How does molecularity differ from the order of a reaction?

The correct answer highlights a fundamental distinction between molecularity and the order of a reaction. Molecularity refers to the number of reactant molecules involved in a single elementary step of a reaction mechanism. It specifically describes the reaction at the molecular level and indicates how many molecules collide to result in a particular reaction.

On the other hand, the order of a reaction reflects the relationship between the concentration of reactants and the rate of the overall reaction. It is determined experimentally and can involve the sum of the powers of the concentration terms in the rate law associated with the reaction, which may not correspond directly to the number of molecules in any single step.

This distinction is significant because molecularity pertains solely to elementary reactions (which are the indivisible steps in the reaction mechanism), while the order considers the entire reaction as observed in the experimental data, which could involve multiple steps or catalytic processes. Thus, while molecularity is about individual steps in a reaction mechanism, the order provides insight into how concentrations of reactants influence the rate across all steps involved in the reaction.