An elementary reaction is a
chemical reaction
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and break ...
in which one or more
chemical species
A chemical species is a chemical substance or ensemble composed of chemically identical molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. These energy levels determine the wa ...
react directly to form
products in a single
reaction step and with a single
transition state
In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked ...
. In practice, a reaction is assumed to be elementary if no
reaction intermediates have been detected or need to be postulated to describe the reaction on a molecular scale. An apparently elementary reaction may be in fact a
stepwise reaction, i.e. a complicated sequence of chemical reactions, with reaction intermediates of variable lifetimes.
In a
unimolecular elementary reaction, a
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
dissociates or
isomerises to form the products(s)
:
At constant temperature, the
rate
Rate or rates may refer to:
Finance
* Rates (tax), a type of taxation system in the United Kingdom used to fund local government
* Exchange rate, rate at which one currency will be exchanged for another
Mathematics and science
* Rate (mathema ...
of such a reaction is proportional to the concentration of the species
:
In a
bimolecular elementary reaction, two
atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas ...
s, molecules,
ions or
radicals, and , 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 and
:
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. An example of this type of reaction is a
cycloaddition reaction.
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.
According to collision theory the probability of three chemical species reacting simultaneously with each other in a
termolecular elementary reaction is negligible. Hence such termolecular reactions are commonly referred as non-elementary reactions and can be broken down into a more fundamental set of bimolecular reactions,
[Aris, R. and Gray, P. and Scott, SK, Modelling cubic autocatalysis by successive bimolecular steps, Chemical Engineering Science 43', 207--211 (1988)] in agreement with the law of mass action. It is not always possible to derive overall reaction schemes, but solutions based on
rate equation
In chemistry, the rate law or rate equation for a reaction is an equation that links the initial or forward reaction rate with the concentrations or pressures of the reactants and constant parameters (normally rate coefficients and partial rea ...
s are often possible in terms of
steady-state or
Michaelis-Menten approximations.
Notes
{{Reaction mechanisms
Chemical kinetics
Physical chemistry