Question -
Answer -
Adsorption is always exothermic.This statement can be explained in two ways.
(i) Adsorptionleads 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, adsorptionis always exothermic.
(ii) ΔH ofadsorption is always negative. When a gas is adsorbed on a solid surface, itsmovement is restricted leading to a decrease in the entropy of the gas i.e., ΔS isnegative. Now for a process to be spontaneous, ΔG should benegative.
ΔG = ΔH − TΔS
Since ΔS is negative,ΔH has to be negative to make ΔG negative. Hence,adsorption is always exothermic.