Chemical changes · GCSE Chemistry
Electrolysis of molten salts
GCSE Chemistry revision on electrolysis of molten salts: mobile ions, cathode reduction, anode oxidation, and half equations such as Pb²⁺ + 2e⁻ → Pb.
Melt the ionic compound so ions can move. Positive ions to the negative cathode (gain electrons, reduction). Negative ions to the positive anode (lose electrons, oxidation). OIL RIG.
The important bits
What you need to know
- 1
Electrolysis decomposes an ionic compound using a d.c. current. The electrolyte must contain mobile ions, so a molten salt works; a solid lattice does not.
- 2
The cathode is the negative electrode. Cations (positive ions) are attracted there and are reduced (gain electrons). Metal is produced from a molten binary salt.
- 3
The anode is the positive electrode. Anions (negative ions) are attracted there and are oxidised (lose electrons). The non-metal is produced, often as a gas or vapour.
- 4
Molten lead bromide: cathode Pb²⁺ + 2e⁻ → Pb(l); anode 2Br⁻ → Br₂(g) + 2e⁻. Brown bromine vapour; molten lead at the cathode. Inert electrodes (graphite or platinum) are used.
- 5
Molten sodium chloride: Na⁺ + e⁻ → Na at the cathode; 2Cl⁻ → Cl₂ + 2e⁻ at the anode. This is how sodium metal is manufactured (Downs cell), not from aqueous NaCl.
- 6
Half equations must balance atoms and charge. Two Br⁻ each lose one electron to make one Br₂ and two electrons on the right-hand side.
- 7
Oxidation is loss of electrons (anode). Reduction is gain of electrons (cathode). OIL RIG. The ionic compound is split into its elements.
- 8
Solid NaCl or PbBr₂ does not conduct: ions are locked in the giant lattice. Melt it, and electrolysis can begin. That is why we heat these salts.
Quotations worth analysing
Short evidence. Real method.
“Pb²⁺ + 2e⁻ → Pb 2Br⁻ → Br₂ + 2e⁻”
Electrons on the left at the cathode (reduction). Electrons on the right at the anode (oxidation). Lead is a metal; bromine is the non-metal vapour.
“OIL RIG: oxidation is loss, reduction is gain (of electrons).”
Apply it every time you write a half equation. If you cannot see electrons changing place, you have not yet described electrolysis.
“Na⁺ + e⁻ → Na 2Cl⁻ → Cl₂ + 2e⁻”
Products are sodium metal and chlorine gas. Aqueous sodium chloride does not give sodium metal — that is a different page. Molten means no water to compete.
Go deeper
Molten binary salts are the cleanest electrolysis questions
Only two ions exist, so the products are predictable: metal at the cathode, non-metal at the anode. Write the ions first, then send them to the electrodes, then write half equations. Lead bromide is the school classic because it melts at a manageable temperature and bromine is visible. Potassium iodide, zinc chloride and sodium chloride follow the same pattern. Inert electrodes mean the electrode material is not the reactant; graphite is cheap and conducts. If the anode were a reactive metal, it could oxidise instead of the anion — that is electroplating or refining, not this molten-salt story.
Go deeper
Balance charge as carefully as you balance atoms
Al³⁺ needs three electrons: Al³⁺ + 3e⁻ → Al. Oxide in molten Al₂O₃ (see extracting aluminium) is 2O²⁻ → O₂ + 4e⁻. Students write Al³⁺ + e⁻ → Al, which does not balance charge, or they put electrons on the wrong side. Check: left-hand charge should equal right-hand charge. For 2Br⁻ → Br₂ + 2e⁻, left is 2−, right is 0 + 2− (from two electrons), so 2− = 2−. If the numbers of electrons in the two half equations differ, multiply to match before you add them to a full equation. At GCSE, separate half equations usually score if they are correct on their own.
Go deeper
Why heat at all? Because structure controls conduction
This page sits next to ionic bonding on purpose. Solid ionic compounds contain ions but the ions cannot move, so a bulb in series stays dark. Molten, the lattice has broken and ions drift to opposite electrodes. That is also why giant covalent solids (except graphite) and simple molecules do not undergo electrolysis: no ions. Metals conduct, but that is delocalised electrons, and the metal is not decomposed into new elements. Electrolysis is specifically the decomposition of an ionic electrolyte. If a question shows a circuit with solid lead bromide and nothing happening, say the ions are not free to move, then say melt it.
See the idea in action
Molten zinc chloride is electrolysed with inert electrodes. Ions are Zn²⁺ and Cl⁻. At the cathode (negative), Zn²⁺ + 2e⁻ → Zn (reduction, zinc metal). At the anode (positive), 2Cl⁻ → Cl₂ + 2e⁻ (oxidation, chlorine gas). If the zinc chloride is solid, there is no electrolysis because ions cannot move. Do not predict hydrogen or oxygen: there is no water in a molten binary salt.
Exam technique
Turn knowledge into marks
Label cathode negative and anode positive, name the ion that moves to each, then write the half equation with electrons on the correct side. For molten binary compounds, products are the metal and the non-metal only.
Common mistakes
Do not give these marks away
- 01
Sending positive ions to the positive electrode, or putting electrons on the wrong side of a half equation.
- 02
Predicting hydrogen from molten NaCl because “sodium is reactive”.
- 03
Saying solid ionic compounds undergo electrolysis because they contain ions.
During electrolysis of molten lead bromide, what is produced at the cathode?
ABromine
BLead
CHydrogen
DOxygen
Show the answer
Lead. Pb²⁺ ions are positive so they move to the negative cathode and are reduced to lead. Bromine is produced at the anode.
Quick questions
If this is the bit you searched
What is electrolysis of a molten salt GCSE Chemistry?
Passing a d.c. current through a molten ionic compound so that it decomposes. The metal forms at the cathode and the non-metal at the anode.
Why must the salt be molten?
Ions must be free to move. In the solid giant lattice they are locked in place, so the solid does not conduct and electrolysis cannot occur.
What are the half equations for molten lead bromide?
Cathode: Pb²⁺ + 2e⁻ → Pb (reduction). Anode: 2Br⁻ → Br₂ + 2e⁻ (oxidation).
What does OIL RIG mean?
Oxidation is loss of electrons, reduction is gain of electrons. Oxidation happens at the anode; reduction happens at the cathode.