126. Taking two examples of heterogeneous catalytic reactions, explain how a heterogeneous catalyst helps in the reaction.

Answer:

The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis.
Adsorption theory of heterogenous catalysis explained in two ways.

(i) The reactants in 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 utilised in enhancing the rate of the reaction.

(ii) The catalytic activity is localised 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 chemical reaction on the catalyst’s surface through formation of an intermediate.
(iv) Desorption of reaction products from the catalyst surface, and 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 up 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. Thus, formed molecules may evaporate leaving the surface for the fresh reactant molecules.

(i) $2{\mathrm{SO}}_2\left(\mathrm{g}\right) + {\mathrm{O}}_2\left(\mathrm{g}\right) \underset{\mathrm{heat}}{\overset{{\mathrm{V}}_2{\mathrm{O}}_5}{\rightleftharpoons }} 2{\mathrm{SO}}_3\left(\mathrm{g}\right)$

(ii) $4{\mathrm{NH}}_3\left(\mathrm{g}\right)+5{\mathrm{O}}_2\left(\mathrm{g}\right) \underset{1120 \mathrm{K}}{\overset{\mathrm{Pt}}{\to }}4\mathrm{NO}\left(\mathrm{g}\right) + 6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

Heterogeneous catalysts are V₂O₅ and platinum respectively. Reacting gases get adsorbed on heterogeneous solid catalyst and probability of collision increases which result in increase in number of effective collisions, resulting an increase in rate of reaction.
Products formed get desorbed from the catalyst and get ‘absorbed and react, leading to increase in rate of reaction.