Industrial Preparation: the Contact Process
The Contact process makes sulphuric acid in four stages:
- Forming sulphur dioxide — by burning sulphur (S + O₂ → SO₂), roasting a metal sulphide (e.g. 2PbS + 3O₂ → 2PbO + 2SO₂), or burning hydrogen sulphide (2H₂S + 3O₂ → 2SO₂ + 2H₂O).
- Forming sulphur trioxide — sulphur dioxide is oxidised over a vanadium(V) oxide catalyst at 450°C and 2–3 atmospheres: 2SO₂(g) + O₂(g) → 2SO₃(g).
- Forming oleum — sulphur trioxide is absorbed into concentrated sulphuric acid rather than water directly (adding it straight to water would boil violently and spray acid droplets around the plant): SO₃(g) + H₂SO₄(conc.) → H₂S₂O₇(l).
- Forming sulphuric acid — the oleum is diluted with water: H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(aq).
Properties of Sulphuric Acid
Physical: concentrated sulphuric acid (“oil of vitriol”) is a colourless, viscous liquid with a density of 1.84 g/cm³; highly corrosive, causing skin burns; hygroscopic (absorbs water vapour from the air); dissolving it in water releases a large amount of heat.
Chemical:
- With bases: being dibasic, it can form two types of salt depending on the ratio used — an acidic salt with 1:1 alkali (NaOH(aq) + H₂SO₄(aq) → NaHSO₄(aq) + H₂O(l)), or a normal salt with 2:1 alkali (2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)).
- With reactive metals (above hydrogen in the reactivity series): releases hydrogen, e.g. H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g).
- With non-metals (concentrated): oxidises them, forming their oxide plus sulphur dioxide, e.g. C(s) + 2H₂SO₄(conc.) → CO₂(g) + 2H₂O(l) + 2SO₂(g).
- With hydrogen sulphide (concentrated): reduces it to sulphur: H₂S(g) + H₂SO₄(conc.) → SO₂(g) + 2H₂O(l) + S(s).
- As a dehydrating agent: removes the elements of water from compounds like sugar and ethanol — C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O(l) (leaving a charred black mass); C₂H₅OH(l) → C₂H₄(g) + H₂O(l). It also strips the water of crystallisation from hydrated salts, e.g. CuSO₄·5H₂O → CuSO₄ + 5H₂O — this dehydrating action is part of why concentrated sulphuric acid is so damaging to skin.
- Displacing more volatile acids from their salts (concentrated): KCl(s) + H₂SO₄(conc.) → KHSO₄(aq) + HCl(g); NaNO₃(s) + H₂SO₄(conc.) → NaHSO₄(aq) + HNO₃(g). With some salts, the displaced acid is unstable and decomposes further, e.g. Na₂SO₃(s) + H₂SO₄(conc.) → Na₂SO₄(aq) + H₂O(l) + SO₂(g), and Na₂CO₃(s) + H₂SO₄(conc.) → Na₂SO₄(aq) + H₂O(l) + CO₂(g).
Uses of Sulphuric Acid
Manufacturing fertilisers like ammonium sulphate and calcium dihydrogen phosphate; making dye and paint pigments; manufacturing explosives (e.g. TNT); car batteries; drying gases; and as a starting material for many other compounds.
Testing for Sulphite and Sulphate Ions
Both give a white precipitate with barium chloride solution — the difference is what happens next when dilute hydrochloric acid is added:
- Sulphate (SO₄²⁻): SO₄²⁻(aq) + BaCl₂(aq) → BaSO₄(s) + 2Cl⁻(aq). The white precipitate (barium sulphate) does not dissolve in the added HCl.
- Sulphite (SO₃²⁻): also gives a white precipitate with barium chloride, but this one does dissolve when dilute HCl is added.
