Why is adsorption always exothermic?

Explanation

When a gas is adsorbed on the surface of the solid, its entropy decreases, i.e. `Delta `S is negative. Now, `Delta G=Delta H-T Delta S`. and for the process to be spontaneous, free energy change, AG must be negative. As T`Delta `S is negative, `Delta `G can be negative only if `Delta H` has sufficiently high negative value. Hence, adsorption is always exothermic.