Question

In which condition, adsorption will not take place?
(A) When ${\rm{\Delta H}}$ is negative
(B) When ${\rm{\Delta H}}$ is positive ${\rm{\Delta S}}$ is positive
(C) When ${\rm{\Delta H}}$ is positive ${\rm{\Delta S}}$ is negative
(D) When ${\rm{\Delta G}}$ is negative

Answer 1

As we know that the absorption and adsorption are two different terms. One is used to explain bulk phenomena and other is used for the surface phenomenon. Surface phenomenon is the attraction of particles on the surface of solid as the particles get attached there due to energy transformation in between them.

Complete step by step answer:
Adsorption is defined as the phenomenon in which the attraction and retention of molecules occur on the surface of solid or liquid. This results in the higher concentration of molecules on the surface.
The substance which is absorbed on the surface is called adsorbate and the solid or liquid surface which is attracting the molecules is known as adsorbent. For example, charcoal, silica gel.
When adsorption takes place, the residual forces on the surface of the adsorbent decreases which means surface energy decreases. Thus, adsorption is an exothermic reaction. The value of ${\rm{\Delta H}}$ is negative.
As the molecules are held on the surface, the molecules are attracted and the entropy decreases. The value of ${\rm{\Delta S}}$ is negative.
So, according to the Gibb’s energy formula which is represented as:
${\rm{\Delta G = \Delta H - T\Delta S}}$
Therefore, ${\rm{\Delta G}}$ must be negative for the process of adsorption to occur.
Thus, the correct option is C
Note:
$\Delta G$ can be negative only if $\Delta {\rm{H}}$ is negative and ${{\rm{1}}^{\rm{0}}}{\rm{ > }}{{\rm{2}}^{\rm{0}}}{\rm{ > }}{{\rm{3}}^{\rm{0}}}$ in magnitude. When ${\rm{\Delta H = T\Delta S}}$ the $\Delta G = 0$ and this state is called adsorption equilibrium.