AP® Chemistry · Unit 5 · Topic 5.2
Rate Law and Reaction Order
A rate law shows how reaction rate depends on concentration, with each reactant raised to a power called its order. That order must be found experimentally, not copied from the balanced equation's coefficients.
A rate law looks like rate = k[A]m[B]n. The exponents m and n are the orders with respect to A and B, and they tell you how sensitive the rate is to changing that concentration. If m = 0, changing [A] does nothing to the rate. If m = 1, doubling [A] doubles the rate. If m = 2, doubling [A] quadruples the rate, because the effect compounds: rate is proportional to [A] squared, so (2x) squared is 4x.
This is why doubling a reactant sometimes does nothing and sometimes quadruples the rate: it depends entirely on that reactant's order, not on intuition about how much of it you added. Zero order means the reactant isn't involved in the slow, rate-determining step at all. First order means it appears once in that slow step. Second order means either it appears twice, or two molecules of it collide in the key step.
The exponents never come from the coefficients in the overall balanced equation. The balanced equation describes the overall stoichiometry, the ratio of reactants consumed and products formed, but says nothing about speed. Rate depends only on what happens in the slowest step of the mechanism, which can involve completely different numbers of each species than the overall equation shows. That is why chemists run experiments, changing one concentration at a time and measuring the effect on rate, to pin down each order. This sets up the next idea: once you know the orders, you can find k and predict rate at any concentration.
Key terms
- Rate law
- An equation showing how reaction rate depends on reactant concentrations, each raised to its order.
- Reaction order
- The exponent on a concentration term in the rate law, found experimentally for each reactant.
- Overall reaction order
- The sum of all the individual exponents in the rate law.
- Rate constant (k)
- The proportionality constant in the rate law, with units that depend on the overall order.
Watch it explained
- How it worksHow does the exponent on a concentration in a rate law describe the reaction's sensitivity?
- How it worksWalk me through what zero, first, and second order each look like as a rate response.
- Common mistakeWhy isn't the exponent in a rate law taken from the coefficient in the balanced equation?
- Going furtherWhat are the units of k for a zero order, a first order, and a second order reaction?
Check yourself
End of topic.