An elementary reaction is a
chemical reaction
A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...
in which one or more
chemical species
Chemical species are a specific form of chemical substance or chemically identical molecular entities that have the same molecular energy level at a specified timescale. These entities are classified through bonding types and relative abundance of ...
react directly to form
products
Product may refer to:
Business
* Product (business), an item that can be offered to a market to satisfy the desire or need of a customer.
* Product (project management), a deliverable or set of deliverables that contribute to a business solution
...
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 w ...
. In practice, a reaction is assumed to be elementary if no
reaction intermediates
In chemistry, a reaction intermediate, or intermediate, is a molecular entity arising within the sequence of a stepwise chemical reaction. It is formed as the reaction product of an elementary step, from the reactants and/or preceding intermed ...
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 that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
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
:
In a
bimolecular
In chemistry, molecularity is the number of molecules that come together to react in an elementary reaction, elementary (single-step) reactionAtkins, P.; de Paula, J. Physical Chemistry. Oxford University Press, 2014 and is equal to the sum of Sto ...
elementary reaction, two
atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
s, molecules,
ion
An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s 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
in 1864. An example of this type of reaction is a
. 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
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,
in agreement with the law of mass action. It is not always possible to derive overall reaction schemes, but solutions based on