Pseudo First Order Reaction - Definition & Examples

Those reactions which are not of 1st order but approximated or appear to be of 1st order due to higher concentration of the reactant/s than other reactants are known as pseudo-first-order reactions. The order of a chemical reaction can be defined as the sum of the power of concentration of reactants in the rate law expression is called the order of that chemical reaction. Reactions can be a first-order reaction, second-order reaction, pseudo-first-order reaction, etc. depending on the concentration of the reactants.

Explanation of Pseudo First-order Reaction 

With the help of the dependency of the rate of reactions on the concentration of reactants, we can determine the order of the reaction. If the order of the reaction is zero, the rate does not depend on the concentrations of reactants. In the same way, if the order of the reaction is one, the rate of reaction is proportional to the first power of the reactant concentration. We will discuss 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 concentrations 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 the reaction will be a 2nd order reaction. But if the concentration of B is much more than the concentration of A then the 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. 

The graph of Pseudo first-order reaction is given below.

(Image will be uploaded soon)

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

Pseudo First Order Reaction Example 

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

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, the 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 it is known as a pseudo-first-order reaction. 

C12H22O11 + H2O  C6H12O6 + C6H12O6

Sucrose      Water      Glucose      Fructose 

Rate of reaction = k [C12H22O11] [H2O]

But [H2O] >>> [C12H22O11]

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

Now the rate of reaction can be written as – 

r = k’ [C12H22O11]

where k’ = k [H2O]

thus, hydrolysis of cane sugar is a 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 the concentration of water remains almost constant during the reaction. 

So, we can write –

Rate of reaction = k’ [CH3COOC2H5]

K’ = k [H2O]

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

Conclusion

In this article, we learned about the pseudo-first-order reaction. With the help of different examples of pseudo-first-order reactions, we get to know different reactions that are not of first-order reaction but appear to be of 1st order due to higher concentration of the reactants.