Final answer:
The order of reaction pertains to how reaction rate depends on reactant concentrations and is determined experimentally, including any number (fractional, whole, or zero), while molecularity is the number of reactant molecules in an elementary step, always an integer, and applies only to single-step reactions.
Step-by-step explanation:
The difference between the order of reaction and molecularity of a reaction involves several key aspects:
- Conceptual Definition: The order of reaction is the exponent to which the concentration of a reactant is raised in the rate law. This defines how the rate of reaction depends on the concentration of reactants. In contrast, molecularity refers to the number of reactant species that come together to collide in a single elementary step.
- Determination: The order of reaction is determined experimentally and can be fractional, whole, or zero. On the other hand, molecularity is a theoretical concept and can only be an integer (one, two, or, rarely, three for unimolecular, bimolecular, or termolecular reactions respectively).
- Complexity of Reactions: Order of reaction is applicable to both elementary and complex reactions, whereas molecularity applies only to elementary reactions, which are reactions occurring in a single step.
- Dependence on Reaction Mechanism: Molecularity is based on the proposed mechanism of a reaction and only involves the molecules participating in the rate-determining step. The overall reaction order can be influenced by multiple steps in a reaction mechanism and is determined by the slowest, or rate-determining, step.
- Relationship with Rate Law: For elementary reactions, the order of reaction is the same as its molecularity. However, for complex, multi-step reactions, the rate law cannot be deduced from the balanced chemical equation and must be found experimentally.