Is the energy barrier the same as the reaction energy change?
You will be able to: Read forward/reverse activation energies and overall energy change from a labeled profile.
Is the energy barrier the same as the reaction energy change?
Walking over a hill to a lower valley still requires an uphill climb first. A reaction can similarly release energy overall while facing an activation barrier.
A useful starting point: Why does warming help more particles cross the barrier? →
Words and symbols before equations
- Reaction coordinate
- A schematic measure of progress in bond rearrangement, not elapsed time.
- Transition state
- High-energy configuration at a pathway maximum.
- Activation energy, Ea
- Energy difference from starting state to the transition state.
- Overall energy change, ΔE
- Product energy minus reactant energy on this diagram.
What this picture assumes
Schematic potential-energy profile: reactants 10, transition state 70 kJ/mol; product energy adjustable. Reaction coordinate is not time. ΔE is the plotted energy difference, not a complete Gibbs-energy prediction.
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.
- Forward Ea=60 kJ/mol; reverse Ea=90 kJ/mol; ΔE=-30 kJ/mol. Moving product energy leaves the forward barrier unchanged.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Read the vertical energy levels of reactants, transition state and products. Forward Ea is E_TS−E_R; reverse Ea is E_TS−E_P.
Overall ΔE is E_P−E_R. A negative value means products lie lower on this energy scale; it is different from the height of the barrier.
An energy-releasing reaction can still be slow if few encounters reach the transition state. Rate and overall energy change answer different questions.
The horizontal axis is reaction coordinate, not time or distance traveled by a particular molecule. These schematic potential-energy levels are not automatically a complete Gibbs-energy or equilibrium analysis. Arrhenius calculations are outside the required AP scope.
A worked example, step by step
A profile has reactants at 10 kJ/mol, transition state at 70 kJ/mol and products at −20 kJ/mol. Find forward Ea, reverse Ea and ΔE.
- Forward Ea = 70−10 = 60 kJ/mol.
- Reverse Ea starts at products: 70−(−20) = 90 kJ/mol.
- ΔE = −20−10 = −30 kJ/mol.
- The products are lower overall, yet a 60 kJ/mol forward barrier still separates reactants from the transition state.
Do not measure both barriers from zero on the page. Each barrier starts from the appropriate starting state.
Can a reaction release energy overall yet be slow?
Compare with an explanation
Yes. An activation barrier can make reaching the transition state rare even when products lie lower.
Predict. Change one thing. Explain.
Keep reactant and transition-state energies fixed while moving product energy. Predict which barrier changes and which remains fixed.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Forward Ea=60 kJ/mol; reverse Ea=90 kJ/mol; ΔE=-30 kJ/mol. Moving product energy leaves the forward barrier unchanged.
Schematic potential-energy profile: reactants 10, transition state 70 kJ/mol; product energy adjustable. Reaction coordinate is not time. ΔE is the plotted energy difference, not a complete Gibbs-energy prediction.
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 E_R=20, E_TS=80 and E_P=40 kJ/mol, calculate both barriers and ΔE, then explain why ΔE alone does not establish the rate.
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Compare with the answer and four-point rubric
- 1 point: Forward Ea = 60 kJ/mol.
- 1 point: Reverse Ea = 40 kJ/mol.
- 1 point: ΔE = +20 kJ/mol.
- 1 point: Rate depends on pathway barriers and kinetic conditions, not solely endpoint energy difference.
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 forward Ea begin?
At reactant energy.
RECALL 2What does ΔE compare?
Product and reactant energy levels.
RECALL 3What does a profile’s width measure?
A schematic reaction coordinate, not necessarily time or physical distance.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Is the energy barrier the same as the reaction energy change?
- Ea,f = E_TS−E_R; Ea,r = E_TS−E_P.
- ΔE = E_P−E_R = Ea,f−Ea,r.
Remember: Do not measure both barriers from zero on the page. Each barrier starts from the appropriate starting state.
Conditions: Schematic potential-energy profile: reactants 10, transition state 70 kJ/mol; product energy adjustable. Reaction coordinate is not time. ΔE is the plotted energy difference, not a complete Gibbs-energy prediction.
Refresh Kid · AP Chemistry Unit 5 · Objectives 5.6.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 5.6, objective 5.6.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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