Question

Define adsorption with an example. Why is adsorption exothermic in nature? Write the types of adsorption based on the nature of forces between adsorbate and adsorbent.

Answer 1

The accumulation of the molecular species at the surface rather than in the bulk of a solid or liquid is known as adsorption. For example, water vapour are adsorbed by silica gel.

When a gas is absorbed on the surface of a solid its entropy decreases, i.e., $\Delta S$ become $-ve$. Since adsorption is a spontaneous process, therefore, $\Delta G(=\Delta H-T\Delta S)$ must be negative. As $-T\Delta S$ is $+ve$, $\Delta G$ can be negative only if $\Delta H$ has sufficiently high $-ve$ value. Hence, adsorption is exothermic in nature.

There are two types of adsorption based on the nature of forces between adsorbate and adsorbent.

i. Physical adsorption, when the accumulation of gas on the surface of a solid occurs due to weak van der Waal forces.

ii. Chemical adsorption, when the gas molecules or atoms are held to the surface of solid by chemical bonds.