Electrolysis – GCSE Combined Science Chemistry

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Electrolysis uses electricity to split a compound apart. It sounds like a big idea, but it follows a few simple rules. We will take them one at a time.

It matters because electrolysis gives us aluminium for drinks cans and aircraft. It also purifies copper for wires and makes chlorine for clean water. Without it, many everyday things would be much harder to make.

By the end you will be able to name the parts of an electrolysis cell. You will predict what forms at each electrode. You will also write half equations and describe how aluminium and copper are made.

Course note: every Combined Science course has one required practical on electrolysis. The next lesson, Electrolysis of aqueous solutions, covers the practical, water, brine and copper electrodes in depth. Writing half equations is Higher tier work.

What electrolysis is

Electrolysis uses an electric current to break down an ionic compound. The compound must be molten or dissolved in water, so that its ions are free to move. A solid ionic compound cannot conduct, because its ions are stuck in place.

The liquid or solution that conducts is the electrolyte. The two rods that carry the current in are the electrodes. The positive electrode is the anode. The negative electrode is the cathode.

Think of magnets, where opposites attract. Positive ions (cations) move to the cathode and negative ions (anions) move to the anode. Inert electrodes, such as graphite, do not take part in the reaction.

Molten ionic compounds

In molten lead bromide the ions are free to move. Lead ions are positive, so they move to the cathode. They gain electrons and become lead atoms.

Bromide ions are negative, so they move to the anode. They lose electrons and become bromine. As a rule, metals form at the cathode and non-metals form at the anode.

The cathode reaction is Pb²⁺ + 2e⁻ → Pb. The anode reaction is 2Br⁻ → Br₂ + 2e⁻. Overall, PbBr₂ → Pb + Br₂. The bars show one product at each electrode.

Aqueous solutions

A solution also has hydrogen ions, H⁺, and hydroxide ions, OH⁻, from the water. At the cathode, a metal more reactive than hydrogen stays as ions, so hydrogen gas forms. A less reactive metal forms as the metal itself.

At the anode, a halide ion (chloride, bromide or iodide) makes the halogen. If there is no halide, oxygen forms.

Sodium chloride solution gives hydrogen at the cathode and chlorine at the anode. Copper sulfate solution with graphite electrodes gives copper at the cathode and oxygen at the anode. The blue colour fades.

Copper electrodes

If the electrodes are made of copper, the anode takes part in the reaction. The copper anode loses atoms as copper ions go into the solution: Cu → Cu²⁺ + 2e⁻. At the cathode, copper ions gain electrons and form copper atoms: Cu²⁺ + 2e⁻ → Cu.

The anode gets smaller and the cathode gets bigger. This is used to purify copper. Impure copper is the anode and pure copper is the cathode.

It is also used for electroplating, which means putting a thin layer of metal on an object. Think of a cheap ring given a thin silver coat.

Extracting aluminium

Aluminium is found as aluminium oxide. Its melting point is very high, over 2000 °C. So the oxide is dissolved in molten cryolite. This lowers the temperature needed to about 950 °C and saves energy. Think of grit on an icy road, which lets ice melt at a lower temperature.

Aluminium forms at the cathode and oxygen forms at the carbon anodes. The oxygen reacts with the hot carbon to make carbon dioxide. So the anodes burn away and have to be replaced.

In the cell, Al³⁺ ions gain electrons at the carbon cathode lining: Al³⁺ + 3e⁻ → Al. The molten aluminium sinks and is tapped off. Oxide ions lose electrons at the carbon anodes: 2O²⁻ → O₂ + 4e⁻. Aluminium is made this way because it is more reactive than carbon, so heating with carbon cannot remove its oxygen.

The process uses a very large amount of electricity.

Half equations

A half equation shows what happens at one electrode. At the cathode, ions gain electrons. This is reduction. Examples are 2H⁺ + 2e⁻ → H₂ and Al³⁺ + 3e⁻ → Al.

At the anode, ions lose electrons. This is oxidation. Examples are 2Cl⁻ → Cl₂ + 2e⁻ and 2O²⁻ → O₂ + 4e⁻. For oxygen from hydroxide ions, 4OH⁻ → O₂ + 2H₂O + 4e⁻.

Check that the atoms and the charges balance. A memory aid is OIL RIG: oxidation is loss of electrons and reduction is gain.

Recap

Electrolysis breaks down a molten or dissolved ionic compound. Positive ions go to the cathode and negative ions go to the anode.

Molten compound: a metal at the cathode and a non-metal at the anode.

Solution: hydrogen at the cathode if the metal is more reactive than hydrogen. A halogen at the anode if a halide is present, otherwise oxygen.

Aluminium oxide is dissolved in cryolite to save energy. Copper is purified with copper electrodes.

Reduction is at the cathode and oxidation is at the anode.