Question

what is pseudo first order reaction? explain with suitable example

Answer 1

We know that order of reaction depends on the dependency of the rate of reaction on the concentration of reactants. That is if the rate is independent of the concentrations of reactants, the order of reaction is zero. Similarly, if the rate of reaction is proportional to the first power of the concentration of the reactant, then the order of reaction is one.

But sometimes the order of a reaction can be made to alter by changing the concentration of the reactants, that is, increasing or decreasing the concentration of one or the other reactant.
example of pseudo first-order reaction is the inversion of cane sugar.

C12H22O11     +      H2O     →       C6H12O6      +      C6H12O6

Cane sugar                                    Glucose                Fructose

Rate = k[C12H22O11]

Answer 2

A pseudo first order reaction is a reaction that is truly second order but can be approximated to be first order under special circumstances. For example, a second order reaction of the type A + B --> C is dependant on the concentrations of both A and B. But monitoring the concentration of two different reactants at the same time is very difficult. If one of the reactants is used in a very large excess then that concentration would hardly change at all and could be considered to be essentially a constant. If [B]>>>[A], then the [B] would be essentially unchanged and the reaction could be considered pseudo first order with respect to [A] as a mathematical simplification. An example of this is the hydrolysis of an ester to a carboxylic acid using water. The reaction is truly second order and the rate depends on the concentration of the ester and water used. But since water is often used as a solvent, then its concentration is in a huge excess compared to the ester and can be considered a constant. Thus the reaction is pseudo first order for the ester.

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