Molecularity and Order in Chemical Reactions

Molecularity and order are two important concepts in the study of chemical reactions. They both describe the number of reactant molecules involved in a particular step of a reaction. However, they differ in terms of the values they can take.

Molecularity: An Integer Value
Molecularity refers to the number of reactant molecules participating in an elementary step of a reaction. It is always expressed as an integer value. Molecularity can be 1, 2, or 3, and rarely 4 or higher. The molecularity of a step is determined by the number of molecules that collide simultaneously and undergo a reaction.

For example, in the reaction A + B → C, if the reaction involves the simultaneous collision of one molecule of A and one molecule of B, the molecularity of that step would be 2. If the reaction involves the collision of two molecules of A and one molecule of B, the molecularity would be 3.

Order: A Fractional Value
Order, on the other hand, describes the relationship between the concentration of a reactant and the rate of a reaction. It is determined experimentally and can take any value, including fractional values. The order of a reaction with respect to a particular reactant is represented by a small, non-negative number.

For example, in the reaction rate equation, Rate = k[A]^m[B]^n, the values of m and n represent the order of the reaction with respect to reactants A and B, respectively. These values can be fractions or integers.

Difference between Molecularity and Order
The main difference between molecularity and order lies in their definitions and values:

- Molecularity refers to the number of reactant molecules involved in an elementary step of a reaction and is always an integer value.
- Order, on the other hand, describes the relationship between reactant concentration and reaction rate and can take any value, including fractional values.

It is important to note that molecularity is a theoretical concept used to describe the mechanism of a reaction at the molecular level, while order is an experimentally determined value that describes the overall rate of a reaction.