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 SN2 (nucleophilic substitution bimolecular) reactions, such as the alkaline hydrolysis of ethyl acetate:
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- 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.
Read more about this topic: Order (chemistry)
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