intermediate accelerate the rate of the reaction. Example The mechanism of Fridel crafts reaction is given below anhydrous AlCl C H +CH Cl C H CH +HCl → The action of catalyst is explained as follows CH Cl+AlCl [CH ] + [AlCl - ] It is an intermediate. C H +[CH + ][AlCl - C H CH +AlCl +HCl ] → Example Thermal decomposition of KClO in presence of MnO proceeds as follows. Steps in the reaction 2KClO 2KCl+3O can be given as XII U10-Surface XII U10-Surface - - - - 2KClO +6MnO 6MnO +2KCl It is an intermediate.
6MnO 6MnO +3O Example : Formation of water due to the reaction of H and O in the presence of Cu proceeds as follows. Steps in the reaction H +½O → H O can be given as 2Cu+ Cu O Cu O+H H O+2Cu It is an intermediate. Example : Oxidation of HCl by air in presence of CuCl proceeds as follows. Steps in the reaction 4HCl+O → 2H O+2Cl can be given as 2CuCl Cl +Cu Cl 2Cu Cl +O 2Cu OCl It is an intermediate.
2Cu OCl +4HCl 2H O+4CuCl This theory describes (i) the specificity of a catalyst and (ii) the increase in the rate of the reaction with increase in the concentration of a catalyst. Limitations (i) The intermediate compound theory fails to explain the action of catalytic poison and activators (promoters). (ii) This theory is unable to explain the mechanism of heterogeneous catalysed reactions. .
Adsorption theory Langmuir explained the action of catalyst in heterogeneous catalysed reactions based on adsorption. The reactant molecules are adsorbed on the catalyst surfaces, so this can also be called as contact catalysis. According to this theory, the reactants are adsorbed on the catalyst surface to form an activated complex which subsequently decomposes and gives the product. The various steps involved in a heterogeneous catalysed reaction are given as follows: .
Reactant molecules diffuse