Elementary Reactions
Infer a step’s rate law from molecularity only when the event is explicitly elementary.
AP Chemistry Topic 5.4 · CED printed p. 98 / PDF p. 105. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Balanced equations and stoichiometric ratios
- Molarity and concentration versus amount
- Bond rearrangement and energy conservation
- Slopes, logarithms and reciprocal graphs
An elementary reaction is one microscopic event, so its reactant coefficients count the participating particles. For an explicitly elementary A + B → products step, the rate is k[A][B]: encounters depend on both particle densities. For 2A → products as a bimolecular elementary step, the rate is k[A]2. A unimolecular elementary conversion has first-order dependence on that species. Molecularity is the number of reactant particles in that one event; it is not the overall reaction’s order. The coefficient inference is permitted only when the step is declared elementary.
Simultaneous productive encounters involving three or more particles are rare because the particles must arrive at the same place at the same time with suitable orientation and energy. An overall equation such as 2A + B → products usually represents several smaller events, not a guaranteed three-particle collision. This is why overall coefficients alone generally fail to give an empirical rate law. A proposed mechanism with such a step needs evidence; rarity is a reason for caution, not an absolute impossibility.
Let's work through it
Compare an explicitly elementary NO + O3 → NO2 + O2 with the overall 2NO + O2 → 2NO2.
- The stated elementary step requires one NO and one O3; its rate law is k[NO][O3].
- The overall equation requires two NO per O2 net, but it does not say that all three collide simultaneously.
- We need experimental rates or a justified mechanism for the overall law. Molecularity can be assigned to a stated elementary step; overall stoichiometry is not sufficient.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
For an elementary 2A → B step, what law follows?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
For an elementary 2A → B step, what law follows?
Worked answer and reason
r =k[A]2 for the elementary event-rate convention. A disappearance rate includes a factor of two that can be absorbed into a separately defined constant.
Two A particles participate in the event; rate convention still matters.
Retrieval 2
Why should 2A + B in an overall equation not automatically become a third-order law?
Worked answer and reason
The net reaction may occur through multiple elementary steps. A three-particle simultaneous collision is also uncommon.
A net atom ledger does not specify one microscopic encounter.
Catch the tempting mistake
- Molecularity and empirical overall order are interchangeable.
- Molecularity describes one elementary event; empirical order describes observed concentration sensitivity.
- Every reaction occurs in one collision.
- Many reactions occur through sequential intermediates and multiple collisions.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 5 problem 10 · ap application · symbolic
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For the teacher
- Use the word elementary explicitly in all coefficient-to-order problems.
- Ask students to identify the evidence needed before applying the same rule to an overall equation.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 5.4.A
- Represent an elementary reaction as a rate law expression using stoichiometry.
- 5.4.A.1
- The rate law of an elementary reaction can be inferred from the stoichiometry of the particles participating in a collision.
- 5.4.A.2
- Elementary reactions involving the simultaneous collision of three or more particles are rare.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Reaction Mechanisms — Scientific check: intermediates, elementary molecularity, slow steps and rapid-equilibrium substitution.
Glossary and deeper reading · 5 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Stoichiometry
Stoichiometry uses a balanced chemical equation to relate amounts of reactants and products. Equation coefficients provide mole ratios rather than direct mass ratios. For example, forming water from hydrogen and oxygen consumes two moles of hydrogen molecules per mole of oxygen molecules.
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Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
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Elementary reaction
An elementary reaction is one step in a proposed mechanism rather than the sum of multiple steps. For a specified elementary step, the rate-law concentration factors follow its reacting-particle event. For example, a bimolecular elementary collision can have a rate proportional to the concentrations of its two reactants.
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Reaction order
Reaction order describes the concentration dependence in a rate law. An order in one reactant is its exponent, and overall order is the sum of the concentration exponents. For example, a second-order dependence on one reactant makes quadrupling the rate follow from doubling that reactant's concentration under fixed conditions.
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Rate law
A rate law relates a reaction rate to reactant concentrations and a rate constant under stated conditions. For an overall reaction, its concentration exponents are established from evidence rather than copied from the balanced equation. For example, doubling one reactant concentration doubles the rate when the reaction is first order in that reactant and other conditions are fixed.