Order of reaction
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{{Main|Rate equation}}
The '''Order of reaction''' <ref>[http://www.iupac.org/goldbook/O04322.pdf IUPAC's Goldbook definition of order of reaction]</ref> with respect to a certain [[reactant]] is defined, in [[chemical kinetics]], as the [[Exponentiation|power]] to which its [[concentration]] term in the [[rate equation]] is raised.
For example, given a chemical reaction A + 2B → C with a rate equation r = k[A]<sup>1</sup>[B]<sup>2</sup>, the reaction '''order with respect to''' A would be 1 and with respect to B would be 2, the '''total reaction order''' would be 2+1=3. It is not necessary that the order of a reaction is a whole number - zero and fractional values of order are possible - but they tend to be integers. Reaction orders can be determined only by [[experiment]]. Their knowledge allows conclusions about the [[reaction mechanism]].
The reaction order is ''not necessarily'' related to the [[stoichiometry]] of the reaction, unless the reaction is [[elementary reaction|elementary]]. Complex reactions may or may not have reaction orders equal to their stoichiometric coefficients
For example <ref>Kenneth A. Connors Chemical Kinetics, the study of reaction rates in solution, 1990, VCH Publishers</ref>:
* The [[alkaline]] [[hydrolysis]] of [[ethyl acetate]] is a complex reaction:
:CH<sub>3</sub>COOC<sub>2</sub>H<sub>5</sub> + OH<sup>-</sup> → CH<sub>3</sub>COO<sup>-</sup> + C<sub>2</sub>H<sub>5</sub>OH.
:It has the following rate equation: r = k[CH<sub>3</sub>COOC<sub>2</sub>H<sub>5</sub>][OH]
* The rate equation for [[imidazole]] catalyzed hydrolysis is
: r = k [imidazole][CH<sub>3</sub>COOC<sub>2</sub>H<sub>5</sub>]
:although no imidazole is present in the stoichiometric [[chemical equation]]
* In the reaction of [[diazonium salt|aryldiazonium]] ions with [[nucleophiles]] in aqueous solution ArN<sub>2</sub><sup>+</sup> + X<sup>-</sup> → ArX + N<sub>2</sub> the rate equation is
: r = k[ArN<sub>2</sub><sup>+</sup>]
Reactions can also have an undefined reaction order with respect to a reactant, for example one can not talk about reaction order in the rate equation found when dealing with a bimolecular reaction between [[reactions on surfaces|adsorbed molecules]]:
::<math>r=k \frac{K_1K_2C_AC_B}{(1+K_1C_A+K_2C_B)^2}</math> .
If the concentration of one of the reactants remains constant (because it is a [[catalyst]] or it is in great excess with respect to the other reactants) its concentration can be included in the rate constant, obtaining a '''pseudo constant''': if B is the reactant whose concentration is constant then <math> r= k [A][B]=k'[A]</math>. The second order rate equation has been reduced to a '''pseudo first order''' rate equation. This makes the treatment in order to obtain an integrated rate equation much easier.
''Zero-order'' reactions are often seen for thermal [[chemical decomposition]]s where the [[reaction rate]] is independent of the concentration of the reactant (changing the concentration has no effect on the speed of the reaction):
In '''broken-order reactions''' the order is a non-integer typical of reactions with a complex [[reaction mechanism]]. For example the [[chemical decomposition]] of [[ethanal]] into [[methane]] and [[carbon monoxide]] proceeds with an order of 1.5 with respect to ethanal. The decomposition of [[phosgene]] to carbon monoxide and [[chlorine]] has order 1 with respect to phosgene itself and order 0.5 with respect to chlorine.
In a '''mixed-order reaction''' the order of a reaction changes in the course of a reaction as a result of changing variables such as [[pH]]. An example is the oxidation of an [[alcohol]] to a [[ketone]] by a [[ruthenate]] (RuO<sub>4</sub><sup>2-</sup>) and a [[hexacyanoferrate]], the latter serving as the [[sacrificial catalyst]] converting Ru(IV) back to Ru(VI) <ref>''Ruthenium(VI)-Catalyzed Oxidation of Alcohols by Hexacyanoferrate(III): An Example of Mixed Order'' Mucientes, Antonio E,; de la Peña, María A. J. Chem. Educ. '''2006''' 83 1643. [http://jchemed.chem.wisc.edu/Journal/Issues/2006/Nov/abs1643.html Abstract]</ref>: the disappearing-rate of the ferrate is zero-order with respect to the ferrate at the onset of the reaction (when its concentration is high and the ruthenium catalyst is quickly regenerated) but changes to first-order when its concentration decreases.
'''Negative order reactions''' are rare, for example the conversion of [[ozone]] (order 2) to [[oxygen]] (order -1).
== References ==
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== External links ==
* [http://www.citycollegiate.com/chemical_kineticsXIa.htm Chemical kinetics, reaction rate, and order] (needs flash player)
* [http://artsandscience.concordia.ca/facstaff/a-c/bird/c206/Labs/html/kinetics-lab-lbwks.html The reaction of crystal violet with sodium hydroxide: a kinetic study].
* [http://itl.chem.ufl.edu/4411/2041/lec_k.html Reaction kinetics, examples of important rate laws] (lecture with audio).
* [http://www.chemguide.co.uk/physical/basicratesmenu.html#top Rates of Reaction]
[[Category:Chemical kinetics]]
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