It's tempting to assume a chemical equation tells you how a reaction's speed depends on the concentration of its ingredients — that doubling a reactant's concentration should double the rate, in rough proportion to its coefficient in the balanced equation. It doesn't work that way. The actual relationship between concentration and rate, called the rate law, has to be determined experimentally, and it frequently bears no simple resemblance to the reaction's own balanced equation at all.
Reaction order isn't written in the chemical equation
A rate law expresses reaction rate as proportional to the concentration of each reactant raised to some power, called its order with respect to that reactant. A reaction might be first order in one reactant (doubling that reactant's concentration doubles the rate), second order in another (doubling it quadruples the rate), or even zero order in a reactant that's present in the equation but doesn't affect the rate at all once its concentration exceeds some threshold. Crucially, these exponents aren't the same as the reactant's stoichiometric coefficients in the balanced chemical equation — they have to be measured directly, typically by running the reaction multiple times at different starting concentrations and observing how the rate actually changes, because the equation alone doesn't reveal them.
Why the equation doesn't reveal the mechanism
The reason rate laws can't be read directly off a balanced equation is that most reactions don't actually happen in one single step matching the overall equation — they proceed through a sequence of simpler elementary steps, some fast and some slow, and the overall rate law is determined almost entirely by whichever step is slowest, the "rate-determining step," rather than by the reaction's net overall stoichiometry. Two reactions that look identical when written as a single balanced equation can proceed through entirely different underlying mechanisms, with different numbers of steps and different rate-determining steps, producing genuinely different rate laws — which is exactly why chemists treat determining a reaction's rate law as an experimental discovery about its actual mechanism, not a calculation performable from the written equation alone.
What we're still unsure about
The experimental methods for determining rate laws, and the underlying logic connecting rate-determining steps to overall kinetics, are well established, standard analytical chemistry — not a live scientific dispute. What remains genuinely difficult, especially for complex reactions involving many intermediate steps, is definitively establishing the full underlying mechanism rather than just the empirical rate law — a measured rate law can often be consistent with more than one plausible mechanism, and distinguishing between competing proposed mechanisms sometimes requires additional, more sophisticated experimental evidence (like detecting short-lived intermediate species directly) that isn't always straightforward to obtain, particularly for fast or complicated multi-step reactions.
This sits inside Reaction Kinetics & Rate Laws, one of seven topics in Physical Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.