6.2.5Additional lessons
Primary Cells — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 136 of 172
Part 1 of 4Voltaic Cell Design
A voltaic cell converts chemical energy into electrical energy using a spontaneous redox reaction.
Two half-cells
- Each half-cell has an electrode dipping in a solution of its own ions, and the two are linked together.
Anode
- Oxidation happens at the anode, which is the negative electrode of a voltaic cell.
Cathode
- Reduction happens at the cathode, which is the positive electrode of a voltaic cell.
Salt bridge
- A salt bridge completes the circuit by letting ions move and keeps each half-cell electrically neutral.
Part 2 of 4The Zinc-Copper Cell
1
Zinc is placed in zinc sulfate solution and copper in copper(II) sulfate solution, joined by a salt bridge and a wire.
2
At the zinc anode, zinc atoms are oxidised: Zn(s) → Zn²⁺(aq) + 2e⁻, so the zinc electrode loses mass.
3
Electrons flow through the external circuit from the zinc to the copper, producing a measurable current.
4
At the copper cathode, Cu²⁺(aq) + 2e⁻ → Cu(s), so copper is deposited and the blue solution fades.
5
The cell is written as Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s), with the anode on the left.
Part 3 of 4Primary Cells and Fuel Cells
Primary Cells and Fuel Cells
Primary cell
- A cell whose reaction is not easily reversed, so it is discarded once the reactants are used up.
Alkaline battery
- Zinc is oxidised at the negative electrode and manganese(IV) oxide is reduced at the positive electrode.
Fuel cell
- Reactants such as hydrogen and oxygen are supplied continuously, so the cell works as long as fuel is fed in.
Acidic H₂/O₂ cell
- Anode
- H₂ → 2H⁺ + 2e⁻;
- cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O, giving only water as product.
- H₂ → 2H⁺ + 2e⁻;
| Feature | Voltaic cell | Fuel cell |
|---|---|---|
| Reactants | Contained in the cell | Supplied continuously |
| Product | Builds up | Removed (H₂O) |
| Life | Ends when reactants are used | Continues while fuel is supplied |
Part 4 of 4Exam Checklist & Tips
6.2.5 – Key Points for the Exam
- Voltaic cell A cell in which a spontaneous redox reaction produces an electric current.
- Anode The electrode where oxidation occurs.
- Cathode The electrode where reduction occurs.
- Salt bridge A tube of ions linking two half-cells to complete the circuit.
- Primary cell A non-rechargeable cell whose reaction cannot be reversed.
Common Mistakes
- Stating that electrons flow through the salt bridge; only ions move through it.
- Saying the anode is positive in a voltaic cell; it is negative.
- Mixing up the direction of electron flow, which goes from anode to cathode in the wire.
- Forgetting that a fuel cell needs a continuous supply of reactants.
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.