How much metal can a measured charge deposit?
You will be able to: Use electron stoichiometry and molar mass to calculate electroplated mass.
How much metal can a measured charge deposit?
Copper ions need two electrons each to become copper atoms. The same passed charge therefore deposits fewer moles of copper than of a one-electron metal ion.
A useful starting point: How does current tell you how many electrons passed? →
Words and symbols before equations
- Electron requirement, z
- Electrons needed per ion in the specified half-reaction.
- Molar mass, M
- Grams per mole of the deposited substance.
- Current efficiency
- Fraction of charge that produces the desired product.
- Electroplating
- Reduction of dissolved ions to a metal layer on a conducting surface.
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. Enough reactant and the specified half-reaction are assumed. Aluminum is an idealized molten system, not aqueous aluminum plating. Competing reactions are represented only through a supplied efficiency; this is not a laboratory procedure.
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.
- Total charge=1920 C; 100% useful charge=1920 C. With z=2, metal amount=0.00995 mol and mass=0.6323 g. Enough reactant and the specified idealized reaction are assumed.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Write the reduction half-reaction first. For Cu²⁺+2e⁻→Cu, z=2, so product moles=q/(2F). Multiply by the metal’s molar mass to obtain grams.
For constant current and 100% efficiency, m=ItM/(zF). If only a fraction η of charge makes the desired product, use m=ηItM/(zF).
Compare species with the ratio M/z, not molar mass alone. A larger electron requirement can offset a larger molar mass for a fixed charge.
The model assumes enough dissolved metal ion and an electrode/source suitable for the specified reaction. It does not predict competing water reactions or provide a real electroplating procedure.
A worked example, step by step
Pass 1930 C through a supplied Cu²⁺ plating system with 100% current efficiency. Use M(Cu)=63.55 g/mol and F≈96500 C/mol.
- Write Cu²⁺+2e⁻→Cu, so z=2.
- Electron amount=1930/96500=0.0200 mol.
- Copper amount=0.0200/2=0.0100 mol.
- Mass=0.0100×63.55=0.6355 g, about 0.636 g.
Divide by the electron requirement before multiplying by molar mass. Charge alone does not specify a unique product mass.
If current doubles while time and efficiency stay fixed, what happens to deposited mass?
Compare with an explanation
It doubles as long as sufficient reactant remains and the assumed reaction continues.
Predict. Change one thing. Explain.
Compare Ag⁺, Cu²⁺ and Al³⁺ for the same passed charge under the stated idealized assumptions. Explain the mass trend using M/z; do not assume aqueous aluminum plating is practical.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Total charge=1920 C; 100% useful charge=1920 C. With z=2, metal amount=0.00995 mol and mass=0.6323 g. Enough reactant and the specified idealized reaction are assumed.
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Enough reactant and the specified half-reaction are assumed. Aluminum is an idealized molten system, not aqueous aluminum plating. Competing reactions are represented only through a supplied efficiency; this is not a laboratory procedure.
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 supplied Ag⁺ plating system receives 965 C at 100% efficiency. Using F=96500 C/mol and M(Ag)=107.9 g/mol, find the deposited mass and compare its electron requirement with Cu²⁺.
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Compare with the answer and four-point rubric
- 1 point: Ag⁺+e⁻→Ag requires z=1.
- 1 point: Electron and silver amounts are both 965/96500=0.0100 mol.
- 1 point: Mass=0.0100×107.9=1.079 g.
- 1 point: Cu²⁺ requires z=2, so the same charge would deposit half as many moles of Cu.
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 1Which coefficient determines electron consumption?
z in the balanced half-reaction.
RECALL 2What does η represent?
Fraction of charge used for the desired product.
RECALL 3What can invalidate unlimited mass growth?
Reactant depletion or changes in current efficiency/reaction.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
How much metal can a measured charge deposit?
- m=ηItM/(zF), with η=1 for ideal 100% efficiency.
- Use the balanced half-reaction to find z.
Remember: Divide by the electron requirement before multiplying by molar mass. Charge alone does not specify a unique product mass.
Conditions: Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Enough reactant and the specified half-reaction are assumed. Aluminum is an idealized molten system, not aqueous aluminum plating. Competing reactions are represented only through a supplied efficiency; this is not a laboratory procedure.
Refresh Kid · AP Chemistry Unit 9 · Objectives 9.11.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 9.11, objective 9.11.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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