I've read in two books, and seen in three separate videos courses, that you cannot determine the rate law order from stoichiometric coefficients.
Then, I have seen homework and "practice" problems that ask you to do exactly that.
Needless to say, I am confused.
This is how I am presently making sense of the issue: Given a balanced equation, and absent any other information, you can (and must) use the stoichiometric coefficients to determine the rate law. So, for example, given the equation $2A+B \rightarrow C + 3D$, the rate law would be $rate = k[A]^2[B]$.
But, this would only hold true if this were an "elementary reaction" (i.e., no equilibrium).
So, I wonder why the books and videos emphasize that you can only determine order by experiment and not from the stoichiometric coefficients. I am guessing that they do so because, in practice, you do not know if the reaction is an elementary reaction or not and therefore experiment is the only way to make that determination. And, because you don't really know if it is an elementary reaction, you can only determine the rate law experimentally.
The bottom line:
- as a practitioner, determine rate law experimentally;
- as a student, given no information other than the balanced equation, determine the rate law from the stoichiometric coefficients;
- as a student, given rate and concentration data, determine the order (and rate law) from the data.
That is a pragmatic, but unsatisfying, solution to the problem but I would prefer a clearer explanation of what is going on.