Order of Reaction

As we know in chemistry the meaning of a chemical reaction is chemical transformation of reactants into products. In chemical reactions the rate of reaction is very important. Order of reaction deals with the relationship between concentration of reactants and rate of reaction.  


What is Order of 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. Order of a reaction is an experimental value. It means it is an experimentally determined parameter. It can have fractional value as well.


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[A]x[B]y

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

order of reaction = x + y


How to Find Order of reaction 

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 – 

  • Order of reaction can be zero – In zero order reaction the concentration of reactant/s doesn’t affect the rate of a reaction. 

  • Order of reaction can be negative integer – Negative integer value of order of reaction indicates that the concentration of the reactants inversely affect the rate of a reaction. 

  • Order of reaction can be positive integer – Positive integer value of order of reaction indicates that the concentration of the reactants directly affect the rate of a reaction. 

  • Order of reaction can be fractional value – Fractional value of order of reaction indicates more intricate relationship between concentration of reactants and rate of reaction. Generally, complex reactions possess fractional value of order of reaction.


Following methods can be used for determination of order of reaction –

  • Differential Method – It is also called initial rates method. In this method concentration of one reactant varies while others are kept in constant concentration and initial rate of reaction is determined. Suppose if three reactants A, B and C are taking part in the reaction then in this method we keep vary concentration of one reactant (for example reactant A) while concentration of other reactants such B and C constant. 

  • Graphical Method – This method is used when only one reactant takes part in the reaction. In this method if we draw a graph between log[A] (where A is a reactant and [A] is concentration of reactant A) and t (time) and it’s a straight line then reaction follows a first order.  In the same way if we draw a graph between 1[A] and t and get a straight line then reaction follows second order. While if we draw a graph between 1[A]2 and t and get a straight line then the reaction is third order reaction. 

  • Integral Method – In this method concentrations of the reactants are compared with the integral form of the rate law. It is used for verification of initial rate methods.


Difference Between Molecularity and Order of Reaction 

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 - 

Molecularity 

Order of reaction 

It is the number of molecules taking part in the rate determining step. 

It shows the relation between concentration of reactants and rate of reaction. 

For determination of molecularity only rate determining step is considered. 

For determination of order of reaction all steps of a reaction are considered. 

It doesn’t depend on pressure and temperature. 

It depends on pressure, temperature and concentration. 

It is always a whole number. 

It can be zero, integer or even a fractional value. 

We can determine molecularity by looking at reaction mechanisms. 

Order of reaction can be determined by experiments. 

Molecularity cannot be a negative number. 

Order of reaction can be a negative number. 


Zero Order 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 -


First Order Reaction 

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 – N2O5 🡪 N2O3 + O2

Rate = k[N2O5]


Second Order Reaction 

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

Example – 2NO2 🡪 2NO + O2 

Rate = k[NO2]2

CH3COOC2H5 + OH- 🡪 CH3COO- + C2H5OH 

Rate = k[CH3COOC2H5] [OH-]


Pseudo First Order Reaction 

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 

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 it is known as pseudo first order reaction.


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