Why is adsorption always exothermic ?

Explanation

When a gas is adsorbed on the surface of a solid, its entropy decreases, i.e., `DeltaS` in negative. Now, `DeltaG = DeltaH-T DeltaS` for a process to be spontaneous. `DeltaG` must be negative. As here, `DeltaS`, is negative, therefore, `TDeltaS` is positive `DeltaG` can be negative only if `DeltaH` is negative Hence, adsorption is always exothermic.