Theories Of Catalysis
Category : JEE Main & Advanced
There are two theories of catalysis which is described as follows.
(1) Intermediate compound theory
(i) This theory was proposed by Clement and Desormes in 1806. According to this theory, the desired reaction is brought about by a path involving the formation of an unstable intermediate compound, followed by its decomposition into the desired end products with the regeneration of the catalyst.
(ii) The intermediate compund may be formed in either of two ways
(a) When the intermediate compound is reactive and reacts with the other reactants.
\[AB+X\underset{\text{intermediate}}{\mathop{\to \,\,BX\,\,\,+}}\,A\]
\[BX+C\to CB+X\] .....(i)
(b) When the intermediate is unstable and decomposes to give the final product.
\[A+B+X\underset{\text{intermediate}}{\mathop{\to ABX\to }}\,AB+X\] .....(ii)
Where, A, B and C are the reactant molecules and X is the molecule of the catalyst. The first type of reaction sums up to, \[AB+C\to CB+A\]
While the second to, \[A+B\to AB\] in many cases, the intermediate compounds postulated to be formed are known compounds and often their presence is detected.
(2) Adsorption theory
(i) This theory is applicable to reactions between gases in the presence of a solid catalyst. Some typical examples are as follows.
(ii) The contact process for the oxidation of \[S{{O}_{2}}\] to \[S{{O}_{3}}\] with atmospheric oxygen in the presence of platinum as the catalyst.
(iii) The Haber's process for the synthesis of ammonia with iron as the catalyst.
(iv) Adsorption results in the loosening of the chemical bonds in the reactant molecules, so that their rupture becomes easier. This is confirmed by the observed lower activation energies for heterogeneous catalytic reactions in the presence of the catalysts as compared to that for the same reaction in the absence of the catalyst.
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