For adsorption of a gas

a. ΔH < 0, ΔS < 0

b. ΔH < 0, ΔS > 0

c. ΔH > 0, ΔS > 0

d. ΔH = 0, ΔS > 0


Answer: (a)

During adsorption, there is always a decrease in residual forces of the surface, i.e.,

there is a decrease in surface energy which appears as heat. Adsorption, therefore, is

invariably an exothermic process, i.e., ΔH < 0. When a gas is adsorbed, the freedom

of movement of its molecules becomes restricted. This amounts to a decrease in the

entropy of the gas after adsorption, i.e., ΔS < 0 is negative.

Hence, the correct option is a.

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