Why can two species change at different rates?
You will be able to: Relate species rates through balanced coefficients and define a normalized reaction rate.
Why can two species change at different rates?
For every two units of one reactant consumed, a reaction may form three units of a product. Their concentrations cannot change at the same numerical rate if the volume is fixed.
A useful starting point: Review what balanced coefficients count →
Words and symbols before equations
- Species rate
- Rate of disappearance or appearance of a named chemical species.
- Stoichiometric coefficient
- Number multiplying a formula in a balanced equation.
- Normalized reaction rate, r
- Species rate divided by its coefficient, with the sign chosen positive forward.
What this picture assumes
Constant-volume 2N₂O₅ → 4NO₂ + O₂. Species-rate ratios come from coefficients; no rate-law order is inferred.
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.
- Normalized r=0.01 M/s. Multiply by coefficients 2, 4 and 1 to recover the three positive species rates.
- 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 2A → 3B at constant volume, consuming 2 mol A forms 3 mol B. Therefore B appears 3/2 as fast as A disappears.
Define r = −(1/2)Δ[A]/Δt = (1/3)Δ[B]/Δt. Dividing by coefficients makes the reported reaction rate independent of the species monitored.
Do not confuse these coefficient factors with rate-law exponents. Coefficients relate consumption and formation; experiments establish overall concentration dependence.
The same relationships apply to instantaneous slopes. If volume changes substantially, concentration changes can also reflect expansion or dilution, so the simple constant-volume interpretation needs care.
A worked example, step by step
For 2N₂O₅ → 4NO₂ + O₂, N₂O₅ disappears at 0.020 M/s. Find normalized reaction rate and both product appearance rates.
- Divide the N₂O₅ disappearance rate by its coefficient: r = 0.020/2 = 0.010 M/s.
- NO₂ appearance rate = 4r = 0.040 M/s.
- O₂ appearance rate = r = 0.010 M/s.
- These rates refer to a constant-volume sample and are linked by stoichiometry, not an assumed rate-law order.
“Rate of disappearance of A” and “reaction rate” may differ by a coefficient. State which quantity you report.
Does the coefficient 2 prove the overall reaction is second order in N₂O₅?
Compare with an explanation
No. It determines the species-rate relationship, not the experimentally measured rate law.
Predict. Change one thing. Explain.
Change the N₂O₅ disappearance rate. Predict NO₂ and O₂ appearance rates before comparing the three bars; identify the common normalized rate.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Normalized r=0.01 M/s. Multiply by coefficients 2, 4 and 1 to recover the three positive species rates.
Constant-volume 2N₂O₅ → 4NO₂ + O₂. Species-rate ratios come from coefficients; no rate-law order is inferred.
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 N₂ + 3H₂ → 2NH₃ at constant volume, H₂ disappears at 0.090 M/s. Calculate r, N₂ disappearance and NH₃ appearance, and distinguish rate ratios from rate-law powers.
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Compare with the answer and four-point rubric
- 1 point: r = 0.090/3 = 0.030 M/s.
- 1 point: N₂ disappears at 0.030 M/s.
- 1 point: NH₃ appears at 2r = 0.060 M/s.
- 1 point: Coefficients determine these ratios; they do not alone determine overall rate-law exponents.
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 1Why divide by coefficients?
To define one normalized rate for the reaction.
RECALL 2Which sign is used for consumed reactants?
A minus sign on their concentration-change slope.
RECALL 3What condition supports simple concentration-rate ratios?
A constant-volume reaction mixture.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Why can two species change at different rates?
- For aA → bB: r = −(1/a)Δ[A]/Δt = (1/b)Δ[B]/Δt.
- At fixed volume, species-rate ratios equal coefficient ratios.
Remember: “Rate of disappearance of A” and “reaction rate” may differ by a coefficient. State which quantity you report.
Conditions: Constant-volume 2N₂O₅ → 4NO₂ + O₂. Species-rate ratios come from coefficients; no rate-law order is inferred.
Refresh Kid · AP Chemistry Unit 5 · Objectives 5.1.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 5.1, objective 5.1.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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