Pseudo First Order Reaction

Order of a reaction is an experimental term. It can not be directly determined by the observed rate law. In order to determine the reaction order, the rate expression must be known. That means, with the help of the dependence of the rate of reactions on the concentration of the reactants, we can easily determine the order of the reaction.

Order of a reaction is defined as- "The summation of exponents of all the reactant's concentration term present in the rate law obtained from the rate determining step is called as an order of reaction". Depending on the concentration of the reactants, the reactions may be first order reactions, second order reactions or pseudo first order reactions etc. 

In this article, we will learn about pseudo first order reaction with the help of various examples of pseudo first order reactions. Here, we will learn that some reactions which are expected to be of high order follow first order kinetics.

 

Pseudo First Order Reaction

A pseudo first order reaction can be defined as a second order (or bimolecular) reaction which behaves like a first order reaction because in this type of reaction, one of the reactant is present in relatively high concentration and it's concentration changes only a little bit during the entire reaction and therefore it's concentration value assumed to be constant during the reaction. Due to this, the reaction behaves like a first order reaction instead of the second order reaction. That's why it is known as pseudo first order reaction rather than second order reaction.

Definition?? 

The overview of this whole statement is that those reactions which are not of first order but approximated or appear to be of first order due to higher concentration of the reactant/s than other reactants are known as "Pseudo First Order Reactions". It follows the same kinetics as a first-order reaction, as depending on the first power of the reactant concentration.

Pseudo first order reaction is used in the "Study of Chemical Kinetics". A pseudo-first-order reaction is actually a higher-order reaction, but it can be approximated or appear to be a pseudo-first-order reaction.

For Example

With the help of different examples of pseudo first order reaction, we will learn about various reactions which are not first order reactions but appear to be of first order due to high concentration of reactants. 

The pseudo first order reaction can be well explained by the following examples;

(a) CH₃COO-C₂H₅ + H₂O ➝ CH₅-COOH + C₂H₅OH

    The rate law for the above reaction is, 

      ➩ rate = k [CH₃COO-C₂H₅]¹ [H₂O]º

Here, 

    water is in excess amount and due to that it's concentration doesn't change during the reaction.

(b) C₁₂H₂₂O₁₁ + H₂O ➝ C₆H₁₂O₆+ C₆H₁₂O₆               ➩rate = k [C₁₂H₂₂O₁₁]¹[H₂O]⁰  

             ➩ rate =k [C₁₂H₂₂O₁₁]¹

        (water is in excess amount) 

Unit of reaction rate is same as of first order reaction. 

What Is Pseudo First Order Reaction???

These types of reaction are of second order but behave like a first order reaction, as change in concentration of one reaction which is in excess as compared to another, does not affect the rate of reaction. Therefore, the rate of the reaction only depend upon the concentration of one reactant. 

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