Explain the mechanism of adsorption.

(N/A) Adsorption arises because the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. Inside the adsorbent,all forces acting between the particles are mutually balanced,but on the surface,the particles are not surrounded by atoms or molecules of their kind on all sides,and hence they possess unbalanced or residual attractive forces.
These forces of the adsorbent are responsible for attracting the adsorbate particles to its surface. The extent of adsorption increases with an increase in surface area per unit mass of the adsorbent at a given temperature and pressure.
Another important factor in adsorption is the heat of adsorption. During adsorption,there is always a decrease in the residual forces of the surface,i.e.,there is a decrease in surface energy which is released as heat. Therefore,adsorption is invariably an exothermic process. Thus,adsorption is accompanied by a decrease in enthalpy as well as a decrease in the 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,i.e.,there is a decrease in Gibbs energy.
On the basis of the equation,$\Delta G = \Delta H - T \Delta S$,if $\Delta H$ has a sufficiently high negative value,then $\Delta G$ can be negative even though $\Delta S$ is negative (making $-T \Delta S$ positive).
Thus,in an adsorption process,which is spontaneous,a combination of these factors makes $\Delta G$ negative. As the adsorption proceeds,$\Delta H$ becomes less negative; ultimately,$\Delta H$ becomes equal to $T \Delta S$ and $\Delta G$ becomes zero. At this state,equilibrium is attained.