Question

Consider a general reaction
$aA{\text{ }} + bB\;\; \to {\text{ }}cC + {\text{ }}dD$. The rate expression for the reaction is Rate= $K{[A]^X}{[B]^Y}$
(i) Establish the significance of in terms of order and molecularity.
Ii ) Write any two differences between order and molecularity.

Answer 1

Rate of reaction means the speed of reaction with which the reaction takes place. The rate of change of concentration of any one of the reactants or products per unit time is known as rate of reaction.

Complete step by step answer:
The reaction that takes place in one step is known as elementary reaction.
(i) The reaction is given as $aA{\text{ }} + bB\;\; \to {\text{ }}cC + {\text{ }}dD$
Its rate is given by Rate= $K{[A]^X}{[B]^Y}$
Now if the reaction is elementary that is it occurs in single step then will be equals to
So x and y will become the molecularity of reaction.
If reaction is not elementary, that is it does not occur in a single step then will not be equal to and hence now x and y have become the order of reaction.

(ii) Order of reaction and molecularity difference

Order of reaction	Molecularity
Order of a reaction may be defined as the sum of powers of concentrations or pressure terms of reactants in the rate law expression	Molecularity of a reaction is determined by the total number of reactant molecules taking part in a single step reaction
Order of a reaction is an experimental quantity. It can be zero and even a fraction	Molecularity can be zero or integer
applicable to elementary as well as complex reaction	It is applicable only for elementary reaction.

Note:
The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction. If one reactant is involved, the reaction is called unimolecular.