📘 IB Understanding
Electrolysis uses a direct current (dc) to decompose an ionic compound (molten or aqueous) into its elements. It is the reverse of a voltaic cell — energy is supplied to force a non-spontaneous reaction.
Electrolysis Cell
Voltaic Cell vs Electrolytic Cell
| Feature | Voltaic Cell | Electrolytic Cell |
|---|---|---|
| Reaction type | Spontaneous | Non-spontaneous |
| Energy conversion | Chemical → Electrical | Electrical → Chemical |
| Anode charge | Negative (−) | Positive (+) |
| Cathode charge | Positive (+) | Negative (−) |
| Salt bridge | Present | Not needed (one container) |
Predicting Products (Aqueous Solutions)
At the Cathode (−)
- If the metal is less reactive than hydrogen: metal deposited
- If more reactive: hydrogen gas produced (2H2O + 2e- → H2 + 2OH-)
At the Anode (+)
- If halide ions present (concentrated): halogen gas
- Otherwise: oxygen gas (2H2O → O2 + 4H+ + 4e-)
⚠️ Exam Tip
In electrolysis, AN OX and RED CAT still apply, but the anode is positive and cathode is negative — the opposite of a voltaic cell!