Intermediate Topic 425 of 1102 Kinetics & Equilibrium
Order of Reaction
The sum of exponents in the rate law, determined experimentally not from stoichiometry.
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In-depth explanation of Order of Reaction
The order of reaction with respect to a given reactant is the exponent of its concentration in the experimentally determined rate law, and the overall order is the sum of all such exponents. An order of zero means the rate does not depend on that concentration, an order of one means the rate is directly proportional to it, and an order of two means doubling the concentration quadruples the rate. Orders are determined by experiment and are generally not equal to the stoichiometric coefficients. They can be fractional, as often happens in complex multi-step mechanisms, or even negative when a species slows the reaction.
Going deeper: Order of Reaction
Key ideas: Find order by changing one concentration at a time: rate ratio = (concentration ratio)^order, so order = log(rate ratio) / log(concentration ratio). A doubled concentration giving a rate ratio of 8 means order 3 in that species. Units of k reveal the overall order directly, which is a useful check on an answer.
Common mistakes and connections: Students equate order with molecularity or with the coefficient, and they treat order as always a whole number. Another mistake is calling a reaction zero order when only one reactant is zero order, since the overall order is the sum. Pseudo orders appear when one reagent is in excess. Order determines which integrated law and half-life formula applies.
Visualization of Order of Reaction
Order tells how the rate scales with concentration: order 0 is flat, order 1 is a straight line, order 2 curves upward. Compare the shapes with the slider. The interactive lesson shows this as a live animation or chart.
Worked example and use case
If doubling [A] quadruples the rate, the reaction is second order in A.
Frequently asked questions about Order of Reaction
What is Order of Reaction?
The sum of exponents in the rate law, determined experimentally not from stoichiometry.
Is Order of Reaction a beginner, intermediate or advanced topic?
Order of Reaction is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Order of Reaction?
Study these earlier topics in Kinetics & Equilibrium first: Average and Instantaneous Rate, Rate Law, Rate Constant.
What comes after Order of Reaction?
The next topics in Kinetics & Equilibrium are Zero Order Reaction and First Order Reaction.
Can I practise Order of Reaction with quizzes?
Yes. The interactive lesson includes a self-check question on Order of Reaction, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Order of Reaction fits in Kinetics & Equilibrium
Order of Reaction is topic 6 of 91 in Kinetics & Equilibrium.