Why is adsorption always exothermic?

  

Adsorption is always exothermic, this statement can be explained by two.

i) Adsorption leads to a decrease in the residual forces on the surface of the adsorption. This causes a decrease in the surface energy of the adsorption therefore adsorption is always exothermic. 

ii) ΔH of adsorption is always negative.

When a gas is adsorbed on a solid surface its movement is restricted leading to a decrease in the entropy of the gas i.e. ΔS

Adsorption is accompanied by decrease in entropy, i.e., ΔS = – ve.

If it is spontaneous then ΔG should be negative.

ΔG = ΔH – TΔS

Since ΔS is – ve, ΔG will be – ve only if

ΔH = – ve, i.e., if the process is exothermic.