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When is the stoichiometric coefficient of a reactant also the order of that reactant? Is that only for elementary reactions? Why is this so? Thanks for any help!
The sum of stoichiometric coefficients for the reactants in the slow step is equal to the order of the overall reaction. For elementary reactions, there is only a single step leading from reaction to product, hence one can directly sum the coefficients of the reactants for that reaction, without worrying about determining the slow step of the reaction.
Reactions are often multistep processes, and the rate of the reaction is equal to the rate of the slowest step, as we assume everything else happens instantaneously as soon as the slow step is completed (think of it as if all the other steps in the reaction are just sitting there idly, waiting for the slow step to finally finish its part). Thus the stoichiometric coefficients in the overall reaction are not necessarily included or used in the rate law. Instead, look at each step individually and use the stoichiometric coefficients of the slowest elementary step (each elementary step tells you how many molecules of each reactant collide to produce the product or intermediate). Add all the stoichiometric coefficients of the slowest step together and you get the reaction order
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