Question

Under what condition can a bimolecular reaction become kinetically first order?

Hint:

Pseudo first-order: One reactant in huge excess $\rightarrow$ its concentration is effectively constant $\rightarrow$ apparent order = 1.

Answer 1

Step 1: Bimolecular rate law.
A bimolecular reaction $A + B \rightarrow $ products has rate $= k[A][B]$, second order overall.
Step 2: Condition for pseudo first-order.
If reactant B is present in a very large excess, its concentration remains essentially constant throughout the reaction.
Step 3: Result of excess B.
The constant $[B]$ is absorbed into a new pseudo rate constant: $k' = k[B]$. The rate law becomes Rate $= k'[A]$, which is first order in A. The reaction is called a pseudo first-order reaction.
Step 4: Example.
Acid hydrolysis of an ester in excess water: $CH_3COOC_2H_5 + H_2O \xrightarrow{H^+} CH_3COOH + C_2H_5OH$. Water is in such excess that $[H_2O]$ is constant, making the reaction appear first order in ester.