Question

Why is adsorption always exothermic

Answer 1

Adsorption is due to the formation of bonds. When a substance is adsorbed, it forms new bond with the adsorbent.

Which results in release of energy or Heat.

Answer 2

Adsorption occurs on the surface which leads to decrease in residual forces on the surface of adsorbent. This decrease in residual forces then leads to a decrease in the surface energy of adsorbent that is one of the reasons that Adsorption is always exothermic.

∆G = ∆H - T∆S

In adsorption, as known, ∆H is always negative. When adsorbed on a surface, the movement of the gas is restricted leading to the decrease in entropy of that gas which causes ∆S to become negative.

Now, if the process has to be spontaneous then, ∆G should also be negative.

When ∆S is negative then it is neceasary to make ∆H negative in order for the reaction to stay balanced.

Both ∆S and ∆H becoming negative makes ∆G negative.