Rate laws are one of the first places in the course where a balanced equation stops being enough information on its own, which surprises students who are used to reading everything they need directly off the equation.
Why the rate law isn't in the balanced equation
A rate law has the form rate = k[A]^m[B]^n, where m and n are the reaction orders with respect to each reactant. These exponents depend on the actual mechanism the reaction follows, not the stoichiometric coefficients, and they can only be determined from real experimental data.
The method of initial rates
Data from three experiments
Find order with respect to A (compare Exp 1 and 2, where B is constant)
Find order with respect to B (compare Exp 2 and 3, where A is constant)
Write the rate law
Overall order
Overall reaction order is simply the sum of the individual orders. In the example above, the overall order is 2, even though it only came from one reactant, since B's order of zero doesn't add anything.
Where students actually lose points here
Reading exponents off the balanced equation
This is the single most common rate law mistake. Reaction order has to come from real data or a proposed mechanism, never assumed from the coefficients.
Comparing experiments where more than one concentration changes
The method of initial rates only works cleanly when comparing two experiments where exactly one concentration changes and the rest stay fixed. Comparing the wrong pair of experiments makes it impossible to isolate a single reactant's order.
Forgetting that an order of zero means no effect at all
A reactant with an order of zero doesn't slow the reaction down, it simply doesn't appear in the rate law, and changing its concentration has no effect on rate whatsoever.
A skill that rewards careful comparison
Rate law problems are usually just careful before-and-after comparisons. Once that pattern is familiar, the calculations themselves are quick.
Common questions
Can you determine a rate law from the balanced equation alone?
No. Reaction order has to be determined experimentally, or from a proposed reaction mechanism's slow step. The coefficients in a balanced equation don't reveal the rate law on their own.
What does an order of zero mean for a reactant?
It means changing that reactant's concentration has no effect on the reaction rate at all. That reactant doesn't appear in the rate law expression.
How do you find overall reaction order?
Add up the individual orders with respect to each reactant in the rate law. A reaction that's second order in one reactant and zero order in another has an overall order of two.