<P> If a reaction rate depends on a single reactant and the value of the exponent is one, then the reaction is said to be first order . In organic chemistry, the class of S 1 (nucleophilic substitution unimolecular) reactions consists of first - order reactions . For example, in the reaction of aryldiazonium ions with nucleophiles in aqueous solution ArN + X → ArX + N, the rate equation is r = k (ArN), where Ar indicates an aryl group . </P> <P> Another class of first - order reactions is radioactive decay processes which are all first order . These are, however, nuclear reactions rather than chemical reactions . </P> <P> 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 r = k (A) 2 (\ displaystyle r = k (A) ^ (2) \,), or (more commonly) to the product of two concentrations r = k (A) (B) (\ displaystyle r = k (A) (B) \,). As an example of the first type, the reaction NO + CO → NO + CO is second - order in the reactant NO and zero order in the reactant CO . The observed rate is given by r = k (NO 2) 2 (\ displaystyle r = k ((\ ce (NO2))) ^ (2) \,), and is independent of the concentration of CO . </P> <P> The second type includes the class of S 2 (nucleophilic substitution bimolecular) reactions, such as the alkaline hydrolysis of ethyl acetate: </P>

What is the order of the reaction in no2
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