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.
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
Iodoacetone