the order of reaction is a concept which raises of the chemical Cinétique.

Let us consider a reaction whose equation assessment is written: has·With + B·B → C·C

has and B being the stoechiometric coefficients which are whole natural.

The reaction speed is defined by:

$v\; =\; \backslash \; frac\; \{-\; 1\}\; \{has\}\; \backslash \; frac\; \{dn\_A\}\; \{dt\}\; =\; \backslash \; frac\; \{-\; 1\}\; \{B\}\; \backslash \; frac\; \{dn\_B\}\; \{dt\}\; =\; \backslash \; frac\; \{1\}\; \{C\}\; \backslash \; frac\; \{dn\_C\}\; \{dt\}$

This speed is function of the concentrations of the reagents. The experimental study of the reaction makes it possible to establish the form of the law speed.

In certain cases, the law speed can be put in the form of a students' rag procession: $v\; =\; K\; \backslash \; cdot\; has\}\; ^\; \backslash \; alpha\; \backslash \; cdot\; B\}\; ^\; \backslash \; beta$ It is said whereas the reaction admits an order and one calls order of reaction the sum σ exhibitors: σ = α + β. The exhibitor α is the order partial of the reaction compared to the species has, β is the order partial of the reaction compared to the species B.

It should be noted that α and β are not necessarily equal to the stoechiometric coefficients, it are not even necessarily whole nor positive.

The coefficient K is called coefficient speed and depends on the temperature.

If the law speed cannot be put in the form of a students' rag procession with the concentrations like factors, it is said that the reaction does not admit an order.