Question

What is a pseudo first order reaction? Give one example of it.

Answer 1

Hint: The sum of power of concentration of reactants in the rate law expression is called the order of that chemical reaction. Reactions can be first order reaction, second order reaction, pseudo first order reaction, etc. depending on the concentration of the reactants.

Complete answer:
Pseudo first order reactions are those reactions which are not first order but appear to be first order due to higher concentration of the reactants than others.
Let us consider the following reaction, \[aA+bB\to cC+dD\]
Rate of the reaction = $k[A]x[B]y$
Where, x and y are concentrations of A and B respectively.
So, the order of the reaction will be x + y.
And, x is the order of the reaction w.r.t A, whereas y is the order of the reaction w.r.t B.
Now, if supposedly, x = 1 and y = 1 then the reaction will be a second order reaction. But if the concentration of B is much more than the concentration of A, then change in concentration of B will be very less so its concentration can be assumed constant.
So, in this condition although the reaction is of second order in nature but can be approximated as first order reaction w.r.t A and is known as pseudo first order reaction.
Thus, pseudo first order reactions are actually higher order reactions. In general, this reaction appears to be the ${{x}^{th}}$ order reaction but is of some different order.
Example of pseudo first order reaction:
Hydration of alkyl halide-
     \[C{{H}_{3}}I+{{H}_{2}}O\to C{{H}_{3}}OH+{{H}^{+}}+{{I}^{-}}\]
Rate of the reaction - $C{{H}_{3}}I$ $k[C{{H}_{3}}I][{{H}_{2}}O]$
Since $C{{H}_{3}}I$ is also used in aqueous solution form, the concentration of water is higher than $C{{H}_{3}}I$.
So, concentration of water doesn’t change much and can be approximated as no change.
Now the rate of reaction would be$=k'[C{{H}_{3}}I]$
Where $k'=k[{{H}_{2}}O]$
Thus, the reaction is an example of pseudo first order reaction.

Note:
This reaction is dependent upon the concentrations of both A and B but one of the components is present in large excess and thus its concentration hardly changes as the reaction proceeds.