_{When adsorbate (gas or liquid mostly) absorbed then their entropy get reduced i.e. Δ}S _{is negative. For every spontaneous process ΔG must be} negative. ΔG = ΔH - TΔS, as TΔS is positive (ΔS is negative) ΔH must be negative in order to make ΔG negative . Hence ΔH is negative that means the reaction is exothermic. Hence every adsorption reactions are exothermic.