When a reaction is elementary,the stoichiometric coefficients always equal the order for each reactant? Is there any exception? Because at Ángel González Ureña's Cinética Química it says that this is "in general cases", but not always. I haven't found any results in google about this topic.

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    $\begingroup$ Elementary reactions always involve a single molecule reacting (e.g. isomerisation) or two colliding and so react. So no exceptions. (Some texts include three molecules colliding but the chance of this is so small it never happens in practice. It is found experimentally that an intermediate is formed, so it is a two step process.) $\endgroup$
    – porphyrin
    Commented Aug 23, 2022 at 13:38
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    $\begingroup$ If the order is not equal to the stoichiometric coefficient, the reaction is not elementary. $\endgroup$
    – Maurice
    Commented Aug 23, 2022 at 14:00
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    $\begingroup$ Yes, like @Maurice, I thought this is the defining property. I guess you could have a multi-step reaction where the orders happen to be equal to the stoichiometric coefficients, so I guess the definition has to be more than that. $\endgroup$
    – Karsten
    Commented Aug 23, 2022 at 15:08


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