Chemistry
Electrolysis
What is electrolysis
Electrolysis: a redox reaction (RR) that takes place at the electrodes when an electric current passes through an electrolyte solution or melt.
Reduction occurs at the cathode, oxidation at the anode.
| Electrode | Process | NaCl solution |
|---|---|---|
| Cathode (−) | reduction | H₂ + OH⁻ |
| Anode (+) | oxidation | Cl₂ |
Faraday's law: m = M·I·t / (n·F), where F = 96500 C/mol.
What is released at the cathode in a solution
- In a solution, water is reduced instead of Li–Al metals (from Li to Al): 2H₂O + 2e⁻ → H₂ + 2OH⁻.
- Less active metals such as Cu and Ag are fully deposited.
What is released at the anode in a solution
- Anions of oxygen-free acids (Cl⁻, Br⁻, I⁻, S²⁻) are oxidized.
- In a solution, SO₄²⁻ and NO₃⁻: water is oxidized. The same applies to F⁻: 2H₂O − 4e⁻ → O₂ + 4H⁺.
Examples
- NaCl solution: H₂ at the cathode, Cl₂ at the anode, NaOH in the solution.
- NaCl melt: Na at the cathode, Cl₂ at the anode.
- CuSO₄ solution: Cu at the cathode, O₂ at the anode (H₂SO₄ accumulates in the solution).
- Aluminium is obtained by electrolysis of an Al₂O₃ melt (in cryolite). Active metals (Na, K, Ca, Al) are obtained only by electrolysis.
- If a soluble (active) anode (copper, nickel) is used, the anode itself dissolves. This is the purification (refining) of metals.
- Uses: electroplating, electrotyping, obtaining metals, production of Cl₂ and NaOH.