Question

What are pseudo unimolecular reactions? Explain with the help of suitable examples.

Answer 1

In these types of reactions, if two or more reactants are involved then the rate of reaction depends only upon the concentration of any one of the reactants and is independent of other reactants. Generally, only one reagent is taken in excess.

Complete step by step answer:
Basically, in a unimolecular reaction, a single molecule rearranges atoms forming different molecules. This is further illustrated as:
$A \to P$
So, the rate of reaction is described as:
$\dfrac{{d[A]}}{{dt}} = - {k_r}[A]$
Where [A] is the concentration of the species, t is the time and ${K_r}$ is the rate constant.
Moreover, the reactions having molecularity two but the order of the reaction is one, then these types of reactions are known as pseudo unimolecular reactions. Usually in such types of reactions, one reagent is taken in excess amount so that it does not determine the rate of the reaction. Some of the examples are Hydrolysis of ester, Hydrolysis of sucrose etc.
Now, let’s discuss pseudo first order reactions. It is basically defined as a second order or bimolecular reaction that is made to behave like a first order reaction. This type of reaction depends upon the concentration of both the components but one of the components is present in large excess and thus its concentration hardly changes as the reaction proceeds.

Note: The Lindeman mechanism is a schematic reaction mechanism for unimolecular reactions. It breaks down an apparently unimolecular reaction into two elementary steps with a rate constant for each elementary step.