Question

Why is adsorption always exothermic?

Answer 1

The phenomenon of adsorption can be expressed as the accumulation of molecular species on the surface rather than in the bulk of liquid or solids. It’s a surface phenomenon and is mainly a consequence of surface energy.

Complete step by step answer:
- As we mentioned above, adsorption is a process that involves the accumulation of a substance in molecular species in higher concentration on the surface rather than in bulk of the species. For the process of adsorption, two components are required, such as Adsorbate; a substance which is deposited on the surface of another substance and Adsorbent; surface of the substance on which adsorbate adsorbs.

- An exothermic reaction is a reaction in which there is a release of energy in the form of heat, light or electricity from the system to its surroundings. The process of adsorption is always exothermic and this can be explained by the following reasons
(i) On adsorption the residual forces on the outside of the adsorbent will reduce. Since the surface particles of the adsorbent are unstable and when the adsorbate is being adsorbed on the surface, the energy of adsorbent will decrease and this will result in the evolution of heat. Hence adsorption is an exothermic process.
(ii) When a gas is adsorbed on a solid surface at a point, its movement is confined which leads to the reduction in the entropy (S) of gas .That is $\Delta S$ is negative.
As we know, for a process to be spontaneous, $\Delta G$ should be negative and the relation can be given as follows :
$\Delta G=\Delta H-T\Delta S$
Since the value of $\Delta S$ is negative, in order to make the process spontaneous ($\Delta G$ negative), the enthalpy of adsorption $\Delta H$ should be always negative. Therefore adsorption will be an exothermic process.

Note: The dissimilarity between the terms absorption and adsorption should be noted. The process of adsorption is a surface phenomenon whereas absorption is a bulk phenomenon. As we discussed, adsorption is an exothermic process while absorption is an endothermic process.