Question

The reaction type for acid catalysed ester hydrolysis will be:

• A. Zero order reaction
• B. First order reaction
• C. Pseudo first order reaction
• D. Second order reaction

Hint:

When a reaction involves one reactant in large excess (like water), treat its concentration as constant — simplifying the order to pseudo first order.

Answer 1

The Correct Option is C

Acid-catalyzed hydrolysis of esters typically involves the reaction of an ester with water in the presence of an acid catalyst.
The actual reaction is bimolecular — involving both ester and water — and would ideally be second order.
However, in practical experiments, the concentration of water is kept very high and essentially remains constant throughout the reaction.
Under such conditions, the rate law simplifies because the concentration of water does not change significantly, and it can be combined into the rate constant.
Thus, the rate law appears to be dependent only on the concentration of the ester:
\[ \text{Rate} = k'[\text{Ester}] \]
where $k' = k[\text{H}_2\text{O}]$ is a pseudo rate constant.
This makes the reaction appear to follow first order kinetics, even though it's actually second order — hence the name pseudo first order reaction.
Incorrect options:
- (1) Zero order: Involves rate independent of reactant concentrations, not applicable here.
- (2) First order: Misleading because water is also involved; not strictly true.
- (4) Second order: True in mechanism, but not in kinetics under excess water conditions.
Therefore, the correct classification is: Pseudo first order reaction.