Electrochemistry

Electrolysis is the decomposition of an ionic compound (the electrolyte) using a direct electric current. The current is passed through the electrolyte via two electrodes:

• Anode — positive electrode. Anions (negative ions) move here. Oxidation occurs.
• Cathode — negative electrode. Cations (positive ions) move here. Reduction occurs.

Electrodes are usually inert (graphite or platinum) so they don't react with the electrolyte.

When an aqueous solution is electrolysed, water also provides H⁺ and OH⁻ ions, so there is competition between ions at each electrode.

At the cathode (which cation is reduced?):

• If the metal is less reactive than hydrogen → the metal ion is discharged (metal deposited).
• If the metal is more reactive than hydrogen → H⁺ is discharged (hydrogen gas produced).

At the anode (which anion is oxidised?):

• If a concentrated halide (Cl⁻, Br⁻, I⁻) is present → the halide ion is discharged (halogen gas).
• Otherwise → OH⁻ is discharged (oxygen gas produced).

Electrolysis of brine also leaves sodium hydroxide (NaOH) in solution.

Write the half-equation for each electrode separately, showing electrons gained or lost.

Cathode (reduction — electrons on the left):

• Cu²⁺ + 2e⁻ → Cu
• 2H⁺ + 2e⁻ → H₂
• Al³⁺ + 3e⁻ → Al

Anode (oxidation — electrons on the right):

• 2Cl⁻ → Cl₂ + 2e⁻
• 2H₂O → O₂ + 4H⁺ + 4e⁻
• 2O²⁻ → O₂ + 4e⁻

Used to coat an object with a thin layer of metal for corrosion protection, improved appearance, or reduced wear.

• Cathode — the object to be plated.
• Anode — the plating metal (it dissolves to replenish ions in solution).
• Electrolyte — a solution of a salt of the plating metal (e.g. silver nitrate for silver plating).

• Anode — impure copper (dissolves: Cu → Cu²⁺ + 2e⁻).
• Cathode — pure copper (gains mass: Cu²⁺ + 2e⁻ → Cu).
• Electrolyte — copper(II) sulfate solution.
• Insoluble impurities fall away as anode sludge and never reach the cathode.
• The concentration of CuSO₄ stays approximately constant because copper leaving the anode replenishes what is deposited at the cathode.

Aluminium is too reactive for carbon reduction, so it is extracted by electrolysis.

• Electrolyte — molten Al₂O₃ (alumina) dissolved in cryolite.
• Cryolite lowers the melting point of Al₂O₃ (from ~2050 °C to ~950 °C), greatly reducing energy costs.
• Cathode — aluminium ions reduced: Al³⁺ + 3e⁻ → Al (liquid aluminium sinks to the bottom).
• Anode — oxide ions oxidised: 2O²⁻ → O₂ + 4e⁻. The carbon anodes react with the oxygen and are slowly burned away, so they need regular replacement.

All three products of brine electrolysis have important industrial uses:

Chlorine and sodium hydroxide react together to form bleach (sodium hypochlorite, NaOCl).