Introduction to Electrolysis
Electrolysis uses electricity to break down an ionic compound into its elements. It is how we extract the most reactive metals (like aluminium), and how we make chlorine and hydrogen from salt water.
Recall which metals this matters for: the ones more reactive than carbon — at the top of the reactivity series — cannot be extracted by reduction with carbon, so electrolysis is the only way to get them.
The reactivity series, regrouped by extraction method. Metals above carbon can only be extracted by electrolysis — that is what this topic is about.
An electrolyte is an ionic compound that is molten or dissolved in water, so its ions are free to move and carry charge. Two electrodes are dipped into it and connected to a power supply:
- the cathode is the negative electrode;
- the anode is the positive electrode.
In a solid ionic compound the ions are locked in a lattice and cannot move, so it does not conduct. Only when molten or dissolved are the ions free to move and carry charge — so only then can it be electrolysed.
Once a current flows, the ions move towards the oppositely-charged electrode, because opposite charges attract:
- positive ions (cations) move to the negative cathode;
- negative ions (anions) move to the positive anode.
At the electrodes the ions are discharged — turned back into neutral atoms — producing the elements.
Inside the electrolyte, charge is carried by moving ions — not by electrons travelling through the liquid. Electrons only flow through the wires and electrodes. Also, the ions are already present in the molten or dissolved compound; electrolysis does not create them, it discharges them at the electrodes.
The ions are free to move in the molten or dissolved electrolyte.
🧪 Exam-style questions
Explain why solid sodium chloride does not conduct electricity, but molten sodium chloride does.
Show answer
- In the solid, the ions are locked in the lattice and cannot move, so there are no free charged particles to carry the charge. 1 mark
- When molten, the ions are free to move and can carry the charge, so it conducts. 1 mark
It is the moving ions that carry the charge through the liquid — the same is true when the compound is dissolved in water.
During electrolysis, which ions move towards the cathode? Tick (✓) one box.
What is an electrolyte? Tick (✓) one box.
Changes at the Electrodes
The discharge of ions and the products are for everyone; the half equations are Higher Tier only.
The simplest case is a molten (liquid) ionic compound (one made of just two elements) — for example molten lead bromide or zinc chloride. The rule is simple:
- The metal is produced at the cathode (negative electrode) — you see solid metal deposits.
- The non-metal is produced at the anode (positive electrode).
This is a redox process. Remember OIL RIG — oxidation is loss of electrons, reduction is gain:
- At the cathode, positive metal ions gain electrons — this is reduction.
- At the anode, negative ions lose electrons — this is oxidation.
Half equations Higher
A half equation shows what happens to one ion, including the electrons (e−). For molten lead bromide, PbBr2:
Pb2+ + 2e− → Pb (cathode — reduction)
2Br− → Br2 + 2e− (anode — oxidation)
Notice the bromine forms a diatomic molecule (Br2) — the halogens always do. The number of electrons lost at the anode must equal the number gained at the cathode.
Make sure the charges balance. On the left of 2Br− the total charge is 2−; adding 2e− to the right gives a matching 2− there too, so the equation balances for both atoms and charge.
🧪 Build a half equation — molten electrolysis
Pick a molten compound and an electrode, then build that electrode’s half equation: choose which side the ion and the product go on, set how many of each, and add electrons until the atoms and charges balance.
🧪 Exam-style questions
Molten zinc chloride is electrolysed. What is produced at the cathode?
Write the half equation for the reaction at the anode when molten zinc chloride is electrolysed, and state whether it is oxidation or reduction.
Show answer
- 2Cl− → Cl2 + 2e− 1 mark
- It is oxidation — the chloride ions lose electrons. 1 mark