Chemical changes · GCSE Chemistry

Electrolysis

Molten and aqueous electrolysis, cathode and anode, half equations, aluminium extraction and the brine cell.

UNDERSTAND→RETRIEVE→REMEMBER
THE MEMORY HOOK
Positive ions go to the negative electrode and gain electrons (reduction). Negative ions go to the positive electrode and lose electrons (oxidation). OIL RIG: oxidation is loss, reduction is gain.

The important bits

What you need to know

  1. 1

    Electrolysis decomposes an ionic compound using electricity. The electrolyte must contain mobile ions, so it is molten or dissolved, not solid.

  2. 2

    The cathode is negative. Cations (positive ions) are reduced there. The anode is positive. Anions (negative ions) are oxidised there.

  3. 3

    In molten binary ionic compounds the products are predictable: the metal at the cathode, the non-metal at the anode. Molten lead bromide gives lead and bromine.

  4. 4

    In aqueous solutions, hydrogen is produced at the cathode if the metal is more reactive than hydrogen. Oxygen is produced at the anode unless a halide is present in reasonable concentration, in which case the halogen is produced.

  5. 5

    Aluminium is extracted by electrolysis of aluminium oxide dissolved in molten cryolite, which lowers the melting point and saves energy. Carbon anodes react with oxygen and must be replaced.

  6. 6

    Electrolysis of brine (concentrated sodium chloride solution) produces hydrogen at the cathode, chlorine at the anode, and sodium hydroxide in solution.

  7. 7

    Higher tier half equations show electrons. Sodium: Na⁺ + e⁻ → Na. Bromide: 2Br⁻ → Br₂ + 2e⁻. Balance atoms and charge.

Go deeper

Molten is simpler than aqueous — start there

When only two ions exist, the metal ion goes to the cathode and the non-metal ion to the anode. Write the half equations with electrons on the correct side. Reduction at the cathode adds electrons. Oxidation at the anode removes them. Aqueous solutions add H⁺ and OH⁻ from water, which compete. A reactive metal such as sodium stays in solution as ions; hydrogen is discharged at the cathode instead. At the anode, halide ions beat OH⁻ if they are concentrated, so chlorine, bromine or iodine appears. If the only negative ions that can be discharged are OH⁻, you get oxygen. Learn that decision tree; it is the whole aqueous topic.

Go deeper

Aluminium and brine are applied electrolysis

Aluminium oxide has a very high melting point. Dissolving it in cryolite lets the process run at a lower temperature, which is an energy and cost point. Carbon anodes are oxidised by the oxygen that forms, making carbon dioxide, so they wear away — a standard evaluation idea. Brine is concentrated sodium chloride solution, not molten salt. Products are chlorine (anode), hydrogen (cathode) and sodium hydroxide left in the cell. All three are valuable industrial chemicals. If a question asks why solid sodium chloride does not conduct, the ions are not free to move. Melt it or dissolve it, and electrolysis can begin.

WORKED EXAMPLE

See the idea in action

Molten lead bromide is electrolysed with inert electrodes. At the cathode, Pb²⁺ + 2e⁻ → Pb (reduction, molten lead forms). At the anode, 2Br⁻ → Br₂ + 2e⁻ (oxidation, brown bromine vapour). The electrolyte must be molten so ions can move. If the same compound is solid, nothing happens because the ions are locked in the lattice.

Exam technique

Turn knowledge into marks

Label cathode negative and anode positive every time. Then say which ion moves where, and whether electrons are gained or lost. For aqueous solutions, apply the reactivity and halide rules before you write products. Balance half equations for atoms and for charge.

Common mistakes

Do not give these marks away

  1. 01

    Sending positive ions to the positive electrode, or mixing up oxidation and reduction.

  2. 02

    Predicting sodium metal at the cathode from aqueous sodium chloride — hydrogen is discharged instead.

  3. 03

    Saying solid ionic compounds undergo electrolysis because they contain ions.

QUICK RETRIEVAL

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 the difference between the cathode and the anode?

The cathode is the negative electrode, where reduction happens. The anode is the positive electrode, where oxidation happens.

Why is cryolite used in aluminium extraction?

It dissolves aluminium oxide and lowers the melting point, reducing the energy needed to keep the electrolyte molten.