Electrolysis

Electrolysis uses electricity to split an ionic compound that is molten or dissolved in water (the electrolyte). 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 solutions, ions from the water compete, so the products depend on how reactive the metal is and which negative ions are there. Electrolysis is used to extract reactive metals such as aluminium, to make chlorine and sodium hydroxide, and to purify and plate metals.

Key facts

  • An electrolyte is a molten or dissolved ionic compound: its ions are free to move.
  • Positive ions go to the cathode (−) and are reduced; negative ions go to the anode (+) and are oxidised. Remember PANIC: Positive Anode, Negative Is Cathode.
  • A molten compound gives the metal at the cathode and the non-metal at the anode: molten lead(II) bromide gives lead and bromine.
  • In a solution, the cathode gives hydrogen unless the metal is less reactive than hydrogen (copper, silver). The anode gives chlorine, bromine or iodine if halide ions are there, otherwise oxygen.
  • Half equations: Pb²⁺ + 2e⁻ → Pb at the cathode; 2Br⁻ → Br2 + 2e⁻ at the anode; 4OH⁻ → O2 + 2H2O + 4e⁻ when oxygen forms.
  • Aluminium is extracted from aluminium oxide dissolved in molten cryolite (to lower the melting point). Oxygen made at the carbon anodes burns them away, so they are replaced.
  • Electrolysis of brine (sodium chloride solution) gives hydrogen, chlorine and sodium hydroxide.

Worked examples

Example 1

Molten lead(II) bromide is electrolysed. What forms at each electrode? Write the half equations.

  1. Molten lead(II) bromide contains Pb²⁺ and Br⁻ ions, free to move.
  2. Pb²⁺ ions go to the cathode and gain electrons: reduction. Pb²⁺ + 2e⁻ → Pb
  3. Br⁻ ions go to the anode and lose electrons: oxidation. Brown bromine vapour forms. 2Br⁻ → Br2 + 2e⁻

Answer: lead at the cathode, bromine at the anode

Example 2

Copper(II) sulfate solution is electrolysed with inert electrodes. What forms at each electrode?

  1. The solution contains Cu²⁺ and SO4²⁻ ions from the salt, and H⁺ and OH⁻ ions from the water.
  2. Cathode: copper is less reactive than hydrogen, so copper is deposited. Cu²⁺ + 2e⁻ → Cu
  3. Anode: there are no halide ions, so hydroxide ions are discharged and oxygen forms. 4OH⁻ → O2 + 2H2O + 4e⁻
  4. Hydrogen ions and sulfate ions stay behind, so the solution becomes sulfuric acid and its blue colour fades.

Answer: copper at the cathode, oxygen at the anode

Example 3

Why is aluminium extracted by electrolysis, and why is the aluminium oxide dissolved in cryolite?

  1. Aluminium is above carbon in the reactivity series, so carbon cannot reduce aluminium oxide. Electrolysis is needed.
  2. Aluminium oxide melts above 2000 °C. Dissolved in molten cryolite it works at about 950 °C, which saves a lot of energy.
  3. Aluminium forms at the cathode and oxygen at the anode. Al³⁺ + 3e⁻ → Al
  4. The oxygen reacts with the hot carbon anodes to make carbon dioxide, so the anodes must be replaced regularly.

Answer: aluminium is more reactive than carbon; cryolite lowers the melting point, saving energy