Question

According to the adsorption theory of catalysis, the rate of reaction increases because :

• A. adsorption lowers the activation energy of the reaction.
• B. concentration of reactant molecules at the active centers of the catalyst becomes high due to adsorption.
• C. adsorption increases the activation energy of the reaction.
• D. None of the above

Answer 1

The correct option is B concentration of reactant molecules at the active centers of the catalyst becomes high due to adsorption.

Adsorption Theory of Heterogeneous Catalyst :
The modern adsorption theory is the combination of two theory, intermediate compound formation theory and the old adsorption theory.
By this theory, the catalytic activity is localised on the surface on the catalyst.

Old adsorption theory:
The reactants in gaseous state or in solution, are adsorbed on the surface of the solid catalyst.​ The increase in concentration of the reactants on the surface increases the rate of reaction. ​Here, the heat of adsorption (exothermic process) is utilised in enhancing the rate of the reaction.​
Hence, option (b) is the correct answer.

Theory:
Intermediate compound formation theory:
A catalyst forms temporary bonds with the reactants resulting in an intermediate complex. ​This exists for a short time ​and decomposes to yield ​products and the catalyst.​

Adsorption Theory of Heterogeneous Catalyst:
The mechanism involve 5 steps.
(i) Diffusion of reactant(s) 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) Formation of reactions product on the catalyst surface.
(v) Desorption of reaction products ​from the catalyst surface, and thereby, making the surface available again ​for more reaction to occur
(vi) Diffusion of reaction products away from the catalyst’s surface.​