6.2.5Additional lessons
Primary Cells — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 88 of 98
Part 1 of 6How a Voltaic Cell Works
A primary cell is a voltaic cell that cannot be recharged, because its reaction is not easily reversed.
Spontaneous redox
- A voltaic (galvanic) cell produces electricity from a spontaneous redox reaction.
Separate half-cells
- The oxidation and reduction occur in different half-cells, so electrons travel through an external circuit.
Anode and cathode
- Oxidation occurs at the anode (negative) and reduction at the cathode (positive).
Salt bridge
- A salt bridge with an inert electrolyte, such as KNO₃, completes the circuit by letting ions move.
Part 2 of 6The Zinc–Copper (Daniell) Cell
The Zinc–Copper (Daniell) Cell
Anode (−)
- Zn(s) → Zn²⁺(aq) + 2e⁻; the zinc electrode loses mass.
Cathode (+)
- Cu²⁺(aq) + 2e⁻ → Cu(s); copper is deposited.
Electron flow
- Electrons move from the zinc to the copper electrode through the wire.
Overall
- Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), giving a voltage of about 1.10 V.
| Feature | Zinc half-cell | Copper half-cell |
|---|---|---|
| Process | Oxidation | Reduction |
| Polarity | Negative | Positive |
| Electrode change | Mass decreases | Mass increases |
Part 3 of 6The Hydrogen–Oxygen Fuel Cell
1
A fuel cell supplies the fuel and oxygen continuously, so it keeps working as long as they are supplied.
2
At the negative electrode hydrogen is oxidised: in acid, H₂ → 2H⁺ + 2e⁻.
3
At the positive electrode oxygen is reduced: O₂ + 4H⁺ + 4e⁻ → 2H₂O.
4
The overall reaction 2H₂ + O₂ → 2H₂O gives water as the only product.
5
Hydrogen storage and its production from fossil fuels are the main limitations.
Part 4 of 6Components of the Cell
Components of the Cell
Electrodes
- Metal strips act as conductors that provide the surface where electrons are lost or gained.
Electrolytes
- Each half-cell contains a solution of the metal's own ions, such as ZnSO₄(aq) for the zinc half-cell.
Voltmeter
- A high-resistance voltmeter measures the potential difference without allowing a large current.
Cell notation
- The cell is written Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s), with the anode on the left and the salt bridge as ||.
Part 5 of 6Worked Example
Question
- A voltaic cell is made from magnesium in Mg²⁺(aq) and copper in Cu²⁺(aq).
- Write the half-equations, the direction of electron flow and the overall equation.
- Magnesium is more reactive, so it is oxidised at the anode: Mg(s) → Mg²⁺(aq) + 2e⁻.
- Copper(II) ions are reduced at the cathode: Cu²⁺(aq) + 2e⁻ → Cu(s).
- Electrons flow from Mg to Cu in the external circuit; add the equations to obtain the overall reaction.
Answer
- Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s);
- electrons flow from the magnesium electrode to the copper electrode.
Part 6 of 6Exam Checklist & Tips
6.2.5 – Key Points for the Exam
- Voltaic cell A cell that generates electricity from a spontaneous redox reaction.
- Anode The electrode where oxidation occurs.
- Salt bridge A link containing mobile ions that completes the circuit.
- Primary cell A non-rechargeable voltaic cell.
- Fuel cell A cell where fuel and oxidant are fed in continuously.
Common Mistakes
- Placing the anode as positive in a voltaic cell; it is negative in a voltaic cell.
- Showing electrons flowing through the salt bridge; only ions move there.
- Thinking that a salt bridge is optional; without it the circuit is incomplete.
- Treating a fuel cell as storing energy like a battery rather than converting a continuous supply.
Last updated:
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.