How can a favorable reaction help an unfavorable one?
You will be able to: Add coupled reactions and their free energies while identifying a shared intermediate.
How can a favorable reaction help an unfavorable one?
A useful product can be difficult to make alone. A chemically connected process can form it through a shared intermediate while another reaction supplies a larger favorable free-energy change.
A useful starting point: Which end of water faces a dissolved ion? →
Words and symbols before equations
- Coupling
- Mechanistic or energetic linkage allowing one process to drive another.
- Intermediate
- Species formed in one step and consumed in another.
- Net reaction
- Sum after canceling species that appear on both sides.
- Additive ΔG°
- Free-energy changes add when equations are combined consistently.
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. Hypothetical linked equations A+X→I and I→B+Y at one temperature. I is a shared bookkeeping intermediate, and the second ΔG° is negative. The diagram assumes a linkage; a negative sum does not establish a real mechanism.
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.
- Net ΔG°=-15 kJ/mol for the assumed linked steps. The combined standard-state process is favored. The energy sum does not establish an actual mechanism.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
If two reactions are linked, add their balanced equations and standard free-energy changes at the same temperature. The net process must have negative ΔG° for standard-state favorability.
A shared intermediate can connect the processes chemically. Merely placing two unrelated reactions in the same container does not guarantee productive coupling.
Reversing a step reverses its ΔG°; scaling a step scales its ΔG°. Apply the same operation to the equation and energy before summing.
Biological ATP-linked reactions are an example of coupling, but actual cellular free energies depend on concentrations and conditions. The explorer uses explicitly hypothetical energy values and a bookkeeping intermediate.
A worked example, step by step
Suppose linked steps A+X→I have ΔG°=+20 kJ/mol and I→B+Y have ΔG°=−35 kJ/mol. Find the net reaction and ΔG°.
- Add both equations.
- Cancel intermediate I, which is produced then consumed.
- Net: A+X→B+Y.
- ΔG°net=+20−35=−15 kJ/mol, favored under the stated standard conditions; an actual chemical linkage is still required.
A negative sum is necessary thermodynamic evidence, not proof that unrelated reactions will automatically couple.
If the favorable step contributes only −10 against +20, is the combined process standard-state favored?
Compare with an explanation
No. The net +10 kJ/mol is still unfavorable in the written direction.
Predict. Change one thing. Explain.
Keep the unfavorable step at +20 kJ/mol and vary the favorable step from −10 to −40. Identify the zero boundary and explain why the diagram explicitly includes an intermediate.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Net ΔG°=-15 kJ/mol for the assumed linked steps. The combined standard-state process is favored. The energy sum does not establish an actual mechanism.
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Hypothetical linked equations A+X→I and I→B+Y at one temperature. I is a shared bookkeeping intermediate, and the second ΔG° is negative. The diagram assumes a linkage; a negative sum does not establish a real mechanism.
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+X→I has ΔG°=+15 kJ/mol; I→B+Y has −25 kJ/mol. Give the net equation and free energy, then explain why a reaction mechanism matters.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: I cancels between the two steps.
- 1 point: Net equation is A+X→B+Y.
- 1 point: ΔG°net=−10 kJ/mol.
- 1 point: A real linkage must direct the favorable process into producing the desired outcome; adding unrelated equations does not establish that mechanism.
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 1How do coupled free-energy changes combine?
Add them with the same equation operations.
RECALL 2What happens to an intermediate in the net equation?
It cancels.
RECALL 3Is an energy sum a mechanism?
No.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How can a favorable reaction help an unfavorable one?
- For consistently combined reactions: ΔG°net=ΣΔG°steps.
- Cancel shared intermediates; preserve stoichiometry and conditions.
Remember: A negative sum is necessary thermodynamic evidence, not proof that unrelated reactions will automatically couple.
Conditions: Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Hypothetical linked equations A+X→I and I→B+Y at one temperature. I is a shared bookkeeping intermediate, and the second ΔG° is negative. The diagram assumes a linkage; a negative sum does not establish a real mechanism.
Refresh Kid · AP Chemistry Unit 9 · Objectives 9.7.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 9.7, objective 9.7.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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