From experiment vs. from balanced equation

[James Yu 3C]

I know that when finding rate of reaction, some parts of the equation can only be found from experiments and some can be learned from the balanced chemical equation like order of reaction is found experimentally. Is it only possible to find experimentally, or are there scenarios where we'd be able to calculate it?

[Nishanth Vemuri 2L]

The rate law requires knowing if the reaction occurs in one step or not. If the reaction is just one step (the chemical equation provided), then we know that the rate law would be the concentrations of the reactants multiplied together, to the power of their respective coefficients. However, if we know that the mechanism takes place in two or more steps, then we will be given whichever step is the slowest. We then use this information to calculate the rate law. Otherwise, if we are not given which step is the slowest through experimental data, we will not know the rate formula.

[Aarush_Maddela_3A]

It is not possible to calculate the rate law from the balanced chemical equation. You must look at experimental data to calculate the rate law. This is because the balanced chemical reaction does not give you information on the proposed reaction mechanism, which is what determines the rate law. As Nishanth said, if the proposed reaction mechanism is known, we simply analyze the slow step to determine the rate law, as the overall rate law is equal to the rate law of the slow step.