Sulphur and Its Oxides

Element: S. Oxides: SO₂ (sulphur dioxide) and SO₃ (sulphur trioxide). Burning: S + O₂ → SO₂. Oxidation: 2SO₂ + O₂ ⇌ 2SO₃ (reversible). Acid formation: SO₃ + H₂O → H₂SO₄. SO₂ is a pungent gas; SO₃ is the oxide that leads to H₂SO₄.

Start from elemental sulphur. Combustion in air/oxygen produces SO₂. In industry and in school equations, SO₂ is then oxidised to SO₃. That oxidation is written as an equilibrium: 2SO₂ + O₂ ⇌ 2SO₃ — the same step that sits at the heart of the Contact Process (catalyst and temperature details belong in the Contact Process topic). SO₃ is highly reactive toward water and forms H₂SO₄. In practice industry absorbs SO₃ into concentrated acid to make oleum rather than dumping SO₃ straight into water, but the school stoichiometry SO₃ + H₂O → H₂SO₄ remains the key acid-formation equation. Keep names sharp: sulphur dioxide SO₂ vs sulphur trioxide SO₃.

Sulphur burns to make SO₂. SO₂ can be turned into SO₃ with oxygen. SO₃ with water makes sulphuric acid H₂SO₄. Learn the three equations by heart.

Equations to write without hesitation: S + O₂ → SO₂; 2SO₂ + O₂ ⇌ 2SO₃; SO₃ + H₂O → H₂SO₄. Note the reversible arrow on the SO₂ → SO₃ step. Link SO₂ pollution themes lightly if asked, but keep the main focus on formulas and balanced equations leading to H₂SO₄.

The SO₂/SO₃ equilibrium is exothermic in the forward direction in the Contact Process narrative, which is why temperature choice is a compromise — details sit with V₂O₅ catalysis in the next topic. Direct SO₃ + water is violently exothermic; industrial preference for oleum is an applied safety/process point often mentioned alongside these equations.

S → SO₂ → SO₃ → H₂SO₄ · S+O₂→SO₂ · 2SO₂+O₂⇌2SO₃ · SO₃+H₂O→H₂SO₄.