Electrolysis

Electrolysis of molten binary ionic compounds

• A binary ionic compound contains just two elements joined by ionic bonding (NaCl, PbBr₂, MgO)
• When melted, all ions become free to move, so the molten compound is an electrolyte
• During electrolysis of any molten binary compound, the products are simply the elements of the compound
  • The metal cation is reduced at the cathode → the metal is deposited there
  • The non-metal anion is oxidised at the anode → the non-metal is released there

More molten examples

• Molten potassium iodide, KI: potassium metal at the cathode, purple iodine vapour at the anode
• Molten magnesium oxide, MgO: magnesium metal at the cathode, colourless oxygen gas at the anode
• Molten aluminium oxide, Al₂O₃ (dissolved in cryolite to lower the melting point): aluminium at the cathode, oxygen at the anode — the basis of industrial aluminium extraction

Electrolysis of aqueous solutions

• An aqueous solution contains the dissolved compound's ions plus ions from water
  • Water self-ionises slightly: H₂O ⇌ H⁺ + OH⁻
  • The cell contains four kinds of ion (metal cation, non-metal anion, H⁺, OH⁻) so which ion is discharged at each electrode has to be worked out
• At the cathode (positive ion arrives), the choice is between the metal ion and H⁺
  • The less reactive of the two is discharged
  • If the metal is above hydrogen in the reactivity series, hydrogen gas is produced and the metal ion stays in solution
  • If the metal is below hydrogen (Cu, Ag), the metal is deposited
• At the anode (negative ion arrives), the choice is between the non-metal anion and OH⁻
  • If the solution contains a halide ion (Cl⁻, Br⁻, I⁻), the halide is discharged and the corresponding halogen (Cl₂, Br₂, I₂) is released
  • If no halide ion is present, oxygen is produced from water (sulfate, nitrate and similar polyatomic anions stay in solution)

Predicting products for the three required aqueous examples