5.8 Why is adsorption always exothermic?
Adsorption is always exothermic. This statement can be explained in two ways.
(i) Adsorption leads to a decrease in the residual forces on the surface of the adsorbent. This causes a decrease in the surface energy of the adsorbent. Therefore, adsorption is always exothermic.
(ii) ∆H 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., ∆S is negative.
Now for a process to be spontaneous, ∆G should be negative.
∆G = ∆H − T∆S
Since ∆S is negative, ∆H has to be negative to make ∆G negative. Hence, adsorption is always exothermic.
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