# Order of reaction

Order of reaction

In chemical kinetics, the order of reaction with respect to certain reactant, is defined as the power to which its concentration term in the rate equation is raised [1].

For example, given a chemical reaction 2A + B → C with a rate equation

r = k[A]2[B]1

the reaction order with respect to A would be 2 and with respect to B would be 1, the total reaction order would be 2 + 1 = 3. It is not necessary that the order of a reaction be 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. Complex reactions may or may not have reaction orders equal to their stoichiometric coefficients

For example [2]:

CH3COOC2H5 + OH → CH3COO + C2H5OH.
It has the following rate equation: r = k[CH3COOC2H5][OH]
• The rate equation for imidazole catalyzed hydrolysis is
r = k[imidazole][CH3COOC2H5]
although no imidazole is present in the stoichiometric chemical equation
• In the reaction of aryldiazonium ions with nucleophiles in aqueous solution ArN2+ + X → ArX + N2 the rate equation is
r = k[ArN2+]

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 adsorbed molecules:

$r=k \frac{K_1K_2C_AC_B}{(1+K_1C_A+K_2C_B)^2}. \,$

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

$r= k [A][B]=k'[A]. \,$

The second-order rate equation has been reduced to a pseudo-first-order rate equation. This makes the treatment to obtain an integrated rate equation much easier.

Zero-order reactions are often seen for thermal chemical decompositions 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 (RuO42−) and a hexacyanoferrate, the latter serving as the sacrificial catalyst converting Ru(IV) back to Ru(VI) [3]: 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

1. ^ IUPAC's Goldbook definition of order of reaction
2. ^ Kenneth A. Connors Chemical Kinetics, the study of reaction rates in solution, 1990, VCH Publishers
3. ^ 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. Abstract

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• reaction mechanism — Introduction       in chemical reactions (chemical reaction), the detailed processes by which chemical substances are transformed into other substances. The reactions themselves may involve the interactions of atoms (atom), molecules (molecule),… …   Universalium

• Reaction rate constant — In chemical kinetics a reaction rate constant k or λ quantifies the speed of a chemical reaction.[1] For a chemical reaction where substance A and B are reacting to produce C, the reaction rate has the form: Reaction: A + B → C k(T) is the… …   Wikipedia

• Order — Contents 1 Ordinality 2 Philosophy 3 Science 4 Mathe …   Wikipedia

• Reaction–diffusion system — Reaction–diffusion systems are mathematical models which explain how the concentration of one or more substances distributed in space changes under the influence of two processes: local chemical reactions in which the substances are transformed… …   Wikipedia

• Order of Canada — Insignia of a Member of the Order of Canada Awarded by the …   Wikipedia

• Order of the Greek Horsemen — logo, circa 1982 Order of the Greek Horsemen is a secret society at the University of Georgia, in Athens, Georgia. Founded in 1955, the organization annually inducts five new members from among the male leaders of the Greek system at the… …   Wikipedia

• reaction formation — n a psychological defense mechanism in which one form of behavior substitutes for or conceals a diametrically opposed repressed impulse in order to protect against it * * * (in psychoanalysis) a defence mechanism by which unacceptable unconscious …   Medical dictionary

• Reaction rate — Iron rusting a chemical reaction with a slow reaction rate. Wood …   Wikipedia

• Reaction mechanism — In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.[1] Although only the net chemical change is directly observable for most chemical reactions, experiments can often be …   Wikipedia

• Reaction formation — In Sigmund Freud s psychoanalytic theory, reaction formation is a defense mechanism in which anxiety producing or unacceptable emotions are replaced by their direct opposites. [cite web url=http://www.psychpage.com/learning/library/counseling/defe… …   Wikipedia