It seems like there might be a slight confusion in your question. “Order” and “molecularity” are terms used in different contexts, particularly in chemistry and kinetics. Let me clarify the differences between them:
Order:
In the context of chemical kinetics, the term “order” refers to the power to which the concentration of a reactant is raised in the rate equation.
The order of a reaction with respect to a specific reactant is determined experimentally and may be an integer, fraction, or zero.
The overall order of a reaction is the sum of the individual orders with respect to each reactant.
Molecularity:
Molecularity refers to the number of molecules or ions involved in the elementary reaction (the smallest step) of a chemical reaction.
For elementary reactions, molecularity is always an integer and represents the number of reactant particles coming together to form the products.
Molecularity is not used to describe overall reactions but rather individual steps within a reaction mechanism.
Units:
Order is a dimensionless quantity. It is determined experimentally and does not have units.
Molecularity is also a dimensionless quantity because it represents the number of molecules participating in an elementary reaction.
Dependence on Reaction Mechanism:
Order is determined experimentally by observing how the rate of a reaction changes with changes in the concentration of reactants.
Molecularity is inherent to the reaction mechanism and is specific to elementary reactions. It describes the number of molecules colliding in a single step.
2A→C, where the molecularity is 2 because two molecules of A are involved in the reaction.
In summary, “order” pertains to the concentration dependence of a reaction rate, while “molecularity” describes the number of molecules involved in an elementary reaction. They are distinct concepts used in different aspects of chemical kinetics.