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The Order of reaction * with respect to a certain reactant is defined, in chemical kinetics, as the power to which its concentration term in the rate equation is raised. For example, given a chemical reaction A + 2B → C with a rate equation r = kA1B2, the reaction order with respect to A would be 1 and with respect to B would be 2, the total reaction order would be 2+1=3. It is not necessary that the order of a reaction is a whole number - zero and fractional values of order are possible - but they tend to be integers. Reaction orders can be determined only by experiment. Their knowledge allows conclusions about the reaction mechanism. The reaction order is not necessarily related to the stoichiometry of the reaction, unless the reaction is elementary. Complex reactions may or may not have reaction orders equal to their stoichiometric coefficients For example: Reactions can also have a undefined reaction order with respect to a reactant, for example one can not talk about reaction order in the rate equation found when dealing with a bimolecular reaction between adsorbed molecules: . If the concentration of one of the reactants remains constant (because it is a catalyst or it is in great excess with respect to the other reactants) its concentration can be included in the rate constant, obtaining a pseudo constant: if B is the reactant whose concentration is constant then . The second order rate equation has been reduced to a pseudo first order rate equation. This makes the treatment in order to obtain an integrated rate equation much easier. Zero-order reactions are often seen for thermal chemical decompositions where the reaction rate is independent of the concentration of the reactant (changing the concentration has no effect on the speed of the reaction):
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