Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- A galvanic (voltaic) cell runs a thermodynamically favourable redox reaction (ΔG° < 0) and produces electrical energy.
- An electrolytic cell uses a power supply to drive a thermodynamically unfavourable reaction (ΔG° > 0), turning electrical energy into chemical energy.
- In both: oxidation at the anode, reduction at the cathode; electrons move through the external wire from anode to cathode.
- Labelling electrodes as positive or negative is not assessed. The signs swap between the two kinds of cell, so rely on anode = oxidation.
Key relationships
| Part or observation | What it tells you |
|---|---|
| Electrode | Conductor where electrons are lost (anode) or gained (cathode); may react, or be inert (graphite, platinum) |
| Half-cell solution | Holds the ions that react or form; lets ions move |
| Salt bridge (e.g. KNO₃) | Completes the circuit with moving ions; keeps each half-cell neutral; keeps solutions apart |
| Voltmeter / power supply | Measures the potential difference (galvanic) / pushes electrons to drive the reaction (electrolytic) |
| Electrode loses mass | A metal there is being oxidised → anode |
| Electrode gains mass | Metal ions are being reduced and deposited → cathode |
| Gas at an electrode | A half-reaction there makes a gas (e.g. Cl₂ at the anode from Cl⁻) |
| Electron count | Electrons lost at the anode = electrons gained at the cathode; use it to link mass changes |
Assumptions behind the numbers
- All the current goes into the half-reactions you wrote (no side reactions).
- The salt-bridge ions do not react with anything in the half-cells.
- Mass changes are of the pure metal deposited or dissolved.
Mistakes to avoid
- Electrons in the salt bridge. Only ions move there.
- Thinking the anode is always negative. True only for galvanic cells; not assessed.
- Reversing electron flow for electrolytic cells. It is still anode → cathode through the external circuit.
- Sending ions the wrong way. Anions → anode; cations → cathode.
- Forgetting to balance electrons before comparing moles at the two electrodes.
- Saying bigger electrodes raise the voltage. They only let the cell run longer.
Quick self-check
- In a galvanic cell, the zinc electrode gets thinner. Is zinc the anode or the cathode? (Anode: zinc is oxidised.)
- Which way do K⁺ ions in a KNO₃ salt bridge move? (Towards the cathode half-cell.)
- 0.0200 mol of electrons passes and Cu²⁺ is reduced at the cathode. What mass of copper forms? (0.0100 mol × 63.55 g mol⁻¹ = 0.636 g)
- What happens to the current if the salt bridge is removed? (It stops: charge builds up in each half-cell.)
Next: practice questions.