Question

Which one of the following is not applicable to the phenomenon of adsorption ?

Answer 1

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, $\Delta H$ of adsorption is always negative.
$\Delta H=-ve$
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., $\Delta S$ is negative.
$\Delta S=-ve$
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, $\Delta G$ must be negative.
$\Delta G=\Delta H-T\Delta S$
$\Delta H$ is sufficiently high negative, in an adsorption process which makes $\Delta G$ negative.

Hence, option (a) is correct.