How do you read several peaks and valleys?
You will be able to: Identify intermediates and transition states and measure each barrier from its own starting level.
How do you read several peaks and valleys?
A journey over two hills contains a valley between them. The higher summit is not necessarily the larger climb from the immediately preceding valley.
A useful starting point: Can a mechanism produce a fractional reaction order? →
Words and symbols before equations
- Intermediate minimum
- A local energy valley between elementary steps.
- Transition-state maximum
- A peak associated with an elementary-step barrier.
- Step activation energy
- Peak energy minus the energy of that step’s starting state.
What this picture assumes
Two-step schematic: R=0, TS₁=50, TS₂=65, P=−10 kJ/mol. Each forward barrier starts at its preceding minimum. Barrier comparisons alone do not determine observed rate without kinetic assumptions.
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.
- First forward barrier=50−0=50 kJ/mol. Second=45 kJ/mol, measured from I=20. Overall ΔE=−10 kJ/mol. Peaks: 2 transition states; internal valley: 1 intermediate.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
A two-step profile has two peaks and one intermediate valley. The valley is a species that can exist for a finite time; a transition state is the high-energy configuration at the peak.
For the first forward step, subtract reactant energy from peak 1. For the second, subtract intermediate energy from peak 2, not the original reactant level.
Overall ΔE depends only on product minus reactant energy. Intermediate energies and peak heights do not change that endpoint difference.
A higher absolute peak is not by itself enough to identify the slowest step. Local barriers, rate constants, populations and conditions matter; simple bottleneck comparisons require additional assumptions.
A worked example, step by step
A profile has R=0, TS₁=50, I=20, TS₂=65 and P=−10 kJ/mol. Find both forward barriers and ΔE.
- First barrier: 50−0 = 50 kJ/mol.
- Second barrier: 65−20 = 45 kJ/mol.
- Overall ΔE = −10−0 = −10 kJ/mol.
- Although TS₂ is the higher absolute peak, its local forward climb is smaller. The picture alone is not a complete kinetic determination.
Measure each barrier from the preceding minimum. Do not label an intermediate as a transition state.
Does the number of peaks generally identify the number of elementary steps in this schematic?
Compare with an explanation
Yes. Each displayed peak represents a transition state for one step, with intermediate valleys between.
Predict. Change one thing. Explain.
Change the intermediate energy while keeping both peak energies and endpoints fixed. Predict which local barrier changes and explain why overall ΔE stays constant.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
First forward barrier=50−0=50 kJ/mol. Second=45 kJ/mol, measured from I=20. Overall ΔE=−10 kJ/mol. Peaks: 2 transition states; internal valley: 1 intermediate.
Two-step schematic: R=0, TS₁=50, TS₂=65, P=−10 kJ/mol. Each forward barrier starts at its preceding minimum. Barrier comparisons alone do not determine observed rate without kinetic assumptions.
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 R=10, TS₁=60, I=25, TS₂=80 and P=−5 kJ/mol, find both forward barriers and ΔE, then distinguish peaks from the internal valley.
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Compare with the answer and four-point rubric
- 1 point: Ea,1=60−10=50 kJ/mol.
- 1 point: Ea,2=80−25=55 kJ/mol.
- 1 point: ΔE=−5−10=−15 kJ/mol.
- 1 point: The two peaks are transition states; I is the intermediate minimum.
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 does step 2’s forward barrier start?
At the intermediate minimum before it.
RECALL 2What sets overall energy change?
Only product and reactant energy levels.
RECALL 3Can peak height alone determine the observed rate?
No; kinetic conditions and the relevant starting populations also matter.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How do you read several peaks and valleys?
- Ea,1=E_TS1−E_R; Ea,2=E_TS2−E_I.
- Overall ΔE=E_P−E_R; peaks are transition states, internal valleys are intermediates.
Remember: Measure each barrier from the preceding minimum. Do not label an intermediate as a transition state.
Conditions: Two-step schematic: R=0, TS₁=50, TS₂=65, P=−10 kJ/mol. Each forward barrier starts at its preceding minimum. Barrier comparisons alone do not determine observed rate without kinetic assumptions.
Refresh Kid · AP Chemistry Unit 5 · Objectives 5.10.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 5.10, objective 5.10.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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