Why is a absorption always exothermic ?

Explanation

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