Molecularity

Molecularity in chemistry is the number of molecules that come together to react in an elementary reaction and is equal to the sum of stoichiometric coefficients of reactants in this elementary reaction. Depending on how many molecules come together, a reaction can be unimolecular, bimolecular or trimolecular.

In a unimolecular reaction, a single molecule rearranges atoms forming different molecules. This is illustrated by the equation

and is described by the first order rate law

where [A] is the concentration of species A, t is time, and k_r is the reaction rate constant.

As can be deduced from the rate law equation, the number of A molecules that decay is proportional to the number of A molecules available. An example of a unimolecular reaction, is the isomerization of cyclopropane to propene:

Unimolecular reactions can be explained by the Lindemann-Hinshelwood mechanism.

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

This can be described by the equation

which corresponds to the second order rate law: d[A]/dt = -k_r [A] [B].

Here, the rate of the reaction is proportional to the rate at which the reactants come together. An example of a bimolecular process, is the first step in binding of H₂ and O₂ to form water:

A termolecular (or trimolecular) reaction in solutions or gas mixtures involves three reactant molecules simultaneously colliding. However the term termolecular is also used to refer to three body association reactions of the type

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:

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

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