Sulphur & Its Compounds

Allotropes of Sulphur

Sulphur exists in different forms known as allotropes. The two main allotropes are:

• Rhombic Sulphur: Yellow, crystalline, and stable at room temperature.
• Monoclinic Sulphur: Needle-shaped crystals that are stable at temperatures above 96°C.

Both allotropes can be converted into each other by heating and cooling.

Sulphuric Acid: The Contact Process

Sulphuric acid, also known as tetraoxosulphate (VI) acid, is produced industrially using the Contact Process. The steps involved are:

1. Burning Sulphur: Sulphur is burned in air to produce sulphur dioxide (SO₂). $$\text{S} + \text{O}_2 \rightarrow \text{SO}_2$$
2. Oxidation: Sulphur dioxide is oxidized to sulphur trioxide (SO₃) using a vanadium(V) oxide catalyst. $$2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$$
3. Absorption: Sulphur trioxide is absorbed in concentrated sulphuric acid to form oleum. $$\text{SO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{S}_2\text{O}_7$$
4. Dilution: Oleum is diluted with water to produce sulphuric acid. $$\text{H}_2\text{S}_2\text{O}_7 + \text{H}_2\text{O} \rightarrow 2\text{H}_2\text{SO}_4$$

Sulphites and Sulphates

Sulphites and sulphates are compounds of sulphur with oxygen, having different oxidation states:

• Sulphites: Contain the trioxosulphate (IV) ion, $\text{SO}_3^{2-}$.
• Sulphates: Contain the tetraoxosulphate (VI) ion, $\text{SO}_4^{2-}$.

Properties of Sulphur Oxides

Sulphur forms two main oxides:

• Sulphur Dioxide (SO₂): A colourless gas with a pungent smell. It is acidic and reacts with water to form sulphurous acid (H₂SO₃).
• Sulphur Trioxide (SO₃): A colourless liquid or solid that reacts with water to form sulphuric acid (H₂SO₄).