Shaillee Kaushal is teaching live on Unacademy Plus
XII-05 SURFACE CHEMISTRY- V
Theories of catalysis Two theories have been proposed for catalysis: (i) Intermediate compound formation theory which explains the homogenous catalysis. (ii) Adsorption theory which explains the heterogenous catalysis
1. intermediate compound formation theory:- This theory was propsed by Clement and Desormes. The catalyst first forms an intermediate compound with one of the reactants.The intermediate compound is formed with less energy consumption than needed for the actual reaction. The intermediate compound being unstable combines with other reactant to form the desired product and the catalyst is regenerated. For example: A + B"AB Which occurs in presence of a catalyst K, may take place as: A+ K (slow reaction) AK Intermediate AK + B AB + K (fast reaction) Rate KIA][K]
Many reactions can be explained on the basis of this theory (i) Catalytic oxidation of Sulphur dioxide to sulphate trioxide 2N0 + 02 2NO2 Intermediate (ii) The formation of methyl benzene (toluene)from benzene and methyl chloride in presence of anhydrous AlCl (CH3)+(All)- intermediate compound AlCl3 + CH3Cl- Products catalyst ii) Formation of di-ethyl ether from ethyl alcohol using sulphuric acid as a catalyst can be explained as: intermediate compound
This theory explains why a catalyst remains unchanged in mass and chemical composition at the end of the reaction and is effective in even small quantities It fails to explain the action of catalytic promoter, catalytic promoters, catalytic poisons and action of finely divided catalysts. Variable oxidation state of transition metals makes them efficient catalyzes. For example catalysts. For example Fe3 catalyzes the reaction between iodide and persulphate ions.
2.Adsorption Theory This theory explains the mechanism of heterogenous catalysis The old point of view was that when a catalyst is in solid state and the reactions are in gaseous state or in solutions, the molecules of the reactants are adsorbed on the surface of the catalyst. The increased concentrations of the reactants on the surface influence the rate of reaction adsorption being exothermic process, the heat of adsorption is taken up by the surface of the catalyst which is used in increasing the chemical activity of the reacting molecules. This theory does not explain the specificity of a catalyst. The chemical adsorption is specific and involves chemical combination of intermediate compound formation theory and the old adsorption theory the mechanism involves five steps (i) Diffusion of reactant to the surface of catalyst. (ii) Some form of association between the catalyst surface and the reactants occurs.This is assumed to be adsorption.
-0-0-0- Adsorpoon of -0-0-0- Reactng II molecules Catalyst surface having free valencies Adsorption of reacting molecules 0 LLIA-B product moleculeso 0-0-O A Desorpoon of _0_0_0_ Product - Catalyst Intermediate