Q.What is a pseudo first order reaction ? Give an example.

The rate law genuinely has two concentration terms, but one reactant is in such large excess that it stays effectively constant, so the reaction is experimentally indistinguishable from a first-order reaction.

Definition: A pseudo first order reaction is a reaction whose actual order (from its rate law) is two or more, but which follows first-order kinetics under the experimental conditions used, because one of the reactants is taken in large excess (or is the solvent) so its concentration does not change appreciably during the reaction.

Derivation: For $A + B \rightarrow \text{products}$ with true rate law $\text{Rate} = k[A][B]$, if $B$ is in large excess, $[B] \approx [B]_0$ (constant), so:

$\text{Rate} = k[B]_0[A] = k'[A]$, where $k' = k[B]_0$

This reduces to a first-order rate expression in $A$ alone.

Example 1 — Acid hydrolysis of ethyl acetate:

$CH_3COOC_2H_5 + H_2O \xrightarrow{H^+} CH_3COOH + C_2H_5OH$

True rate law: $\text{Rate} = k[CH_3COOC_2H_5][H_2O]$. Water (solvent, large excess) has near-constant concentration, giving $\text{Rate} = k'[CH_3COOC_2H_5]$.

Example 2 — Inversion of cane sugar: …