Sulphur and Its Compounds

Sulphur Dioxide (SO₂)

Method of preparation of sulphur dioxide

Sulphur dioxide is formed together with a little (6-8%) sulphur trioxide when sulphur is burnt in air or oxygen:

S (s) + O₂ (g) → SO₂ (g)

In the laboratory, it is readily generated by treating a sulphite with dilute sulphuric acid.

SO₃^2- (aq) + 2H⁺ (aq) → H₂O (l) + SO₂ (g)

Industrially, it is produced as a by-product of the roasting of sulphide ores.

4FeS₂ (s) + 11O₂ (g) → 2Fe₂O₃ (s) + 8 SO₂ (g)

The gas after drying is liquefied under pressure and stored in steel cylinders, because it is a strong oxidising and reducing agent.

Physical properties of sulphur dioxide

It is a colourless gas with pungent smell, highly soluble in water. It liquefies at room temperature under a pressure of two atmospheres and boils at 263 K.

Chemical reactions of sulphur dioxide

Structure of dulphur dioxide

The molecule of SO₂ is angular. It is a resonance hybrid of the two canonical forms:

Reaction of sulphur dioxide with water

Sulphur dioxide, when passed through water, forms a solution of sulphurous acid.

SO₂ (g) + H₂O (l) ⇔ H₂SO₃ (aq)

Reaction of sulphur dioxide with lime water

It turns lime water milky due to the formation of calcium sulphite.

Ca(OH)₂ + SO₂ → CaSO₃ (milky) + H₂O

However, in excess of SO₂ milkiness disappears due to the formation of calcium bisulphite.

CaSO₃ + SO₂ + H₂O → Ca(HSO₃)₂

Reaction of sulphur dioxide with sodium hydroxide

It reacts readily with sodium hydroxide solution, forming sodium sulphite, which then reacts with more sulphur dioxide to form sodium hydrogen sulphite.

NaOH + SO₂ → Na₂SO₃ + H₂O;

2Na₂SO₃ + SO₂ + H₂O→ NaHSO₃

Reaction of sulphur dioxide with chlorine

Sulphur dioxide reacts with chlorine in the presence of charcoal (which acts as a catalyst) to give sulphuryl chloride, SO₂Cl₂.

SO₂ (g) + Cl₂ (g) → SO₂Cl₂

Reaction of sulphur dioxide with oxygen

It is oxidised to sulphur trioxide by oxygen in the presence of vanadium (V) oxide catalyst.

SO₂ (g) + O₂ (g)  2SO₃ (g)

Reducing properties of sulphur dioxide

Moist sulphur dioxide behaves as a reducing agent. For example, it converts iron (III) ions to iron (II) ions and decolourises acidified potassium permanganate (VII) solution; the latter reaction is a convenient test for the gas.

2Fe³⁺ + SO₂ + 2H₂O → 2Fe²⁺ + SO₄^2- + 4H⁺

5SO₂ + 2MnO₄^- + 2H₂O → 5SO₄^2- + 4H⁺ + 2Mn²⁺

Uses of sulphur dioxide

Ø In refining petroleum and sugar

Ø In bleaching wool and silk and

Ø As an anti-chlor, disinfectant and preservative

Ø Sulphuric acid, sodium hydrogen sulphite and calcium hydrogen sulphite (industrial chemicals) are manufactured from sulphur dioxide.

Ø Liquid SO₂ is used as a solvent to dissolve a number of organic and inorganic chemicals.