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Question

With the help of an example, explain what is meant by a pseudo-first-order reaction.


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Solution

Pseudo-first-order reaction

A reaction which appears to be second-order reaction but is actually a first-order reaction is called a pseudo-first-order reaction.

This condition occurs in a chemical reaction between two substances, when one of the reactants is present in a large amount. The concentration of the reactant present in excess does not get altered much during the reaction. So, it behaves as a first-order reaction.

Example

Hydrolysis of esters.

Reaction –

CH3COOC2H5+H2OCH3COOH+C2H5OH

Ethyl ethanoate Ethanoic acid Ethanol

Rate of reaction =

k[CH3COOC2H5][H2O]

But if [H2O]>>>[CH3COOC2H5]

Concentration of water will remain almost constant during the reaction.

Therefore, rate of the reaction

=k[CH3COOC2H5]

Where, K=k[H2O]

Thus, hydrolysis of ester is a
pseudo -first-order reaction.


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