# 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

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

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

 Order (n) Unit of rate constant 0 1 2 3 mole litre-1Sec-1 Sec-1 litre mole -1Sec-1 litre2 mole -2Sec-1

For a reaction,

A      →   product

The rate law expression is,

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.

(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

Iodoacetone

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