Molecularity

Bimolecular reactions

In a bimolecular reaction, two molecules collide and exchange energy, atoms or groups of atoms.

This can be described by the equation

$${\displaystyle \text{A}+\text{B}⟶\text{P}}$$

which corresponds to the second order rate law: ${\displaystyle \frac{d[\text{A}]}{dt}=-k_r[\text{A}][\text{B}]}$.

Here, the rate of the reaction is proportional to the rate at which the reactants come together. An example of a bimolecular reaction is the S_N2-type nucleophilic substitution of methyl bromide by hydroxide ion:

$${\displaystyle {\text{CH}}_3^{}\text{Br}+{\text{OH}}^{-}⟶{\text{CH}}_3^{}\text{OH}+{\text{Br}}^{-}}$$

Termolecular reactions

A termolecular (or trimolecular) reaction in solutions or gas mixtures involves three reactants simultaneously colliding, with appropriate orientation and sufficient energy. However the term trimolecular is also used to refer to three body association reactions of the type:

$${\displaystyle \text{A}+\text{B}\stackrel{\text{M}}{\to }\text{C}}$$

Where the M over the arrow denotes that to conserve energy and momentum a second reaction with a third body is required. After the initial bimolecular collision of A and B an energetically excited reaction intermediate is formed, then, it collides with a M body, in a second bimolecular reaction, transferring the excess energy to it.

The reaction can be explained as two consecutive reactions:

$${\displaystyle {\text{A}+\text{B}⟶\text{AB}}^{\ast }}$$ $${\displaystyle {\text{AB}}^{\ast }+\text{M}⟶\text{C}+\text{M}}$$

These reactions frequently have a pressure and temperature dependence region of transition between second and third order kinetics.

Catalytic reactions are often three-component, but in practice a complex of the starting materials is first formed and the rate-determining step is the reaction of this complex into products, not an adventitious collision between the two species and the catalyst. For example, in hydrogenation with a metal catalyst, molecular dihydrogen first dissociates onto the metal surface into hydrogen atoms bound to the surface, and it is these monatomic hydrogens that react with the starting material, also previously adsorbed onto the surface.

Reactions of higher molecularity are not observed due to very small probability of simultaneous interaction between 4 or more molecules.

Difference between molecularity and order of reaction

It is important to distinguish molecularity from order of reaction. The order of reaction is an empirical quantity determined by experiment from the rate law of the reaction. It is the sum of the exponents in the rate law equation. Molecularity, on the other hand, is deduced from the mechanism of an elementary reaction, and is used only in context of an elementary reaction. It is the number of molecules taking part in this reaction.

This difference can be illustrated on the reaction between nitric oxide and hydrogen:

$${\displaystyle 2\text{NO}+2{\text{H}}_2^{}⟶{\text{N}}_2^{}+2{\text{H}}_2^{}\text{O},}$$

where the observed rate law is ${\displaystyle v=k{[\text{NO}]}^2[{\text{H}}_2^{}]}$, so that the reaction is third order. Since the order does not equal the sum of reactant stoichiometric coefficients, the reaction must involve more than one step. The proposed two-step mechanism has a rate-limiting first step whose molecularity corresponds to the overall order of 3:

Slow: $${\displaystyle 2\text{NO}+{\text{H}}_2^{}⟶{\text{N}}_2^{}+{\text{H}}_2^{}{\text{O}}_2^{}}$$ Fast: $${\displaystyle {\text{H}}_2^{}{\text{O}}_2^{}+{\text{H}}_2^{}⟶2{\text{H}}_2^{}\text{O}}$$

On the other hand, the molecularity of this reaction is undefined, because it involves a mechanism of more than one step. However, we can consider the molecularity of the individual elementary reactions that make up this mechanism: the first step is trimolecular because it involves three reactant molecules, while the second step is bimolecular because it involves two reactant molecules.