Rate Laws
Rate = k[A]ᵐ[B]ⁿ links rate to concentration.
About this topic
A rate law relates rate to the concentrations of reactants: rate = k[A]^m[B]^n, where k is the rate constant and the exponents m and n are the reaction orders, found by experiment, not from coefficients. The overall order is m + n. A first-order reaction doubles in rate when concentration doubles; a second-order quadruples; zero-order is independent of concentration.
To find orders use the method of initial rates: compare experiments where one concentration changes while others stay fixed. If doubling [A] doubles the rate, the order in A is 1; if it quadruples the rate, the order is 2; if there is no change, it is 0. The constant k can then be calculated from any trial, and it depends on temperature and catalysts. Units of k depend on the overall order.
Worked example: for A + B, rate is 0.010 M/s at [A] = 0.10 and [B] = 0.10, 0.020 when [A] doubles, and 0.040 when [B] doubles. So the order is 1 in A and 2 in B, rate = k[A][B]^2, and k = 0.010 / (0.10 x 0.010) = 10 per M squared per second.
Rate laws reveal reaction mechanisms, since the slowest step determines the rate. Radioactive decay and many drug eliminations are first order. A misconception is that orders come from the balanced equation; they come only from data.
Key ideas
- Orders found by experiment
- k depends on temperature
- Half-life for first-order reactions
Quick check
Use cases: where Rate Laws helps
- Homework: review the key idea behind Rate Laws before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Rate Laws without notes.
- Visual learning: play with the interactive lab to see how Rate Laws changes when you adjust its inputs.
- Teaching a friend: explain Rate Laws in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Rate Laws?
Rate = k[A]ᵐ[B]ⁿ links rate to concentration.
What will I learn in Rate Laws?
Orders found by experiment. k depends on temperature. Half-life for first-order reactions.
Is Rate Laws part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Rate Laws lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Rate Laws?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.