Adsorption, if spontaneous, is exothermic. Explain.

Explanation

When a gas is adsorbed on the surface of a solid, its entropy decreases, i.e., `DeltaS` is negative. Now, `DeltaG=Delta-TDeltaS` and for the process to be spontaneous, free energy change, `DeltaG` must be negative.
`TDeltaS` is negative, i.e., `-TDeltaS` is `+ve` , therefore, `DeltaG` can be negative only if `DeltaH` is negative. Hence, adsorption is always exothermic.