What are pseudo first order reactions? Give one example and explain why it is pseudo first order.

Question: 

What are pseudo first order reactions? Give one example and explain why it is pseudo first order.

Explanation:

Pseudo first-order reaction: 

Reaction which appears to be a second-order reaction, but actually is the first-order reaction is called pseudo-first-order reaction. This condition occurs in a chemical reaction between two substances when one reactant is present in a large amount. The concentration of reactant present in excess does not get altered much during the course of the reaction. Due to this reaction behaves as the first-order reaction.

Example:

Hydrolysis of esters

CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH

Rate law for this reaction is;

Rate K[CH₃COOC₂H₅][H₂O]

But the concentration of water does not change during the course of the reaction. 

So; [H₂O] is constant.

Therefore rate=K₁[CH₃COOC₂H₅]

Where, K=K'[H₂O]

So, the reaction behave as a first order reaction.