Question

Assertion: Acid catalyzed hydrolysis of esters is a pseudo first order reaction.
Reason: Water is present in excess in a given reaction.
A.Both assertion and Reason are correct and Reason is the correct explanation for Assertion.
B.Both assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C.Assertion is correct but Reason is incorrect.
D.Reason is correct but Assertion is incorrect.

Answer 1

Hint: Sum of the exponents gives the overall order of a reaction. The order of a complex reaction is given by the order of the slowest step in the sequence of various steps involved in that reaction.

Complete answer:
Pseudo first order reactions are those reactions which are not truly first order but under certain conditions behave as a first order also known as pseudo-unimolecular reactions.
Consider acid catalyzed reactions as follows:
Hydrolysis of ethyl acetate
\[C{{H}_{3}}COO{{C}_{2}}{{H}_{5}}+{{H}_{2}}O\to C{{H}_{3}}COOH+{{C}_{2}}{{H}_{5}}OH\]
Inversion of sugar
\[{{C}_{12}}{{H}_{22}}{{O}_{11}}+{{H}_{2}}O\to {{C}_{6}}{{H}_{12}}{{O}_{6}}+{{C}_{6}}{{H}_{12}}{{O}_{6}}\]
Both the reactions are bimolecular and supposed to be of second order
Rate = \[K[C{{H}_{3}}COO{{C}_{2}}{{H}_{5}}][{{H}_{2}}O]\] or Rate = \[K[{{C}_{12}}{{H}_{22}}{{O}_{11}}][{{H}_{2}}O]\]
But reactions are found to be of first order because experimentally it is found that it is actually carried out taking dilute aqueous solutions of ester (RCOOR’) or sugar and acid (HCl) such that in the reaction mixture water exists in large excess as compared to ester. We can also say that water almost achieves its maximum concentration of 55.5 M so there is no appreciable change.
\[\therefore \] Rate = \[K[C{{H}_{3}}COO{{C}_{2}}{{H}_{5}}][{{H}_{2}}O]\]
Becomes Rate = \[K'[C{{H}_{3}}COO{{C}_{2}}{{H}_{5}}]\] where \[K'\] called pseudo rate constant.
Molecularity of the above reaction is two but order is one. Hence option A is the correct answer.

Note:
Molecularity and order are two different things. Molecularity is the number of molecules that come together to react in a chemical equation. The Order of reaction tells the relationship between the rate of a chemical reaction and the concentration of the species taking part in it. In order to calculate the reaction order the rate expression of the reaction is very necessary. Once the rate equation is obtained, the entire composition of the mixture of all the species in the reaction can be understood.