Question

Why the process of adsorption is always exothermic

Answer 1

Adsorption is always exothermic. This statement can be explained in two ways.

(i)Adsorption leads to a decrease in the residual forces on the surface of the adsorbent. This causes a decrease in the surface energy of the adsorbent. Therefore, adsorption is always exothermic.

(ii)ΔH of adsorption is always negative. When a gas is adsorbed on a solid surface, its movement is restricted leading to a decrease in the entropy of the gas i.e., ΔS is negative. Now for a process to be spontaneous, ΔG should be negative.

ΔG= ΔH-TΔS

Since ΔSis negative, ΔH has to be negative to make ΔG negative. Hence, adsorption is always exothermic.

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Answer 2

Answer:

Endothermic reactions are those in which heat is absorbed by the system and exothermic reactions are those in which heat is released by the system.

$\Delta H$ for Endothermic reaction is positive and $\Delta H$ for Exothermic reaction is negative.

Entropy is the measure of randomness or disorder of a system. If a system moves from an ordered arrangement to a disordered arrangement, the entropy is said to decrease and vice versa.

$\Delta S$ is positive when randomness increases and $\Delta S$ is negative when randomness decreases.

When the gas is adsorbed on solid, the randomness of particles decreases and hence  $\Delta S$ is negative.  

As the adsorption is spontaneous : $\Delta G=-ve$

Using Gibbs Helmholtz equation:

$\Delta G=\Delta H-T\Delta S$

$\Delta G=\Delta H-T(-ve)$

$\Delta G=\Delta H+ve$

For $\Delta G$ to be negative, $\Delta H$ has to be negative.

$\Delta H=-ve$

Thus the value of $\Delta H$ is negative and the reaction is exothermic.