Sulphuric acid

Preparation — the Contact process: sulphuric acid can be manufactured by the lead-chamber process, the cascade process or the contact process; the contact process, described here, is the modern industrial method. It proceeds in three steps: (i) sulphur dioxide is first generated by burning sulphur, or by roasting iron pyrites, in oxygen/air (S + O2 -> SO2; 4FeS2 + 11O2 -> 2Fe2O3 + 8SO2); (ii) the sulphur dioxide is then catalytically oxidised to sulphur trioxide by air, using a catalyst such as vanadium pentoxide (V2O5) or platinised asbestos; (iii) the sulphur trioxide is absorbed into concentrated sulphuric acid to give oleum (fuming sulphuric acid, H2S2O7), which is then diluted carefully with water to give sulphuric acid (SO3 + H2SO4 -> H2S2O7; H2S2O7 + H2O -> 2H2SO4). To maximise yield, the plant is typically operated at about 2 bar pressure and 720 K, and the sulphuric acid obtained by this process is over 96% pure.

Physical properties: pure sulphuric acid is a colourless, viscous liquid (density 1.84 g/mL at 298 K); its high boiling point and viscosity arise from extensive intermolecular hydrogen bonding. It freezes at 283.4 K and boils at 590 K, is highly soluble in water, and, owing to its strong affinity for water, functions as a powerful dehydrating agent; when dissolved in water it forms both a monohydrate (H2SO4.H2O) and a dihydrate (H2SO4.2H2O), and the dissolution process is strongly exothermic. This dehydrating power is well illustrated by its action on organic compounds such as sucrose (table sugar), oxalic acid and formic acid, from which it chemically removes the elements of water, charring sucrose to carbon (C12H22O11 + H2SO4 -> 12C + H2SO4.11H2O), converting formic acid to carbon monoxide (HCOOH + H2SO4 -> CO + H2SO4.H2O), and converting oxalic acid to a mixture of carbon monoxide and carbon dioxide ((COOH)2 + H2SO4 -> CO + CO2 + H2SO4.H2O).

Chemical properties: sulphuric acid is highly reactive, capable of behaving both as a strong acid and as an oxidising agent. It is thermally stable under ordinary conditions but decomposes at very high temperature to give water and sulphur trioxide (H2SO4 -> H2O + SO3). Being a strong dibasic acid, it forms two series of salts, the normal sulphates and the acid bisulphates, depending on the amount of base used (H2SO4 + NaOH -> NaHSO4 + H2O; H2SO4 + 2NaOH -> Na2SO4 + 2H2O; H2SO4 + 2NH3 -> (NH4)2SO4). As an oxidising agent, hot concentrated sulphuric acid decomposes to liberate nascent oxygen (H2SO4 -> H2O + SO2 + [O]), which is responsible for its oxidising action on elements such as carbon, sulphur and phosphorus (C + 2H2SO4 -> CO2 + 2SO2 + 2H2O; S + 2H2SO4 -> 3SO2 + 2H2O; P4 + 20H2SO4 -> 4H3PO4 + 20SO2 + 8H2O) and on hydrogen sulphide (H2S + H2SO4 -> S + SO2 + 2H2O); it similarly oxidises bromide and iodide ions to bromine and iodine (H2SO4 + 2HBr -> SO2 + 2H2O + Br2; H2SO4 + 2HI -> SO2 + 2H2O + I2). With metals its reactivity depends on both the metal and the acid's concentration: dilute sulphuric acid reacts with tin, aluminium and zinc to give the corresponding sulphates and liberate hydrogen gas (Zn + H2SO4 -> ZnSO4 + H2; 2Al + 3H2SO4 -> Al2(SO4)3 + 3H2), whereas hot concentrated sulphuric acid instead oxidises copper and lead to their sulphates while itself being reduced to sulphur dioxide, with no hydrogen evolved (Cu + 2H2SO4 -> CuSO4 + 2H2O + SO2; Pb + 2H2SO4 -> PbSO4 + 2H2O + SO2); sulphuric acid does not attack noble metals such as gold, silver and platinum. With salts, sulphuric acid can displace more volatile acids, giving metal bisulphates or sulphates and liberating the volatile acid or, if that acid is itself oxidisable, an oxidation product instead (KCl + H2SO4 -> KHSO4 + HCl; KNO3 + H2SO4 -> KHSO4 + HNO3; Na2CO3 + H2SO4 -> Na2SO4 + H2O + CO2; 2NaBr + 3H2SO4 -> 2NaHSO4 + 2H2O + Br2 + SO2, the last example showing oxidation of bromide rather than simple displacement, since HBr itself would be oxidised by the concentrated acid). With aromatic organic compounds such as benzene, it gives sulphonic acids (C6H6 + H2SO4 -> C6H5SO3H + H2O).

Uses: sulphuric acid is used in the manufacture of fertilisers such as ammonium sulphate and superphosphate, and of other important chemicals including hydrochloric acid and nitric acid; it is also used as a drying agent and in making pigments, explosives and detergents.

Test for sulphate/sulphuric acid: a dilute solution of sulphuric acid, or an aqueous solution of a soluble sulphate, gives a white precipitate of barium sulphate with barium chloride solution (BaCl2 + H2SO4 -> BaSO4(white precipitate) + 2HCl), a test that (unlike most other anion tests) is unaffected by dilute mineral acid. Sulphate can also be detected using lead acetate solution, which gives a white precipitate of lead sulphate ((CH3COO)2Pb + H2SO4 -> PbSO4(white precipitate) + 2CH3COOH). …