Question

The value of rate constant of a pseudo first order reaction.
A.depends only on temperature
B.depends on the concentration of reactants present in small amounts
C.depends on the concentration of reactants present in excess
D.independent of the concentration of reactants

Answer 1

Hint: To solve this question, we require knowledge about the kinetics of a pseudo first order reaction and the factors it depends on. The reaction in which if the concentration of one of the reactants is in large excess, its concentration is practically constant throughout the reaction. This type of reaction is called pseudo first order reactions.

Complete step-by-step answer:
We know that, in second order reactions, the rate constant depends on the concentration of both the reactants. In the case of first order reactions, the rate constant is dependent upon only one reactant.
Sometimes in a second order reaction, if the concentration of one of the reactants is present in large excess. Then throughout the reaction, the decrease in its concentration is so less that it is practically constant. So, though the reaction occurs with two reactants, the concentration of one of the reactants does not change. Thus, the reaction occurs like a first order reaction, i.e. the rate constant is only dependent upon the concentration of the reactant which is present in small amounts. That is why this type of reaction is called a pseudo first order reaction.

$\therefore $ The correct option is option B, i.e. the rate constant of pseudo first order reaction depends on the concentration of the reactants present in small amounts.

Note: Most of the pseudo first order reactions are hydrolysis reactions in which the water molecule is in large excess. For example, hydrolysis of esters or alkyl halides, inversion of cane sugar etc. The reactions in which the molecularity is more than two but the rate determining step consist of only two reactants is known as pseudo second order reaction.