Order (chemistry) - Second Order

Second Order

A reaction is said to be second order when the overall order is two. The rate of a second-order reaction may be proportional to one concentration squared, or (more commonly) to the product of two concentrations . As an example of the first type, the reaction NO₂ + CO → NO + CO₂ is second-order in the reactant NO₂ and zero order in the reactant CO. The observed rate is given by, and is independent of the concentration of CO.

The second type includes the class of S_N2 (nucleophilic substitution bimolecular) reactions, such as the alkaline hydrolysis of ethyl acetate:

CH₃COOC₂H₅ + OH− → CH₃COO− + C₂H₅OH.

This reaction is first-order in each reactant and second-order overall: r = k

If the same hydrolysis reaction is catalyzed by imidazole, the rate equation becomes r = k. The rate is first-order in one reactant (ethyl acetate), and also first-order in imidazole which as a catalyst does not appear in the overall chemical equation.