{S} < 0\).
From equation, \(\Delta \mathrm{G}=\Delta \mathrm{H}-\mathrm{T} \Delta \mathrm{S}, \Delta \mathrm{G}\) can be negative if process is spontaneous. Since, adsorption process is a spontaneous process, therefore, \(\Delta \mathrm{G} < 0\).

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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