When may coefficients become rate-law powers?
You will be able to: Write the event-rate law of a stated elementary step and distinguish it from an overall reaction.
When may coefficients become rate-law powers?
If one elementary event requires an A particle and a B particle to meet, opportunities depend on both populations. An overall equation may hide several steps, so it cannot be interpreted the same way automatically.
A useful starting point: Review experimental overall rate laws →
Words and symbols before equations
- Elementary step
- A single molecular event in a proposed mechanism.
- Molecularity
- Number of reacting particles in that elementary event.
- Unimolecular
- One reacting particle.
- Bimolecular
- Two reacting particles.
What this picture assumes
Event-rate constants use numerical k=0.10 with units s⁻¹ for A and M⁻¹ s⁻¹ for two-particle steps. For 2A the disappearance rate is twice the event rate. Elementary status is supplied.
Read the picture in three steps
- Read the species and labels first. Identify what each symbol and line represents. Read the units and fixed conditions before comparing quantities.
- A → products is supplied as elementary. r_event=0.05 M/s from k[A]; A disappearance=0.05 M/s. Unimolecular.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
For an elementary A → products step, event rate is k[A]. For elementary A+B → products, it is k[A][B]. For elementary 2A → products, event rate is k[A]².
The molecularity describes particles taking part in that one step. Simultaneous collisions of three or more particles are much less likely than two-particle encounters.
An overall equation such as 2A+B → products need not be an elementary three-body event. Its rate law must come from experiment or a supported mechanism.
For elementary 2A, A disappears twice as fast as events occur. If r=k_event[A]², then −d[A]/dt=2k_event[A]²; integrated formulas using k for disappearance absorb that factor into their k.
A worked example, step by step
The step NO(g)+O₃(g) → NO₂(g)+O₂(g) is specified as elementary. Write its event-rate law and predict the effect of doubling only [O₃].
- One NO and one O₃ participate, so the step is bimolecular.
- Write r=k[NO][O₃].
- At fixed [NO] and temperature, doubling [O₃] doubles r.
- This inference uses the supplied elementary-step statement; it would not follow solely from an arbitrary overall equation.
Read “elementary” carefully. Coefficients of an overall reaction are not generally rate-law exponents.
Is a balanced equation enough to establish that a step is elementary?
Compare with an explanation
No. Elementary status is part of the proposed mechanism and needs evidence, not just balance.
Predict. Change one thing. Explain.
Switch elementary A, A+B and 2A cases. Compare molecularity, event rate and A disappearance rate; identify when the factor of two is required.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
A → products is supplied as elementary. r_event=0.05 M/s from k[A]; A disappearance=0.05 M/s. Unimolecular.
Event-rate constants use numerical k=0.10 with units s⁻¹ for A and M⁻¹ s⁻¹ for two-particle steps. For 2A the disappearance rate is twice the event rate. Elementary status is supplied.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using concentration–time slopes, rate-law dependence, encounter geometry or the stated mechanism. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionFor a stated elementary 2X → Y step, write molecularity, event-rate law, X disappearance rate and the effect of doubling X at fixed temperature.
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Compare with the answer and four-point rubric
- 1 point: The step is bimolecular.
- 1 point: r_event=k_event[X]².
- 1 point: −d[X]/dt=2r_event=2k_event[X]².
- 1 point: Doubling X quadruples both rates while keeping their factor-of-two relationship.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1When can step coefficients give exponents?
For a specified elementary step.
RECALL 2What is molecularity?
The number of reacting particles in one elementary event.
RECALL 3Why are three-body elementary events uncommon?
Simultaneous suitable encounters of three particles are relatively unlikely.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
When may coefficients become rate-law powers?
- Elementary A+B: r=k[A][B]; elementary 2A: r=k_event[A]².
- For 2A consumed per event, disappearance rate=2r.
Remember: Read “elementary” carefully. Coefficients of an overall reaction are not generally rate-law exponents.
Conditions: Event-rate constants use numerical k=0.10 with units s⁻¹ for A and M⁻¹ s⁻¹ for two-particle steps. For 2A the disappearance rate is twice the event rate. Elementary status is supplied.
Refresh Kid · AP Chemistry Unit 5 · Objectives 5.4.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 5.4, objective 5.4.A. CED effective Fall 2024 and June 2026 clarifications checked September 16, 2026. Unit 5: Kinetics, Topics 5.1–5.11. Focused lesson names, examples, models and assessments are original Refresh Kid teaching materials, not additional official topics or official AP questions. Official corrections.
The model states its assumptions beside the diagram. Arrhenius calculations are not assessed in the current AP framework; temperature and activation energy are taught qualitatively here. Collection of intermediate-detection data is not assigned. Integrated rate laws explicitly use the monitored species’ disappearance constant, while event and normalized reaction rates are labeled separately. Pre-equilibrium models state their timescale assumptions and use free concentrations. Original illustrative data and geometry are not measured kinetics.
Teaching resources: The Organic Chemistry Tutor video titles/descriptions and topic coverage were checked for optional links; no claim is made to have watched every video. No creator scripts, examples, worksheets or artwork were copied. GitHub’s 3D website collection and its Three.js camera-control example informed the idea of controllable spatial inspection. Scientific diagrams, geometry and interactions here are original. The self-hosted Three.js runtime retains its MIT license. Camera rotation changes the view, not the chemistry.
Independent teacher review and observation of students remain pending. Implementation checks do not certify scientific accuracy, accessibility or learning effectiveness. This is a review edition.
Optional official resource: Released AP Chemistry questions and scoring guides. This archive contains questions across units; it is not an assignment of every question to this lesson.
The teaching sequence is informed by the IES learning guide; this exact implementation has not been evaluated with learners.
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