How do experiments reveal a rate law?
You will be able to: Determine concentration exponents from controlled initial-rate comparisons.
How do experiments reveal a rate law?
Doubling one ingredient might double a reaction’s starting rate, quadruple it, or leave it unchanged. The experimental pattern tells us which power of concentration belongs in the rate law.
A useful starting point: Why can smaller pieces react faster? →
Words and symbols before equations
- Initial rate
- Rate measured near the start before concentrations change appreciably.
- Rate law
- An experimentally supported expression such as r = k[A]ᵐ[B]ⁿ.
- Order
- Exponent describing dependence on one concentration.
- Overall order
- Sum of the concentration exponents.
What this picture assumes
Illustrative empirical r = (1.0 M⁻² s⁻¹)[A]²[B] at fixed temperature and conditions. Compare to baseline [A]=0.10 M, [B]=0.20 M. Not inferred from overall coefficients.
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.
- r=(1.0 M⁻² s⁻¹)(0.1 M)²(0.2 M)=0.002 M/s. Relative to 0.0020 M/s baseline, factor=1. Overall order=3.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Compare experiments at the same temperature and conditions, changing one reactant concentration while holding the others fixed. The rate ratio then isolates that reactant’s effect.
If doubling [A] doubles r, the A order is 1; if it quadruples r, order is 2; if r stays constant, order is 0 over the tested conditions.
For r = k[A]ᵐ[B]ⁿ, use r₂/r₁ = ([A]₂/[A]₁)ᵐ when [B] is fixed. The overall balanced equation does not generally supply m or n.
A measured rate law applies to its experimental range and conditions. Noninteger orders are possible in general, although these introductory data use simple integer powers.
A worked example, step by step
At fixed temperature: experiment 1 has [A]=0.10 M, [B]=0.20 M, r=0.0020 M/s; experiment 2 doubles A and r becomes 0.0080; experiment 3 doubles B from experiment 1 and r becomes 0.0040. Find the law.
- Experiments 1→2 keep B fixed: A doubles while rate quadruples, so m=2.
- Experiments 1→3 keep A fixed: B doubles while rate doubles, so n=1.
- Write r = k[A]²[B], overall order 3.
- Check: doubling both predicts 2² × 2 = 8 times the original rate.
Do not compare two experiments as if only A changed when B changed too. Account for every changed factor.
If doubling A changes no rate, must A be absent from the overall equation?
Compare with an explanation
No. Zero order describes the measured dependence under those conditions, not whether A participates.
Predict. Change one thing. Explain.
Use the supplied model r = k[A]²[B]. Double A alone, then B alone, then both. Explain the rate ratios while temperature and k stay fixed.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
r=(1.0 M⁻² s⁻¹)(0.1 M)²(0.2 M)=0.002 M/s. Relative to 0.0020 M/s baseline, factor=1. Overall order=3.
Illustrative empirical r = (1.0 M⁻² s⁻¹)[A]²[B] at fixed temperature and conditions. Compare to baseline [A]=0.10 M, [B]=0.20 M. Not inferred from overall coefficients.
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 questionAt fixed T, doubling X with Y fixed doubles rate; tripling Y with X fixed leaves rate unchanged. Write the rate law and overall order, and predict the effect of doubling both.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: X is first order.
- 1 point: Y is zero order in the measured range.
- 1 point: r = k[X][Y]⁰ = k[X], overall order 1.
- 1 point: Doubling both doubles rate; the Y change contributes a factor of 1.
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 1Where do overall reaction orders come from?
Experimental rate behavior or a supported mechanism.
RECALL 2What is overall order?
The sum of concentration exponents.
RECALL 3What must stay fixed in an isolated comparison?
Other relevant concentrations, temperature and conditions.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How do experiments reveal a rate law?
- r = k[A]ᵐ[B]ⁿ; overall order = m+n.
- At fixed B and T: r₂/r₁ = ([A]₂/[A]₁)ᵐ.
Remember: Do not compare two experiments as if only A changed when B changed too. Account for every changed factor.
Conditions: Illustrative empirical r = (1.0 M⁻² s⁻¹)[A]²[B] at fixed temperature and conditions. Compare to baseline [A]=0.10 M, [B]=0.20 M. Not inferred from overall coefficients.
Refresh Kid · AP Chemistry Unit 5 · Objectives 5.2.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 5.2, objective 5.2.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.
Want to work through this with a tutor?
Bring your question about How do experiments reveal a rate law? Your explanation and answers remain free to access.