When a gas is adsorbed on the surface of a solid, its entropy decreases, i.e., ΔS is negative. Now, ΔG = ΔH-TΔS For a process to be spontaneous, ΔG must be negative. As here, ΔS is negative, therefore, TΔS is positive ΔG can be negative only if ΔH is negative Hence, adsorption is always exothermic.
