Rate Molecularity Order

Molecularity in chemistry is the number of colliding molecular entities that are involved in a single reaction step. While the order of a reaction is derived experimentally, the molecularity is a theoretical concept and can only be applied to elementary reactions. In elementary reactions, the reaction order, the molecularity and the stoichiometric coefficient are the same, although only numerically, because they are different concepts.

A reaction involving one molecular entity is called unimolecular.

A reaction involving two molecular entities is called bimolecular.

A reaction involving three molecular entities is called termolecular. Termolecular reactions in solutions or gas mixtures are very rare, because of the improbability of three molecular entities simultaneously colliding. However the term termolecular is also used to refer to three body association reactions of the type.

Order of reaction

In chemical kinetics, the order of reaction with respect to a certain reactant, is defined as the power to which its concentration term in the rate equation is raised.

For example, given a chemical reaction 2A + B → C with a rate equation

r = k [A]² [B]¹

the reaction order with respect to A would be 2 and with respect to B would be 1, the total reaction order would be 2 + 1 = 3. It is not necessary that the order of a reaction be a whole number - zero and fractional values of order are possible - but they tend to be integers. Reaction orders can be determined only by experiment. Their knowledge allows conclusions about the reaction mechanism.

The reaction order is not necessarily related to the stoichiometry of the reaction, unless the reaction is elementary. Complex reactions may or may not have reaction orders equal to their stoichiometric coefficients.

Rate law

The rate law or rate equation for a chemical reaction is an equation which links the reaction rate with concentrations or pressures of reactants and constant parameters (normally rate coefficients and partial reaction orders). To determine the rate equation for a particular system one combines the reaction rate with a mass balance for the system.

For a generic reaction mA + nB → C with no intermediate steps in its reaction mechanism (that is, an elementary reaction), the rate is given by

r = k [A]^m [B]ⁿ                      

where [A] and [B] express the concentration of the species A and B, respectively (usually in moles per liter (molarity)); m and n are not the respective stoichiometric coefficients of the balanced equation; they must be determined experimentally. k is the rate coefficient or rate constant of the reaction.