Assertion (A) Adsorption is an exothermic process. Reason (R) Physisorption of a gas on a solid surface is…
Assertion (A) Adsorption is an exothermic process.
Reason (R) Physisorption of a gas on a solid surface is normally reversible.
Both $A$ and $R$ are correct and $R$ is the correct. explanation of $\mathrm{A}$.
Both $A$ and $R$ are correct but $R$ is not the correct explanation of $\mathrm{A}$.
$\mathrm{A}$ is correct but $\mathrm{R}$ is incorrect.
$A$ is incorrect but $R$ is correct.
Solution
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.