Which one of the following is not applicable to the phenomenon of adsorption? (i) ∆H > 0 (ii) ∆G < 0 (iii) ∆S < 0 (iv) ∆H < 0
The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption. During adsorption, there is always a decrease in residual forces of the surface, i.e., there is the decrease in surface energy which appears as heat. Adsorption, therefore, is invariably an exothermic process. In other words, ΔH of adsorption is always negative. When a gas is adsorbed, the freedom of movement of its molecules become restricted. This amounts to decrease in the entropy of the gas after adsorption, i.e., ΔS is negative. Adsorption is thus accompanied by the decrease in enthalpy as well as a decrease in entropy of the system. For a process to be spontaneous, the thermodynamic requirement is that, at constant temperature and pressure, ΔG must be negative.
