Question

In pseudo-unimolecular reactions:
A. One of the reactants is present in large excess
B. Both the reactants have same concentration
C. both the reactants are present in low concentration
D. one of the reactants less reactive

Answer 1

The pseudo-unimolecular reaction is defined as the reaction, where the rate of reaction is dependent on the concentration of only one reactant as other reactant concentration is taken constant.

Complete step by step answer:
The pseudo-unimolecular reaction is defined as the reaction where the molecularity of the reaction is less than or equal to 2 but the order of reaction is one.
In a pseudo-unimolecular reaction, the rate of the reaction depends on the concentration of only one reactant and the other reactant is present in the excess amount and therefore the value of the other reactant is taken as constant.
Example: Reaction between ethyl acetate and water
$C{H_3}COO{C_2}{H_5} + {H_2}O \to C{H_3}COOH + {C_2}{H_5}OH$
In this reaction, one mole of ethyl acetate reacts with one mole of water to give one mole of acetic acid and one mole of ethanol.
In this reaction, the hydrolysis of ester is done with excess of water, where carboxylic acid and alcohol is formed. As water is present in excess amounts, its concentration at the start of the reaction and end of the reaction is almost the same, so the value is kept constant. The rate is dependent on the ester.
The rate of the reaction is given as shown below.
$Rate = k[C{H_3}COO{C_2}{H_5}]$
The order of reaction one, but the molecularity of the reaction is two.
Therefore, the correct option is A.

Note:
When in a given reaction, the concentration is not altered by the time then the value of concentration is taken as constant. The reaction looks like a second order reaction but after knowing the concentration the actual order of reaction can be depicted.