Notes On Pseudo First Order Reactions - CBSE Class 12 Chemistry

If the sum of the powers of the concentration terms in the rate law expression is equal to one, then it is a first-order reaction.

A chemical reaction in which one of the reactants is present in excess and shows an order different from that of the expected order, is called a pseudo order reaction.

If the reaction is bimolecular, but still the rate of reaction depends only on one concentration term, then the reaction is a pseudo first-order reaction.

Ex: Acid-catalysed hydrolysis of methyl acetate in aqueous solution.

Hydrolysis of sucrose

The rate equation for acid-catalysed hydrolysis of methyl acetate in aqueous solution can be written as

The acid catalysed hydrolysis of methyl acetate in aquesous is expected to be second order reaction.

Rate = k[CH3COOCH3][H2O]

But experimentally, it is found to be first order, therefore it is pseudo first order reaction.

Rate = k' [CH3COOCH3]

Summary

If the sum of the powers of the concentration terms in the rate law expression is equal to one, then it is a first-order reaction.

A chemical reaction in which one of the reactants is present in excess and shows an order different from that of the expected order, is called a pseudo order reaction.

If the reaction is bimolecular, but still the rate of reaction depends only on one concentration term, then the reaction is a pseudo first-order reaction.

Ex: Acid-catalysed hydrolysis of methyl acetate in aqueous solution.

Hydrolysis of sucrose

The rate equation for acid-catalysed hydrolysis of methyl acetate in aqueous solution can be written as

The acid catalysed hydrolysis of methyl acetate in aquesous is expected to be second order reaction.

Rate = k[CH3COOCH3][H2O]

But experimentally, it is found to be first order, therefore it is pseudo first order reaction.

Rate = k' [CH3COOCH3]

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