Answer:
Adsorption is always exothermic, this statement can be explained by two.
i) Adsorption leads to a decrease in the residual forces on the surface of the adsorption.
This cause a decrease in the surface energy of the adsorption therefore adsorption is always exothermic.
 ii) ΔH of adsorption is always negtive.
When a gas is adsorbed on a soild surface its movement is restricted leading to decrease in the entropy of the gas i.e. ΔS
Adsorption is accompanised by decrease in entropy, i.e., ΔS = – ve. If it is spontaneous then ΔG should be negative.
ΔG = ΔH – TΔS
Since ΔS is – ve, ΔG will be – ve only if
ΔH = – ve, i.e., if process is exothermic.