Question

Why is adsorption always exothermic?

Answer 1

ΔH is always negative because during adsorption, the surface energy decreases in the form of heat. Furthermore, ΔS is negative because when a gas is adsorbed on a solid surface, its movement is restricted; this leads to a decrease in the entropy of the gas. Now, for a process to be spontaneous, ΔG should be negative.

∴ ΔG = ΔH − TΔS

Because ΔS is negative, TΔS becomes positive. Therefore, ΔH has to be highly negative for the spontaneous process to make ΔG negative. Hence, adsorption is always exothermic.