Order of Reaction:
The dependence of the reaction rate on concentration may be expressed in terms of order of a reaction. To calculate the order of reaction we must carry out the experiment to find out the dependence of rate on the concentration of each individual reactant at a given temperature. Therefore, the sum of the powders or exponents to which the concentration terms are raised in the rate law expression.
For a general reaction, the rate of a reaction,
aA + bB ———> Products
The order of the above reaction is equal to (m + n). These powers or exponents i.e. m and n have no relation to the stoichiometric coefficients a and b of a balanced equation. Order of the reaction with respect to A is m and that of B is n. If the sum of the power is equal to one, the reaction is called first order reaction. If the sum of the power is equal to two or three, the order reaction is second or third order reaction respectively. The order of a reaction can also be zero or fractional. For examples
1) First order reaction: If rate of reaction depends upon the first power of the concentration of reacting species, decomposition of Br2.
Br2 —————>2Br
– d[Br2] /dt = k1 [Br2]
2) Pseudo-first order reaction:
When the molar concentration of one of the reactants in a bimolecular reaction is in large excess then the rate of reaction depends on the molar concentration of one of the reactant is called Pseudo-first order reaction. For examples, hydrolysis of sucrose in excess of water to give glucose and fructose
C12H22O11 + H2O ———> C6H12O6 + C6H12O6
Sucrose Glucose Fructose
4) Zero order reaction:
There are certain reactions in which the concentrations of reactants do not change with time as a result of this the rate of reaction remain constant through. Such reaction in which the rate of reaction is independent of concentration of the reactant molecules, called Zero order reaction. For a general reaction,
A ———->Products
– d[A]/ dt = k [A]° = k