[LackOfNames34]

I'm studying for an exam and have come across a weird "loophole" that I don't quite get. What I initially wrote in my notes was that you could not simply use the coefficients of a balanced equation as the order of a reaction for rate law. But I'm now seeing that we CAN use them for elementary reactions. Did I get it wrong from the start or is this true? And if so, why is this the case?

[Borek]

Stoichiometric coefficients are not guaranteed to be OK in general, which doesn't mean they are never correct. Elementary reactions are one of these cases when they work OK.

[Meter]

The reason we CAN'T use it for non-elementary reactions is because in these reactions consist of multiple elementary reactions (the schematization of these reactions is known as a reaction mechanism). Thus reading the reaction order directly from the stoichiometric coefficient will often yield wrong results as the net reaction does not give you the full picture of how the reactants and intermediates behave.

An elementary reaction can not be broken down into smaller reactions (otherwise it would not be elementary!) and as such, what you see is what you get.

[penta-d]

I agree with both Borek and Meter. Here is a web page that might be helpful on what elementary reactions are, and why they are considered elementary. It touches on rates also, but not a lot of detail there:

https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Kinetics/03%3A_Rate_Laws/3.02%3A_Reaction_Mechanisms/3.2.01%3A_Elementary_Reactions