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

chapter 6. CHEMICAL KINETICS class 12 chemistry textbook solution

Answer:-

A reaction with a higher-order true rate law but experimentally found to behave as a first-order reaction is called a pseudo first-order reaction.

For example:

C₁₂H₂₂O_11(aq) + H₂O → C₆H₁₂O_{6 + C₆H₁₂O₆}

Where:

This is an example of a pseudo first-order reaction because water takes part in the reaction. The true rate law:

rate = k'[C₁₂H₂₂O₁₁][H₂O]

indicates that the reaction must be second order. However, since [H₂O] is constant, the rate law becomes:

rate = k[C₁₂H₂₂O₁₁]

Thus, the second-order true rate law is converted into a first-order rate law.