Explain the adsorption theory of heterogeneous catalysis.

(N/A) This theory explains the mechanism of heterogeneous catalysis. The old theory,known as the adsorption theory of catalysis,stated that the reactants in a gaseous state or in solutions are adsorbed on the surface of the solid catalyst.
The increase in concentration of the reactants on the surface increases the rate of reaction. Adsorption being an exothermic process,the heat of adsorption is utilized in enhancing the rate of the reaction.
The modern adsorption theory is a combination of the intermediate compound formation theory and the old adsorption theory. The catalytic activity is localized on the surface of the catalyst. The mechanism involves five steps:
$(i)$ Diffusion of reactants to the surface of the catalyst.
$(ii)$ Adsorption of reactant molecules on the surface of the catalyst.
$(iii)$ Occurrence of a chemical reaction on the catalyst's surface through the formation of an intermediate.
$(iv)$ Desorption of reaction products from the catalyst surface,thereby making the surface available again for more reaction to occur.
$(v)$ Diffusion of reaction products away from the catalyst's surface.
The surface of the catalyst,unlike the inner part of the bulk,has free valencies which provide the seat for chemical forces of attraction. When a gas comes in contact with such a surface,its molecules are held there due to loose chemical combination. If different molecules are adsorbed side by side,they may react with each other,resulting in the formation of new molecules.