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Oxidation: the loss of electrons or the gain of oxygen

Reduction: the gain of electrons or the loss of oxygen

Example: The electrolysis of lead (II) bromide, PbBr₂

At the cathode (negative electrode):   Pb²⁺ (l) + 2e^–  →  Pb (l)         (reduction)

At the anode (positive electrode):       2Br^– (l)      →  Br₂ (g) + 2e^–       (oxidation)

Example: The electrolysis of aluminium oxide, Al₂O₃

At the cathode:   Al³⁺ + 3e^–    →    Al         (reduction)

At the anode:      2O^2-    →    O₂ + 4e^–       (oxidation)

Example: The electrolysis of sodium chloride solution (NaCl (aq))

At the cathode:   2H⁺ (aq) + 2e^–  →  H₂ (g)       (reduction)

At the anode:      2Cl^– (aq)    →  Cl₂ (g) + 2e^–       (oxidation)

Example: The electrolysis of copper sulfate solution (CuSO₄ (aq))

At the cathode:   Cu²⁺ (aq) + 2e^–  →  Cu (s)      (reduction)

At the anode:      4OH^– (aq)     →  O₂ (g) + 2H₂O (l) + 4e^–       (oxidation)