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: Concept
A bimolecular reaction involves two reactant molecules in the rate-determining step: Rate $= k[A][B]$ (second order overall).

Step 2: Analysis
If one of the reactants (say B) is present in a very large excess compared to A, its concentration remains essentially constant throughout the reaction and does not change appreciably. In this case, its concentration can be absorbed into the rate constant: $k' = k[B]$ where $k'$ is the pseudo rate constant.

Step 3: Conclusion
The reaction then appears to follow first-order kinetics with respect to A: Rate $= k'[A]$. This is called a

pseudo first-order reaction. Example: acid hydrolysis of an ester in excess water.

Final Answer: When one reactant is in a very large excess (pseudo first-order conditions). Example: Hydrolysis of an ester in excess water.