What is electrolysis?

The word means "splitting with electricity". Electrolysis makes reactions happen that wouldn't happen on their own, which is how aluminium is extracted from its ore, how chlorine and sodium hydroxide are made, how objects are electroplated, and how hydrogen can be produced from water.

How electrolysis works

• Electrolyte: an ionic compound that is melted or dissolved, so its ions can move. A solid ionic compound can't be electrolysed, because its ions are locked in place.
• Electrodes: two conducting rods, often graphite or platinum, dipped into the electrolyte and connected to a DC power supply.
• Cathode: the negative electrode. Positive ions (cations) are attracted to it.
• Anode: the positive electrode. Negative ions (anions) are attracted to it.

At the cathode, positive ions gain electrons: that is reduction. At the anode, negative ions lose electrons: that is oxidation. Two memory aids help: PANIC (Positive Anode, Negative Is Cathode) and OIL RIG (Oxidation Is Loss, Reduction Is Gain, of electrons). Our guide to oxidation numbers shows how to track these changes.

Electrolysis of a molten compound

With a molten compound, there are only two kinds of ion, so the products are simple: the metal forms at the cathode and the non-metal at the anode. Molten lead bromide, PbBr₂, is the classic example.

These are called half equations. The electrons balance: two are gained at the cathode for every two lost at the anode.

Electrolysis of aqueous solutions

In a solution, water also provides ions, a few hydrogen ions (H⁺) and hydroxide ions (OH⁻), so the ions compete. AQA's GCSE gives the rules:

• At the cathode: hydrogen forms if the metal is more reactive than hydrogen. Otherwise, the metal forms. So copper chloride solution gives copper, but sodium chloride solution gives hydrogen.
• At the anode: oxygen forms, unless the solution contains halide ions (chloride, bromide or iodide), in which case the halogen forms.

Electrolysis of brine

Electrolysing concentrated sodium chloride solution gives three useful products at once: hydrogen at the cathode, chlorine at the anode, and sodium hydroxide left in the solution. Chlorine is used to treat water and make plastics such as PVC, and sodium hydroxide is used to make soap, paper and many other chemicals.

Electrolysis of water

Pure water barely conducts, so a little acid or salt is added. Water splits into hydrogen at the cathode and oxygen at the anode, in a volume ratio of 2 to 1, matching the formula H₂O.

Powered by renewable electricity, this can make hydrogen fuel without carbon emissions, which is why the US Department of Energy describes electrolysis as a promising way to produce hydrogen.

Uses of electrolysis

• Extracting reactive metals: aluminium is extracted by electrolysing aluminium oxide dissolved in molten cryolite, which lowers the melting point and saves energy. Aluminium forms at the cathode and oxygen at the carbon anodes, which burn away and must be replaced.
• Electroplating: coating an object with a thin layer of metal, such as silver on cutlery or chromium on taps. The object is the cathode and the plating metal is the anode.
• Purifying copper: impure copper is the anode and pure copper builds up on the cathode, giving copper pure enough for electrical wiring.
• Making chemicals: chlorine, sodium hydroxide and hydrogen from brine.

Electrolysis calculations

In more advanced chemistry, the amount of product depends on how much charge flows. Charge is current × time, and one mole of electrons carries a charge of 96,485 coulombs, the Faraday constant (NIST).

Example: copper plating

A current of 2.0 A flows through copper(II) sulfate solution for 30 minutes. What mass of copper forms at the cathode?

1. Charge: Q = It = 2.0 × 30 × 60 = 3,600 C.
2. Moles of electrons: 3,600 ÷ 96,485 = 0.0373 mol.
3. Each Cu²⁺ needs 2 electrons (Cu²⁺ + 2e⁻ → Cu), so moles of copper = 0.0373 ÷ 2 = 0.0187 mol.
4. Mass: 0.0187 × 63.5 = 1.18 g of copper.

Doubling the current or the time doubles the mass. Our guide to the mole explains the moles-to-mass step.

Common mistakes

• Mixing up the electrodes. In electrolysis, the cathode is negative and the anode is positive.
• Saying solid ionic compounds can be electrolysed. Their ions can't move until they are melted or dissolved.
• Forgetting water's ions in aqueous solutions. Hydrogen or oxygen often forms instead of the metal or non-metal.
• Writing half equations with unbalanced electrons. Electrons lost at the anode must equal electrons gained at the cathode.
• Calling it oxidation at the cathode. Reduction happens at the cathode, oxidation at the anode.

Read next

Chemistry · Reactions and electrolysisElectrolysis diagram: labelled, with how to draw oneA labelled electrolysis diagram: power supply, anode, cathode and electrolyte, which way the ions move, how to draw it, and the products of common setups.