Pseudo First Order Reaction - Chemical Kinetics

In this section, we will learn about the pseudo first order of reaction and their rate equations. To begin with, reactions that appear to be second order in nature but are approximated as a first-order of reaction on close analysis. For example, a second – order of reaction is given by the equation,

A + B —-> C + D

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. So, if component B is in large excess and the concentration of B is very high as compared to that of A, the reaction is considered to be pseudo-first order reaction with respect to A and if component A is in large excess and the concentration of A is very high as compared to that of B, the reaction is considered to be pseudo first order with respect to B.

Pseudo first order reaction

Here is an example to help you understand the concept more clearly.

Consider the hydrolysis of ethyl acetate, during the hydrolysis, the concentration of ethyl acetate is 0.02 mol/L whereas the amount of water is 20 mol/L as the process of hydrolysis involves a large amount of water. Let us say, the process of hydrolysis attains completion in time t. The reaction can be represented as

CH3COOC2H5 + H2O —–> CH3COOH + C2H5OH

(ethyl acetate) (Water) (Acetic acid) (ethyl alcohol)

Components CH3COOC2H5OH H2O CH3COOH C2H5OH
Concentration t=0 0.02 20 0 0
Concentration t 0 19.8 0.02 0.02

For the above-mentioned reaction, the rate equation can be given as,

Rate = k [CH3COOC2H5] [H2O]

Here we see that the concentration of water is very high and thus does not change much during the course of the reaction. Thus the rate of the reaction can be said to be independent of the change in the concentration of H2O. Putting the term for change of concentration of water in the above reaction to be zero we can write the effective rate equation as,

Rate = k [CH3COOC2H5]

Here, the term kt takes into account the value of the constant concentration of water.

where K = K’ [H2O]

We see that the reaction behaves as a first-order reaction. Such reactions are termed as pseudo-first order reactions.

Another example of pseudo first-order reaction is the inversion of cane sugar, given by the following reaction,

C12H22O11 + H2O —–> C6H12O6 + C6H12O6

(Cane sugar) (Water) (Glucose) (Fructose)

The rate equation can be given as,

Rate = k [C12H22O11]

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Practise This Question

Time required for 99.9% completion of a first order reaction is nearly ?