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2013-08-20T19:20:16Z

129.109.5.239: /* Interpretation */

An '''elementary reaction''' is a [[chemical reaction]] in which one or more of the [[chemical species]] react directly to form products in a single [[reaction step]] and with a single [[transition state]].<ref>{{GoldBookRef | file = E02035 | title = elementary reaction}}</ref>

In a [[unimolecular]] elementary reaction, a [[molecule]] A [[Dissociation (chemistry)|dissociates]] or [[Isomerisation|isomerises]] to form the products(s)
:<math>\mbox{A} \rightarrow \mbox{products.}</math>
At constant temperature, the [[reaction rate|rate]] of such a reaction is proportional to the concentration of the species A
:<math>\frac{d[\mbox{A}]}{dt}=-k[\mbox{A}].</math>
In a [[bimolecular]] elementary reaction, two [[atom]]s, [[molecule]]s, [[ion]]s or [[Radical (chemistry)|radical]]s, A and B, react together to form the product(s)
:<math>\mbox{A + B} \rightarrow \mbox{products.}</math>
The rate of such a reaction, at constant temperature, is proportional to the product of the concentrations of the species A and B
:<math>\frac{d[\mbox{A}]}{dt}=\frac{d[\mbox{B}]}{dt}=-k[\mbox{A}][\mbox{B}].</math>
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.<ref>Gillespie, D.T., A diffusional bimolecular propensity function, The Journal of Chemical Physics '''131''', 164109 (2009)</ref>

According to [[collision theory]] the probability of three chemical species reacting simultaneously with each other in a termolecular elementary reactions 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,<ref>Cook, GB and Gray, P. and Knapp, DG and Scott, SK, Bimolecular routes to cubic autocatalysis, The Journal of Physical Chemistry '''93''', 2749--2755 (1989)</ref><ref>Aris, R. and Gray, P. and Scott, SK, Modelling cubic autocatalysis by successive bimolecular steps, Chemical Engineering Science '''43'''', 207--211 (1988)</ref> in agreement with the [[law of mass action]]. However it is not always possible to derive overall reaction schemes but solutions based on [[rate equation]]s are possible in terms of [[steady state (chemistry)|steady-state]] or [[Michaelis-Menten kinetics|Michaelis-Menten]] approximations.

==Notes==
<references/>

{{Reaction mechanisms}}

[[Category:Chemical kinetics]]
[[Category:Physical chemistry]]

[[ja:反応速度#単純反応と複合反応]]

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