Order of reaction
In chemical kinetics, the order of reaction with respect to a given substance (such as reactant, catalyst or product) is defined as the index, or exponent, to which its concentration term in the rate equation is raised. For the typical rate equation of form , where [A], [B], ... are concentrations, the reaction orders (or partial reaction orders) are x for substance A, y for substance B, etc. The overall reaction order is the sum x + y + .... For many reactions, the reaction orders are not equal to the stoichiometric coefficients.
![r\;=\;k[{\mathrm {A}}]^{x}[{\mathrm {B}}]^{y}...](https://wikimedia.org/api/rest_v1/media/math/render/svg/cd049bdd01976b0ae9d5044f7597c0498187879b)
For example, the chemical reaction between mercury (II) chloride and oxalate ion
has the observed rate equation
In this case, the reaction order with respect to the reactant HgCl2 is 1 and with respect to oxalate ion is 2; the overall reaction order is 1 + 2 = 3. The reaction orders (here 1 and 2 respectively) differ from the stoichiometric coefficients (2 and 1). Reaction orders can be determined only by experiment. Their knowledge allows conclusions to be drawn about the reaction mechanism, and may help to identify the rate-determining step.
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