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Lila Lidberg

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The order of reaction is defined as the dependence of the concentration of all reactants in a chemical reaction on the rate law expression. For example, in a first order chemical reaction, the rate of reaction is entirely dependent on the concentration of one reactant in the 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. depending on the concentration of the reactants. Order of a reaction is an experimental value. It means it is an experimentally determined parameter. It can have fractional value as well.

There are some characteristics of order of reaction that are enumerated as follows:

If experimental rate law expression is given for a reaction, then we can deduce the order of that reaction as well. For example, consider a reaction –

aA + bB \ P

and rate law is given as –

rate = k(Ax)(By)

order of reaction for the above reaction on the basis of given rate law can be written as follows –

order of reaction = x + y

Order of reaction is determined by experiment. Although if we know rate law expression determined experimentally then we can determine order of reaction using rate law. Order of reaction can be an integer or fractional value. Following orders of reactions are possible –

Molecularity and order of reaction both give information about the chemical reaction but are very different from each other as one tells about the number of molecules taking part in reaction while another one tells about the relationship between rate of reaction and concentration of reactants. For your better understanding we are providing you here pointwise difference between molecularity and order of reaction-

In these reactions the rate of reaction doesn’t depend upon the concentration of reactants. It means change in concentration of reactants doesn't affect the rate of reaction.

Example -


In these reactions the rate of reaction depends on the concentration of one reactant only. There can be many reactants in the reaction but concentration of only one reactant will affect the rate of reaction. Concentration of other reactants will have no effect on order of reaction.

Example – \

Rate = k

In these reactions the rate of reaction depends on the concentration of two different reactants or square of concentration of one reactant.

Example – \

Rate = k2

\ \ \

Rate = k

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.

Example – Hydration of alkyl halide

\ \ \

Rate of reaction = k

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


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’

Where k’ = k

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

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