Why does a cell’s voltage change as its composition changes?
You will be able to: Explain the sign of the Nernst correction and the approach to equilibrium.
Why does a cell’s voltage change as its composition changes?
A cell’s concentration changes as reactants are consumed and products form. Its measured driving voltage need not stay at the standard table value.
A useful starting point: How much free energy corresponds to a cell voltage? →
Words and symbols before equations
- Q
- Current reaction quotient, omitting pure solids and liquids.
- E
- Cell potential for the stated current conditions.
- E°
- Standard cell potential, with Q=1.
- Equilibrium condition
- Q=K, where the reversible cell potential is zero.
- Nernst relation
- E=E°−(RT/nF)ln Q.
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 cell: log₁₀K=4, T=298 K and n=2. E°=RT ln(K)/(nF) is chosen consistently. Graph is reversible potential for the written reaction, not loaded terminal voltage; beyond Q=K the favored direction reverses.
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.
- Q=10^0, K=10⁴ and E°=0.118 V. Q<K: the written reaction is forward-favored. E=0.118 V supports qualitative sign reasoning.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
At fixed T and reaction convention, compare Q with one: Q<1 makes ln Q negative, so E exceeds E°. Q>1 makes the correction subtract from E°. This is sign reasoning, not simply a calculator exercise.
For a forward-favored standard reaction with K>1, moving Q from 1 toward K reduces its positive driving potential. At Q=K, E=0. Beyond K, E becomes negative for the written direction; the reverse direction is favored.
A working cell is not an equilibrium system, so use the quotient, driving-force relation and direction toward equilibrium rather than claiming Le Châtelier’s principle directly applies to a running cell.
The explorer supplies a hypothetical cell with log₁₀K=4 at 298 K and n=2, setting E° consistently. It plots reversible potential, not the terminal voltage under load; resistance, current and overpotentials are not included.
A worked example, step by step
For Zn+Cu²⁺→Zn²⁺+Cu, write Q and predict how increasing [Zn²⁺] at fixed [Cu²⁺] and T affects the written cell potential.
- Pure solids are omitted, so Q≈[Zn²⁺]/[Cu²⁺] using normalized concentrations.
- Increasing the numerator increases Q.
- ln Q increases, so the subtracted Nernst correction becomes larger.
- The written E decreases; whether the direction remains forward depends on Q relative to K.
E=0 at Q=K, not necessarily at Q=1. Standard state and equilibrium are different conditions.
Must a larger Q always mean a smaller magnitude of E?
Compare with an explanation
No. Once Q exceeds K, E is negative and its magnitude can grow as the reverse driving force grows.
Predict. Change one thing. Explain.
For the supplied cell, inspect log₁₀Q=0, 2, 4 and 6. Explain the relation to standard conditions, decreasing forward driving force, equilibrium and reversal.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Q=10^0, K=10⁴ and E°=0.118 V. Q<K: the written reaction is forward-favored. E=0.118 V supports qualitative sign reasoning.
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Hypothetical cell: log₁₀K=4, T=298 K and n=2. E°=RT ln(K)/(nF) is chosen consistently. Graph is reversible potential for the written reaction, not loaded terminal voltage; beyond Q=K the favored direction reverses.
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 hypothetical cell has K=10⁴. Compare Q=1, Q=10², Q=10⁴ and Q=10⁶ qualitatively, holding the written reaction and temperature fixed.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: Q=1 gives the standard potential, positive because K>1.
- 1 point: Q=10² is closer to equilibrium and has a smaller positive E.
- 1 point: Q=10⁴ gives E=0.
- 1 point: Q=10⁶ gives negative E for the written reaction, favoring the reverse direction.
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 Q=1 represent?
The standard quotient condition.
RECALL 2What does Q=K represent?
Equilibrium and zero reversible potential.
RECALL 3What does negative E mean for the written reaction?
The reverse direction is favored under those conditions.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Why does a cell’s voltage change as its composition changes?
- E=E°−(RT/nF)ln Q; use the sign of ln Q qualitatively.
- Q=1 gives E=E°; Q=K gives E=0.
Remember: E=0 at Q=K, not necessarily at Q=1. Standard state and equilibrium are different conditions.
Conditions: Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Hypothetical cell: log₁₀K=4, T=298 K and n=2. E°=RT ln(K)/(nF) is chosen consistently. Graph is reversible potential for the written reaction, not loaded terminal voltage; beyond Q=K the favored direction reverses.
Refresh Kid · AP Chemistry Unit 9 · Objectives 9.10.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 9.10, objective 9.10.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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