Why is adsorption always exothermic ?

Explanation

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