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Question
Why is adsorption always exothermic?
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Solution
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.
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