Pseudo First Order Reaction

A Pseudo first-order reaction can be defined as a second-order or bimolecular reaction that is made to behave like a first-order reaction. This reaction occurs when one reacting material is present in great excess or is maintained at a constant concentration compared with the other substance.

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

Example of 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)

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 being 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 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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