Question

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

Answer 1

Hint: As the name ‘pseudo’ suggested the meaning of false or something that is not correct. So, the name suggested by pseudo first order reaction is also delivering something wrong meaning which is totally different from the actual meaning.

Complete answer:
Pseudo first order reaction is not actually a first order reaction but sometimes seems to be like a second order reaction. Some points regarding pseudo first order reaction are as follow:
-As we know that in first order reactions or uni-molecular reactions, the rate of the whole reaction depends upon the concentration of one reactant.
-And in a second order reaction or bi-molecular reaction, the rate of the whole reaction depends upon the concentration of two reactants.
-Pseudo order reactions are those reactions in which two reactants are present but among those two, concentration of one reactant is present in excess quantity than the other then in this condition concentration of that excess reactant can be neglected. And the whole rate of the reaction depends only on one reactant.
Example of pseudo first order reaction is as follow:
-Sucrose on reacting with excess water produces glucose and fructose which is an example of pseudo first order reaction and rate of this reaction depends upon the concentration of sucrose only.
$\mathop {{{\text{C}}_{{\text{12}}}}{{\text{H}}_{{\text{22}}}}{{\text{O}}_{{\text{11}}}}}\limits_{{\text{(Sucrose)}}} {\text{ + }}\mathop {{{\text{H}}_{\text{2}}}{\text{O}}}\limits_{{\text{(Water)}}} \to \mathop {{{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{12}}}}{{\text{O}}_{\text{6}}}{\text{ + }}}\limits_{{\text{(Glucose)}}} \mathop {{{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{12}}}}{{\text{O}}_{\text{6}}}}\limits_{{\text{(Fructose)}}} $

Note:
Here some of you may think in pseudo first order reactions only one reactant is present but that assumption is wrong, because in this reaction two reactants are present and rate only depends on one reactant.