Question

Write the difference between order and molecularity of reaction.

Answer 1

Hint: A chemical reaction is that in which one or more of the chemical species react directly to form products in a single reaction step and with a single transition state. For a simple chemical reaction that occurs in one step, the molecularity tells us how many molecules affect the rate of that reaction. And order tells about number of molecules of the reactant whose concentration changes during the chemical change. Use this basic knowledge and curate the differences.

Complete answer:
Reaction: we should know about reaction if it takes place in a single step than it is elementary reaction. And if a reaction takes place in two or more steps then we say it as a complicated reaction. And this sequence of steps through which a complicated reaction takes place is called reaction – mechanism. Each step in a mechanism is an elementary step reaction.
One simple idea about the molecularity of reaction that we can get is the minimum number of molecules that are required for feasibility of reaction is called Molecularity.

	Chemical reaction	Molecularity
1	$PC{{l}_{5}}\to PC{{l}_{3}}+C{{l}_{2}}$	Uni-molecular
2	\[NO+{{O}_{3}}\to N{{O}_{2}}+{{O}_{2}}\]	Bi-molecular
3	\[2S{{O}_{2}}+{{O}_{2}}\to 2S{{O}_{3}}\]	Tri-molecular

We define order of reaction as the number of molecules of the reactant whose concentration changes during the chemical change.

	Molecularity	Order
	Molecularity is the number of ions or molecules that take part in the rate-determining step.	Number of molecules of the reactant whose concentration changes during the chemical change
	It is always a whole number	It can either be a whole number or a fraction
	The molecularity of the reaction is determined by looking at the reaction mechanism	The order of the reaction is determined by the experimental methods
	It is theoretical concept	It is experimental concept
	It is meaningful only for simple reactions or individual steps of a complex reaction. It is meaningless for an overall complex reaction.	It is meant for the reaction and not for its individual steps

Note: We should note here that it is important to distinguish molecularity from order of reaction. Order of reaction is calculated by experiment from the rate law of reaction. To determine order we should sum up the exponents from the rate law of the equation.

We cannot consider the molecularity of reaction, it involves a difficult mechanism. Only the molecularity of individual elementary reactions should be considered.