Order of Reaction:
The order of reaction refers to the exponent of the concentration terms in the rate equation of a chemical reaction. It determines how the rate of the reaction is affected by changes in the concentration of reactants. The order of reaction can be determined experimentally by conducting a series of reactions with different initial concentrations of reactants and measuring the corresponding rates.

Molecularity:
Molecularity is a concept used in the collision theory of chemical reactions. It refers to the number of molecules or ions that must collide in order to produce a chemical reaction. Molecularity can only be determined theoretically based on the proposed mechanism of the reaction.

Differences:

1. Definition:
- The order of reaction is the exponent of the concentration terms in the rate equation.
- Molecularity is the number of molecules or ions involved in a single elementary step of a reaction.

2. Determination:
- The order of reaction can be determined experimentally by measuring the rates of reaction at different concentrations.
- Molecularity can only be determined theoretically based on the proposed mechanism of the reaction.

3. Exponent:
- The order of reaction can be any positive or negative integer or even fractional values.
- Molecularity is always a whole number, either 1, 2, or 3.

4. Change in Rate:
- The order of reaction determines how the rate of reaction changes with changes in the concentration of reactants.
- Molecularity does not directly affect the rate of reaction, but it provides information about the number of molecules involved in a single step.

5. Experimental vs Theoretical:
- The order of reaction is determined experimentally by conducting experiments with different initial concentrations.
- Molecularity is determined theoretically based on the proposed mechanism of the reaction.

In summary, the order of reaction and molecularity are two different concepts in chemical kinetics. The order of reaction is determined experimentally and represents the exponent of the concentration terms in the rate equation, while molecularity is a theoretical concept that represents the number of molecules involved in a single step of the reaction mechanism.