order and molecularity of reaction And molecularity differ from order of reaction

Order of reaction: The sum of the powers of the concentration terms inthe rate equation of a chemical reaction is called‘ order of that reaction.

The order of reaction denotes the dependency of the rate of reaction on the variation of concentration of reactant or reactants in the slowest step . Hence it may have any value-positive, negative, zero, integral or fractions. The order of reaction is always less than or equal to the molecularity of reaction but never greater than the molecularity It is obtained from the rate law equation .which is determined experimentally. For the reaction:

nA + mB ------> Products

The rate of reaction k [A] [3]”y

The value of x and y may not be equal to m and n respectively because the values of x and y are determined experimentally. So experimental rate law differs from the theoretical rate law as a reaction is completed usually in mm e than one step and the sloWest step is the rate determining step. so the rate equation has no empirical relation with the overall chemical equation.

e.g. (i) A + B ---1st order /reaction -> products

Rate of reaction = k[A] [B]° =k [A]

(ii) A + B ----2nd order/reaction --> Products

Rate of reaction = k[A] [B]

The sum of powers of conc. terms in rate equation
=1 + 1= 2

Therefore it is 2nd order reaction.
The chlorination of methane in presence of sunlight is an example of zero order reaction.

ch4 + cl2 ----zero order / reaction --> Products

Because theme of reaction does not depend upon the variation of concentration of either of reactants CH4 or C12, hence the rate of reaction

=k[CH4]° [CL2]^0= k

Molecularity of reaction . The number of reactants participating in collision to, bring about the reaction or needed to form an activated complex in the slowest step of a chemical reaction is called molecularioz of that reaction.

Therefore it is always a Whole number and cannot be zero or fraction. Reactions may be uni, bi and ter-molecular depending upon whether one, two and three reactant molecules are Involved in the slowest step of a chemical reaction. since there is very little probability of collision of four or more reactants each having energy equal to or greater than the threshold energy, hence molecularlty more than three is always almost rate.

Moleculanty of reaction is obtained purely on theoretical consideration i.e. from the mechanism of reaction. Therefore it is not the real quantity like order of reaction. For a complex reaction, the molecularity of reaction is expressed for each step and hence overall value is meaningless.

The possible mechanism for the decomposition of ozone.

O3 --fast-> 0 2+ 0  ...(i)
03 + 0 --slow --> 4 O .....(ii)

Therefore step (i) is a unimolecular reaction and step (ii) is a bimolecular . reaction becauseone and two reactants are involved in the steps (i) and (ii) respectively. 

order and molecularity of reaction.