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Explain intermediate compound formation theory of catalysis with an example. |
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Answer» Solution :The intermediate compound formation theory: A catalyst acts by providing a new PATH with low energy of activation. In homogeneous catalysed reactions a catalyst may combine with one or more reactant to form an intermediate which reacts with other reactant or DECOMPOSE to give products and the catalyst is regenerated. Consider the reactions: `A+B to AB` (1) `A+C to AC` ( intermediate ) (2) C is the catalyst `AC+B to AB +C` (3) Activation ENERGIES for the reactions (2) and (3) are lowered compared to that of (1). Hence the formation and decomposition of the intermediate accelerate the rate of the reaction. Example: The mechanism of Fridel crafts reaction is given below `C_6H_5+CH_3Cl overset("anhydrous" AlCl_3) to C_6H_5CH_3+HCl` The actionof catalyst is explainedas follows `CH_3Cl +AlCl_3 tounderset(" It is an intermediate ")([CH_3]^(+))[AlCl_4]^(-)` `C_6H_6 +[CH_3]^(+) [AlCl_4]^(-) to C_6H_5CH_3 + AlCl_3 +HCl` Thistheroy describes , (i) The specificity of a catalyst . (ii) The increasein the rate of the reaction wiht increasein 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. |
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