Theory of Catalysis

The following theories are proposed to explain the mechanism of catalysis:

(i)  Intermediate compound formulation theory:

According to this theory one of the reactants combines with catalyst to form intermediate product, which carries out the reaction,


A + C \to AC


AC + B \to AB + C


A + B + C \to AB + C


where A and B are reactants, C is the catalyst and AC is the intermediate product.

4KClO_3 +2MnO_2 \to 2KMnO_4 + 2KCl + Cl_2 + 4O_2


2KMnO_4 \to K_2MnO_4 + MnO_2 + O_2


K_2MnO_4 + Cl_2 \to 2KCl + MnO_2 + O_2




or \hspace{2mm} 2KClO_3 \overset{MnO_2}{\rightarrow} 2KCl + 3O_2


(ii)  Adsorption theory:

According to this theory, reactants are adsorbed on the surface of the catalyst and form a film. Due to high concentration of the reactants on the film, reaction proceeds at a faster rate.


(iii)   Modern adsorption theory:

According to this theory, reactants are adsorbed at the active centers i.e. free valencies etc. on the solid surface and form activated complex which under strain forms new molecules and leaves the surface. This explains, why the finely divided catalyst has greater activity.


(iv)    Energy activation theory:

According to this theory, catalyst changes the value of activation energy which can be crossed by the reactants easily and consequently products are formed.

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