Describe the difference between molecularity and reaction order.

Molecularity refers to the number of molecules involved in an elementary reaction, while reaction order is the power to which reactant concentration is raised in the rate equation.

Molecularity is a term used in the detailed study of chemical reactions. It is defined as the number of molecules, atoms or ions that must collide for a reaction to occur. This is a theoretical concept and is always a whole number. For example, a unimolecular reaction involves one molecule, a bimolecular reaction involves two molecules, and a termolecular reaction involves three molecules. It's important to note that termolecular reactions are rare because they require simultaneous collision of three particles.

On the other hand, reaction order is an experimental concept that describes how the rate of a reaction changes with the concentration of the reactants. It is determined from the rate equation of a reaction, which is usually established through experimental data. The order of a reaction is the sum of the powers to which the concentrations of the reactants are raised in the rate equation. For example, for a reaction with rate equation rate = k[A]^m[B]^n, the order of the reaction is m + n. Unlike molecularity, reaction order can be zero or any positive or negative integer or fraction.

While both molecularity and reaction order deal with the reactants in a chemical reaction, they approach it from different perspectives. Molecularity is a theoretical concept based on the mechanism of the reaction, while reaction order is an empirical concept based on experimental observations. It's crucial to understand that the molecularity of a reaction doesn't necessarily determine its order. For instance, a bimolecular reaction isn't always second order. The reaction order is determined by the rate-determining step, which may not involve all the reactants in a multi-step reaction. Therefore, while molecularity and reaction order are related, they are not interchangeable terms.