Electrolysis

Electrolysis is the redox process that occurs when direct current passes through a solution or melt of an electrolyte: reduction at the cathode (–) and oxidation at the anode (+). In molten NaCl, Na⁺ + e⁻ → Na at the cathode and 2Cl⁻ – 2e⁻ → Cl₂ at the anode, i.e. 2NaCl → 2Na + Cl₂. In solution water also takes part. At the cathode, water rather than the cation is reduced for active metals up to aluminium (2H₂O + 2e⁻ → H₂ + 2OH⁻); metals after hydrogen (Cu, Ag, Au) are deposited; those in the middle (Zn, Fe, Ni, Pb) give both metal and hydrogen. At an inert anode (graphite, Pt) anions of oxygen-free acids (Cl⁻, Br⁻, I⁻, S²⁻) are oxidised, while for anions of oxygen acids (SO₄²⁻, NO₃⁻, PO₄³⁻) water is oxidised and O₂ is released; a soluble anode (Cu, Ni, Zn) dissolves itself. Examples: 2NaCl + 2H₂O → H₂ + Cl₂ + 2NaOH (table-salt solution gives hydrogen, chlorine and alkali); 2CuSO₄ + 2H₂O → 2Cu + O₂ + 2H₂SO₄; in Na₂SO₄ solution water is split: 2H₂O → 2H₂ + O₂. Electrolysis produces Na, Mg, Ca and Al, refines metals and plates objects with nickel, chromium or gold. Faraday's law: m = M·I·t / (n·F), where F = 96500 C/mol and n is the number of electrons. Electrolysis experiments are shown only by the teacher with a low-voltage source; chlorine is a poisonous gas.