Electrochemistry

Redox and oxidation numbers

Oxidation number rules: elements 0; monatomic ions = charge; O usually −2 (−1 in peroxides); H +1 (−1 in metal hydrides); F always −1; sum = overall charge. Oxidation = increase in oxidation number (loss of electrons); reduction = decrease. Half-equations in acid: balance atoms other than O and H, add H₂O for O, H⁺ for H, then electrons for charge. Combine by matching electrons. Disproportionation: one element is both oxidised and reduced. Redox titrations: $\mathrm{MnO_4^-}$ (self-indicating, purple → colourless) and iodine–thiosulfate (starch indicator).