Why is adsorption always exothermic?
ΔH is always negative because during adsorption, the surface energy decreases in the form of heat. Also, 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. So, ΔH has to be highly negative for the spontaneous process to make ΔG negative. Hence, adsorption is always exothermic
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