Why can a favored reaction appear not to happen?
You will be able to: Distinguish a thermodynamic driving force from a kinetic barrier.
Why can a favored reaction appear not to happen?
A fuel can remain in a closed container without visibly reacting even when its oxidation would be thermodynamically favored. A difficult reaction pathway can make the rate extremely small.
A useful starting point: How can formation data give a reaction’s free energy? →
Words and symbols before equations
- Activation barrier
- Energy barrier associated with a reaction pathway.
- Kinetic control
- A favored process occurs too slowly to observe appreciably under the conditions.
- Catalyst
- A substance that provides a faster pathway and is regenerated overall.
- Equilibrium
- State with no net driving force for the reaction, distinct from a kinetically trapped state.
What this picture assumes
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Schematic free-energy profile, endpoints 0 and −20 kJ/mol and forward barriers 80 or 40 kJ/mol. These are teaching values, not a measured mechanism. Horizontal reaction progress is not time.
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.
- Uncatalyzed schematic pathway: forward barrier 80 kJ/mol; reverse barrier 100 kJ/mol; endpoints differ by −20 kJ/mol. Changing the pathway does not change the endpoint driving force or K at fixed T.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Thermodynamics compares the starting and ending states under specified conditions. Kinetics concerns the pathway and how often particles cross its barrier.
A large barrier can make a process slow even with negative ΔG. No visible change is therefore insufficient evidence for equilibrium.
A catalyst can lower pathway barriers and speed the approach toward equilibrium. It does not change the endpoint free-energy difference or equilibrium constant at fixed temperature.
The explorer shows a schematic free-energy profile with reaction progress, not time, on the horizontal axis. Barrier heights are teaching values; no measured rate is inferred from the drawing alone.
| Question | Thermodynamics | Kinetics |
|---|---|---|
| What is predicted? | Favored direction under stated conditions | How quickly composition changes |
| Relevant quantity | Gibbs free energy change | Activation barrier and rate law |
| Catalyst at fixed T | Does not change ΔG° or K | Can increase approach rate |
A worked example, step by step
Two supplied pathways have the same product level 20 kJ/mol below reactants. Their forward barriers are 80 and 40 kJ/mol. Compare favorability and expected rate tendency.
- Both pathways connect the same endpoints, so their free-energy change is −20 kJ/mol.
- Both are thermodynamically favored under the stated conditions.
- The lower-barrier pathway can allow faster reaction if other kinetic factors are comparable.
- No exact reaction time follows from the barrier drawing; catalyst action does not change the endpoints.
Thermodynamically favored does not mean instant, and an unchanged sample is not necessarily at equilibrium.
Can a catalyst make an unfavorable equilibrium suddenly product-favored?
Compare with an explanation
No. It can accelerate approach but does not change K at fixed temperature.
Predict. Change one thing. Explain.
Toggle the catalyst while keeping endpoint levels fixed. Identify the changing barrier and unchanged ΔG. Explain why the horizontal axis cannot be read as elapsed seconds.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Uncatalyzed schematic pathway: forward barrier 80 kJ/mol; reverse barrier 100 kJ/mol; endpoints differ by −20 kJ/mol. Changing the pathway does not change the endpoint driving force or K at fixed T.
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Schematic free-energy profile, endpoints 0 and −20 kJ/mol and forward barriers 80 or 40 kJ/mol. These are teaching values, not a measured mechanism. Horizontal reaction progress is not time.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using energy and entropy contributions, electron and ion bookkeeping, or the stated cell reaction. 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 questionA sample has no visible change for a day despite a known negative ΔG under its conditions. Explain a possible cause, why equilibrium is not established by that observation, and a catalyst’s role.
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Compare with the answer and four-point rubric
- 1 point: A large barrier may make the reaction too slow to observe.
- 1 point: No visible change alone does not establish equilibrium.
- 1 point: A catalyst can provide a lower-barrier pathway.
- 1 point: It does not alter the endpoint ΔG or equilibrium constant at fixed T.
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 1What does a barrier influence?
Reaction rate and pathway access.
RECALL 2What does negative ΔG indicate?
Favored direction under the specified conditions.
RECALL 3Is reaction progress the same as time?
No.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Why can a favored reaction appear not to happen?
- Endpoint difference governs thermodynamic direction; pathway barriers influence rate.
- At fixed T, a catalyst does not change ΔG° or K.
Remember: Thermodynamically favored does not mean instant, and an unchanged sample is not necessarily at equilibrium.
Conditions: Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Schematic free-energy profile, endpoints 0 and −20 kJ/mol and forward barriers 80 or 40 kJ/mol. These are teaching values, not a measured mechanism. Horizontal reaction progress is not time.
Refresh Kid · AP Chemistry Unit 9 · Objectives 9.4.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 9.4, objective 9.4.A. CED effective Fall 2024 and June 2026 clarifications checked September 17, 2026. Unit 9: Thermodynamics and Electrochemistry, Topics 9.1–9.11. 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. Numerical thermodynamic examples state standard conditions, temperature, reaction scaling and unit conventions. Supplied data and schematic geometry are teaching models. Standard ΔG° describes standard-state favorability and relates to K; actual direction depends on composition. Thermodynamic favorability does not predict rate. Nonstandard cell potential is taught through Q, distance from equilibrium and qualitative Nernst reasoning; algorithmic substitution alone does not demonstrate the assessed understanding. Electrode positive/negative labeling is excluded from assessed scope. Oxidation at the anode and reduction at the cathode remain essential. Faraday calculations assume the stated current efficiency and electron stoichiometry. Rotatable particle models are schematic inventories, not measured molecular trajectories. Virtual models do not replace required supervised laboratory work.
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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