Pseudo First Order Reaction - Definition & Example

Pseudo First Order Reaction

Order of a chemical reaction can be defined as 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. depends on the concentration of the reactants. In this article we will discuss about pseudo first order reactions in detail. 

Suppose a reaction is – aA + bB 🡪 cC + dD 

Rate according to rate law expression = k [A]x [B]y

Where x and y are concentration of A and B respectively. 

So, order of reaction will be = x + y

We can say x is the order of reaction with respect to A and y is the order of reaction with respect to B. 

Now if suppose x=1 and y = 1 then reaction will be 2nd 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 2nd order in nature but can be approximated as 1st order reaction with respect to A and known as pseudo 1st order reaction. 

Thus, pseudo first order reaction is actually of higher order reaction but can be approximated or appears to be pseudo first order reaction. We can say in general pseudo order reactions are those reactions which appears to be of xth order reaction but can be approximated or are of some different order. 

Pseudo First Order Reaction Definition 

Those reactions which are not of 1st order but approximated or appears to be of 1st order due to higher concentration of the reactant/s than other reactant are known as pseudo first order reactions. 

Pseudo First Order Reaction Example 

Pseudo first order reaction can be well explained by following examples – 

1. Hydration of Alkyl halide 

CH3I + H2O 🡪 CH3OH + H+ + I-

Rate of reaction = k [CH3I] [ H2O]

As methyl iodide is also used in aqueous solution form so the concentration of water is far higher than methyl iodide. 

 [CH3I] <<< [ H2O]

So, concentration of water doesn’t change much and can be approximated as no change or constant. 

Now we can write – Rate of reaction = k’ [CH3I]

Where k’ = k [H2O]

Thus, the reaction appears to be first order, but it is actually of second order that’s why known as pseudo first order reaction. 

2. Hydrolysis of Cane Sugar  

C12H22O11 + H2O 🡪 C6H12O6 + C6H12O6

Sucrose      Water Glucose   Fructose 

Rate of reaction = k [C12H22O11] [H2O]

But [H2O] >>> [C12H22O11]

So, concentration of water can be approximated as constant as its concentration doesn’t change a lot during the reaction. 

Now rate of reaction can be written as – 

r = k’ [C12H22O11]

where k’ = k [H2O]

thus, hydrolysis of cane sugar is pseudo first order reaction. 

3. Hydrolysis of ester – 

Reaction – CH3COOC2H5 + H2O         🡪 CH3COOH + C2H5OH 

                Ethyl ethanoate  Water         Ethanoic acid Ethanol 

Rate of reaction = k [CH3COOC2H5] [H2O]

But [H2O] >>> [CH3COOC2H5]

So, we can say concentration of water remains almost constant during the reaction. 

So, we can write –

Rate of reaction = k’ [CH3COOC2H5]

K’ = k [H2O]

Thus, hydrolysis of ester is a pseudo first order reaction.