Electrolysis

Chemistry · Chemical changes. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Chemistry topics

The idea

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

Positive ions move to the negative electrode, the cathode, and gain electrons. Negative ions move to the positive electrode, the anode, and lose electrons. In a molten compound of two elements, the metal forms at the cathode and the non-metal at the anode.

In a solution, water also gives H⁺ and OH⁻ ions, so hydrogen or oxygen may form instead. Aluminium is more reactive than carbon, so it has to be extracted by electrolysis.

Rules to remember

Cathode (negative): positive ions gain electrons (reduction), e.g. Pb²⁺ + 2e⁻ → Pb
Anode (positive): negative ions lose electrons (oxidation), e.g. 2Br⁻ → Br₂ + 2e⁻
Solution, cathode: hydrogen forms if the metal is more reactive than hydrogen; if not, the metal forms (copper, silver)
Solution, anode: a halogen forms if halide ions are present; if not, oxygen forms
Oxygen at the anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻
Aluminium oxide is dissolved in molten cryolite to lower the melting point and save energy
Aluminium: Al³⁺ + 3e⁻ → Al at the cathode; oxygen at the carbon anodes, which burn away to carbon dioxide and are replaced

Common mistake

Students say sodium forms when sodium chloride solution is electrolysed. Sodium is more reactive than hydrogen, so hydrogen forms at the cathode. Sodium forms only from the molten compound.

Worked example

Potassium bromide solution is electrolysed with inert electrodes. What forms at each electrode?

The solution contains K⁺ and Br⁻ ions from the salt, and H⁺ and OH⁻ ions from the water.
Cathode: K⁺ and H⁺ are attracted. Potassium is more reactive than hydrogen, so hydrogen forms: 2H⁺ + 2e⁻ → H₂.
Anode: Br⁻ and OH⁻ are attracted. Bromide is a halide ion, so bromine forms: 2Br⁻ → Br₂ + 2e⁻.

Answer: Hydrogen at the cathode and bromine at the anode.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Keep going

← Reactions of acids Exothermic and endothermic reactions →Take a Chemistry test