How can identical electrodes still produce a voltage?
You will be able to: Use concentration differences to identify electrode roles and electron direction.
How can identical electrodes still produce a voltage?
Two copper electrodes can form a cell even though both use the same metal. Different Cu²⁺ concentrations provide the imbalance that the cell can reduce.
A useful starting point: Why does a cell’s voltage change as its composition changes? →
Words and symbols before equations
- Concentration cell
- Cell with the same electrode couple on both sides but different activities or concentrations.
- Dilute half-cell
- Side with the lower dissolved metal-ion concentration.
- Concentrated half-cell
- Side with the higher metal-ion concentration.
- Standard difference
- E°cell=0 when identical standard reduction couples are subtracted.
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. Identical Cu/Cu²⁺ couples at 298 K, dilute concentration approximation, n=2 and E°cell=0. Potential is signed for electrons moving left→right. Junction potentials and resistance are omitted; ratio settings are independently supplied states.
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.
- Right/left Cu²⁺ ratio=100; identical standard potentials cancel to E°=0. Left oxidation makes ions; right reduction consumes ions; electrons move left→right. Signed E=0.059 V for the left→right convention.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
For Cu/Cu²⁺ with a dilute left solution and concentrated right solution, oxidation on the dilute side produces Cu²⁺, while reduction on the concentrated side consumes Cu²⁺. Both changes reduce the concentration difference.
The dilute side is therefore the anode and the concentrated side the cathode for this setup. Electrons travel externally from dilute-side electrode to concentrated-side electrode.
The identical standard potentials cancel, but nonstandard concentrations can give nonzero E. At equal activities, the concentration-cell driving force becomes zero.
The explorer changes supplied initial concentration ratios; it is not a closed-system time simulation. Its idealized potential omits liquid-junction effects and assumes compatible ionic conduction.
A worked example, step by step
A Cu/Cu²⁺ cell has 0.010 M Cu²⁺ on the left and 1.00 M on the right. Identify electrode roles and explain the favored change.
- Oxidation at the dilute left side creates Cu²⁺.
- Reduction at the concentrated right side consumes Cu²⁺.
- Left is anode, right is cathode, and external electrons move left to right.
- Both changes reduce the concentration difference; equal concentrations would remove this driving force.
Identical electrodes imply zero standard cell potential, not zero potential under every concentration condition.
What if the left side is now more concentrated?
Compare with an explanation
The right becomes the dilute anode, the left the cathode, and electron direction reverses.
Predict. Change one thing. Explain.
Move the right/left concentration ratio through values below, equal to and above one. Predict the electrode-role swap and electron direction before reading the labels.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Right/left Cu²⁺ ratio=100; identical standard potentials cancel to E°=0. Left oxidation makes ions; right reduction consumes ions; electrons move left→right. Signed E=0.059 V for the left→right convention.
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Identical Cu/Cu²⁺ couples at 298 K, dilute concentration approximation, n=2 and E°cell=0. Potential is signed for electrons moving left→right. Junction potentials and resistance are omitted; ratio settings are independently supplied states.
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 questionAn identical-metal concentration cell has a tenfold higher metal-ion concentration on the left than right. Identify the anode, cathode, electron direction and equal-concentration limit.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: The dilute right side oxidizes and is the anode.
- 1 point: The concentrated left side reduces and is the cathode.
- 1 point: External electrons travel right to left.
- 1 point: At equal activities, the ideal concentration-cell potential is zero.
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 supplies the driving force?
The concentration/activity difference.
RECALL 2Where are ions generated?
At the dilute anode in this metal/ion model.
RECALL 3What happens when concentrations match?
The ideal potential reaches zero.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How can identical electrodes still produce a voltage?
- For identical metal/ion couples, E°cell=0.
- Dilute side oxidizes and concentrated side reduces until the concentration driving force is removed.
Remember: Identical electrodes imply zero standard cell potential, not zero potential under every concentration condition.
Conditions: Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Identical Cu/Cu²⁺ couples at 298 K, dilute concentration approximation, n=2 and E°cell=0. Potential is signed for electrons moving left→right. Junction potentials and resistance are omitted; ratio settings are independently supplied states.
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.
Want to work through this with a tutor?
Bring your question about How can identical electrodes still produce a voltage? Your explanation and answers remain free to access.