Adsorption is always exothermic. This statement can be explained in two ways.
(i) Adsorption leads to a decrease in the residual forces on the surface of the
adsorbent. This causes a decrease in the surface energy of the adsorbent. Therefore,
adsorption is always exothermic.
(ii) ΔH of adsorption is always negative. When a gas is adsorbed on a solid surface, its movement is restricted leading to a decrease in the entropy of the gas i.e., ΔS is negative. Now for a process to be spontaneous, ΔG should be negative.
ΔG = ΔH − TΔS Since ΔS is negative, ΔH has to be negative to make ΔG negative. Hence, adsorption is always exothermic.
