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Assertion (A) Adsorption is an exothermic process.
Reason (R) Physisorption of a gas on a solid surface is normally reversible.
Options:
Reason (R) Physisorption of a gas on a solid surface is normally reversible.
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Both $A$ and $R$ are correct but $R$ is not the correct explanation of $\mathrm{A}$.
Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion. The correct reason for given assertion is as follows : When a gas is adsorbed on a solid surface, its movement is restricted. This leads to a decrease in the entropy of the gas i.e. $\Delta S$ is negative. Now, for a process to be spontaneous, $\Delta G$ should be negative.
We know that, $\Delta G=\Delta H-T \Delta S$
Now, $\Delta S$ is negative, $\Delta H$ has to be negative to make $\Delta G$ negative. Hence, adsorption is always exothermic.
We know that, $\Delta G=\Delta H-T \Delta S$
Now, $\Delta S$ is negative, $\Delta H$ has to be negative to make $\Delta G$ negative. Hence, adsorption is always exothermic.
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