Intermediate Topic 434 of 1102 Kinetics & Equilibrium
Elementary Reactions
Single-step processes for which the rate law follows directly from stoichiometry.
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In-depth explanation of Elementary Reactions
An elementary reaction is a single step that proceeds exactly as written at the molecular level, with no intermediate stages. For such a step the rate law can be written directly from the stoichiometry, with each reactant exponent equal to its coefficient. Most overall reactions are not elementary; they are made up of several elementary steps, and only the individual steps have predictable rate laws. Recognizing elementary steps is essential for building mechanisms and for predicting how a reaction responds to changes in concentration. Elementary steps are also what collision theory and transition state theory describe at the molecular level.
Going deeper: Elementary Reactions
Key ideas: For an elementary step write the rate law directly: A + B -> products gives k[A][B], 2A -> products gives k[A]^2. Reverse steps have their own rate constants. Check a candidate mechanism by confirming that every step is chemically reasonable, with one or two particles colliding, and that the steps sum to the overall equation.
Common mistakes and connections: The commonest error is treating the overall equation as elementary and reading orders from its coefficients. A reaction that does follow the coefficients need not be elementary, since coincidence is possible. Students also forget that elementary steps are reversible in principle. This topic provides the building blocks for mechanisms, collision theory and microscopic reversibility.
Visualization of Elementary Reactions
NO + O3 to NO2 + O2 happens in one step through a single barrier, so its rate law k[NO][O3] follows directly from the equation. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The reaction NO + O3 -> NO2 + O2 is elementary, so its rate is k[NO][O3].
Frequently asked questions about Elementary Reactions
What is Elementary Reactions?
Single-step processes for which the rate law follows directly from stoichiometry.
Is Elementary Reactions a beginner, intermediate or advanced topic?
Elementary Reactions is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Elementary Reactions?
Study these earlier topics in Kinetics & Equilibrium first: Method of Initial Rates, Integrated Rate Laws, Molecularity.
What comes after Elementary Reactions?
The next topics in Kinetics & Equilibrium are Reaction Mechanism and Rate-Determining Step.
Can I practise Elementary Reactions with quizzes?
Yes. The interactive lesson includes a self-check question on Elementary Reactions, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Elementary Reactions fits in Kinetics & Equilibrium
Elementary Reactions is topic 15 of 91 in Kinetics & Equilibrium.