What is electrolysis?
Electrolysis means "splitting with electricity". We pass a direct current (DC) through a liquid that contains ions. This liquid is the electrolyte.
- An ionic compound only conducts when its ions can move: when it is molten (melted) or dissolved in water.
- Two rods called electrodes dip into it. The negative one is the cathode. The positive one is the anode.
- Positive ions (cations) are pulled to the cathode. Negative ions (anions) are pulled to the anode. Opposite charges attract.
- Electrodes are usually inert (they do not react), such as graphite or platinum.
In the wires, electrons carry the current. In the electrolyte, moving ions carry it.
Electrolysis of molten ionic compounds
A molten ionic compound has only two kinds of ion, so the products are easy to predict:
- The metal forms at the cathode.
- The non-metal forms at the anode.
Example: molten lead(II) bromide, PbBr₂, gives grey lead at the cathode and brown bromine vapour at the anode. Molten sodium chloride gives sodium and chlorine.
You must heat the solid until it melts first. A solid gives no current, because its ions are fixed in the lattice.
Half equations at the electrodes
A half equation shows what happens at one electrode, with electrons written as e⁻.
- Cathode (reduction, gain of electrons): Pb²⁺ + 2e⁻ → Pb
- Anode (oxidation, loss of electrons): 2Br⁻ → Br₂ + 2e⁻
More examples: Cu²⁺ + 2e⁻ → Cu · 2H⁺ + 2e⁻ → H₂ · 2Cl⁻ → Cl₂ + 2e⁻ · 4OH⁻ → O₂ + 2H₂O + 4e⁻ · Al³⁺ + 3e⁻ → Al · 2O²⁻ → O₂ + 4e⁻
Check: atoms balance and charges balance on both sides. Electrons can be shown taken away on the left instead (2Br⁻ − 2e⁻ → Br₂); both are accepted.
Electrolysis of aqueous solutions
Water splits a tiny bit into H⁺ and OH⁻ ions. So a solution has two cations and two anions competing.
At the cathode: hydrogen is made, unless the metal is less reactive than hydrogen (copper, silver, gold). Then the metal forms.
At the anode: oxygen is made, unless the solution has a halide ion (Cl⁻, Br⁻, I⁻). Then the halogen forms.
| Solution | Cathode | Anode |
|---|---|---|
| Sodium chloride (brine) | hydrogen | chlorine |
| Copper(II) sulfate | copper | oxygen |
| Dilute sulfuric acid | hydrogen | oxygen |
| Copper(II) chloride | copper | chlorine |
In brine, Na⁺ and OH⁻ stay behind, so the solution becomes sodium hydroxide, a useful alkali.
Using electrolysis to extract metals
Metals more reactive than carbon (like aluminium, sodium, magnesium) cannot be got by heating their oxide with carbon. We use electrolysis, which needs a lot of electricity, so it is expensive.
- Aluminium oxide melts at over 2000 °C. It is dissolved in molten cryolite so it melts near 950 °C. This saves energy.
- Cathode: Al³⁺ + 3e⁻ → Al (liquid aluminium collects at the bottom).
- Anode: 2O²⁻ → O₂ + 4e⁻. The hot oxygen reacts with the carbon anode to make carbon dioxide, so the anodes must be replaced often.
Try it: a home electrolysis check (with an adult)
Put two pencil leads (graphite) in a cup of salty water and connect them to a 9 V battery using clip wires. Watch for bubbles on both leads. More bubbles form at the negative lead (hydrogen). Do it near an open window, for only a minute, and never use mains electricity. Then predict: what would you see with sugar water? (Nothing: sugar has no ions.)
Key formulas and definitions
- Cathode (−): cations gain electrons → reduction, e.g. Cu²⁺ + 2e⁻ → Cu
- Anode (+): anions lose electrons → oxidation, e.g. 2Cl⁻ → Cl₂ + 2e⁻
- Molten compound: metal at cathode, non-metal at anode
- Solution, cathode: H₂ unless metal is less reactive than hydrogen
- Solution, anode: O₂ unless a halide is present (then Cl₂, Br₂ or I₂)
- Water gives oxygen at the anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻
Worked examples
1. Predict the products when molten zinc chloride, ZnCl₂, is electrolysed.
Molten → only Zn²⁺ and Cl⁻. Cathode: zinc (Zn²⁺ + 2e⁻ → Zn). Anode: chlorine (2Cl⁻ → Cl₂ + 2e⁻).
2. Predict the products for aqueous potassium bromide, KBr(aq).
Cathode: potassium is more reactive than hydrogen, so hydrogen forms (2H⁺ + 2e⁻ → H₂). Anode: bromide is a halide, so bromine forms (2Br⁻ → Br₂ + 2e⁻). Potassium hydroxide is left in solution.
3. Predict the products for aqueous silver nitrate, AgNO₃(aq).
Cathode: silver is less reactive than hydrogen, so silver forms (Ag⁺ + e⁻ → Ag). Anode: nitrate is not a halide, so oxygen forms (4OH⁻ → O₂ + 2H₂O + 4e⁻).
4. Why must the carbon anodes in aluminium extraction be replaced, and why is cryolite used?
Oxygen made at the anode reacts with the hot carbon: C + O₂ → CO₂, so the anode wears away. Cryolite lowers the melting temperature from over 2000 °C to about 950 °C, saving energy and money.
Common mistakes
- Saying a solid ionic compound conducts. It only conducts when molten or dissolved, because the ions must move.
- Swapping cathode and anode. The cathode is negative and attracts positive ions (cations).
- Saying sodium forms at the cathode in salt water. Sodium is more reactive than hydrogen, so hydrogen forms.
- Writing electrons on the wrong side: reduction (cathode) has electrons on the left; oxidation (anode) has electrons on the right.