Intermediate Topic 423 of 1102 Kinetics & Equilibrium
Rate Law
An equation relating rate to concentrations raised to experimentally determined powers.
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In-depth explanation of Rate Law
The rate law expresses the reaction rate as a product of reactant concentrations, each raised to an experimentally determined power, multiplied by the rate constant. For a reaction involving A and B the general form is rate = k[A]^m[B]^n, where m and n are the reaction orders with respect to A and B. These exponents cannot be deduced from the balanced equation and must be measured, because the actual molecular pathway may involve several steps. The sum m + n gives the overall order of the reaction. Rate laws are the central link between experiment and mechanism, since a proposed mechanism is acceptable only if it predicts the observed law.
Going deeper: Rate Law
Key ideas: Write rate = k[A]^m[B]^n and find m and n from experiments, either by comparing initial rates or by testing integrated plots. Solids and pure liquids do not appear, and a catalyst can appear even though it is absent from the balanced equation. Once orders are known, use any run to compute k and then predict rates at new concentrations.
Common mistakes and connections: The classic error is reading orders from the coefficients of the balanced equation, which is valid only for an elementary step. Students also forget that the rate law can contain products or catalysts, for example inhibition by a product. A rate law links to mechanisms, since it constrains which elementary steps are allowed.
Visualization of Rate Law
Rate = k[A]^n: with a second order dependence, doubling the concentration quadruples the rate. The exponents must come from experiment, not from the equation. The interactive lesson shows this as a live animation or chart.
Worked example and use case
For 2NO + O2 -> 2NO2 experiments show rate = k[NO]^2[O2], so the reaction is third order overall.
Frequently asked questions about Rate Law
What is Rate Law?
An equation relating rate to concentrations raised to experimentally determined powers.
Is Rate Law a beginner, intermediate or advanced topic?
Rate Law is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Rate Law?
Study these earlier topics in Kinetics & Equilibrium first: Chemical Kinetics, Reaction Rate, Average and Instantaneous Rate.
What comes after Rate Law?
The next topics in Kinetics & Equilibrium are Rate Constant and Order of Reaction.
Can I practise Rate Law with quizzes?
Yes. The interactive lesson includes a self-check question on Rate Law, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Rate Law fits in Kinetics & Equilibrium
Rate Law is topic 4 of 91 in Kinetics & Equilibrium.