In a unimolecular elementary reaction, a molecule A dissociates or isomerises to form the products(s) At constant temperature, the rate of such a reaction is proportional to the concentration of the species A In a bimolecular elementary reaction, two atoms, molecules, ions or radicals, A and B, react together to form the product(s) The rate of such a reaction, at constant temperature, is proportional to the product of the concentrations of the species A and B The rate expression for an elementary bimolecular reaction is sometimes referred to as the law of mass action as it was first proposed by Guldberg and Waage in 1864.
This rate expression can be derived from first principles by using collision theory for ideal gases.
For the case of dilute fluids equivalent results have been obtained from simple probabilistic arguments.
[2] According to collision theory the probability of three chemical species reacting simultaneously with each other in a termolecular elementary reaction is negligible.
It is not always possible to derive overall reaction schemes, but solutions based on rate equations are often possible in terms of steady-state or Michaelis-Menten approximations.