Order of Reaction

Let’s consider a reaction,

            A    +    B     →        product

The rate of reaction of above reaction is given by rate law expression,

Rate order of reaction

where k is constant called rate constant and is rate of reaction when concentration of all reactants is unit molarity (1m). Here m is order of reaction with respect to A, n is order of reaction w.r.t B and (m +n) is overall order of reaction. So, order of a reaction is the sum of the power to which the concentration of reactants is raised in the experimental rate law expression.

 Unit of Rate Constant

Let’s consider a reaction,

A     →       product

Rate = k [A]n

order of reaction 

The unit of rate constant depends upon the order of reaction.      

Order (n) Unit of rate constant
mole litre-1Sec-1
litre mole -1Sec-1
litre2 mole -2Sec-1

For a reaction,

A      →   product

The rate law expression is,

order of reaction

 when n = 0 The reaction is said to be a zeroth order reaction and for such reaction the rate of reaction is independent of concentration of reactant.

E.g. (i) Enzyme catalysed bio-chemical reaction.

       (ii) Decomposition of HI on the surface of gold.

       (iii) Reaction between H2 and Cl2 to give HCl in presence of light.

When, n =1,

            The reaction is said to be a first order reaction and for such reaction the change rate is equal to the change in concentration of reactant.

 E.g. (i) Radioactive disintegration (decay)

       (ii) Decomposition of H2O2

            2H2O2              →           2H2O + O2

       (iii) Decomposition of N2 O5

            2N2O5              →           2NO2 + O2

       (iv) Acid catalyzed hydrolysis of water

  CH3COOCH3 + H2O     H+ →    CH3COOH     + CH3OH

For 2nd order reaction, there can be two possibilities.

            (i) 2A               →            product

            Rate = k [A]2

            (ii)  A + B           →         product

For their reaction we can have different possible rate laws.

            Rate = k [A]1 [B]

Or        Rate = k [A]2 [B]0

Or,       Rate = k [A]0 [B]2

E.g., (i) Base catalyzed hydrolysis of ester

C2H5COOCH3 + NaOH           →                      C2H5COONa + CH3OH

Methyl propanoate                                          Sudium propionate


(ii) Decomposition of NO2 to NO

            2NO2                →            2NO + O2

(iii) Iodination of Acetone

            CH3COOCH3  + I2         OH- →       CH2ICOCH3   + HI