Sulphur: allotropes, properties and oxides

Sulphur is the yellow element that gives matches their head, hardens rubber for tyres and is the raw material for sulphuric acid, the most widely made industrial chemical. Burned, it gives sulphur(IV) oxide, a choking gas that preserves fruit juice but also causes acid rain. This lesson covers sulphur, its allotropes and sulphur(IV) oxide.

Learning objectives

By the end of this lesson you should be able to:

• state the occurrence of sulphur and describe its extraction by the Frasch process
• describe the allotropes of sulphur and the transition temperature
• describe the effect of heat on sulphur
• state the chemical properties and uses of sulphur
• describe the preparation, test, properties and uses of sulphur(IV) oxide
• explain how sulphur(IV) oxide bleaches

Occurrence and extraction

• Free (native): in underground deposits and volcanic regions.
• Combined: in sulphide ores (galena PbS, zinc blende ZnS, iron pyrites FeS₂) and sulphates (gypsum CaSO₄·2H₂O); in crude oil and natural gas as hydrogen sulphide and organic sulphur compounds. Today most sulphur is recovered from oil and gas during refining.

The Frasch process (for underground deposits): three concentric pipes are sunk into the deposit.

1. Superheated water (about 170 °C, under pressure) is pumped down the outer pipe and melts the sulphur (melting point 115 °C).
2. Hot compressed air is blown down the innermost pipe.
3. A froth of molten sulphur, air and water is forced up the middle pipe and solidifies in large vats. The sulphur is about 99.5% pure.

Allotropes of sulphur

The transition temperature is 95.5 °C: below it rhombic sulphur is stable, and above it monoclinic. Left at room temperature, monoclinic sulphur slowly changes back into rhombic.

Plastic (amorphous) sulphur forms when nearly boiling sulphur is poured into cold water. It is a brown, rubbery, elastic solid of long tangled chains. It slowly hardens back into rhombic sulphur.

Effect of heat on sulphur

Chemical properties of sulphur

1. Burns in air with a blue flame, forming sulphur(IV) oxide:

2. Combines with metals on heating, forming sulphides: Fe + S → FeS (the mixture glows); Cu + S → CuS.

3. With hydrogen: H₂ + S → H₂S.

4. Oxidised by concentrated nitric or sulphuric acid to sulphuric acid.

Uses of sulphur:

• making sulphuric acid (most important)
• vulcanising rubber: sulphur cross-links rubber chains, making car tyres hard and durable
• matches and fireworks (and gunpowder)
• fungicides and insecticides for crops
• medicines and skin ointments (for scabies and fungal infections)
• making carbon disulphide and dyes

Sulphur(IV) oxide, SO₂

Laboratory preparation: add dilute hydrochloric acid to sodium sulphite, or heat copper with concentrated sulphuric acid:

Dry it with concentrated sulphuric acid; collect by downward delivery (it is denser than air and very soluble in water).

Test: it turns orange acidified potassium dichromate(VI) paper green. It also decolourises purple acidified potassium manganate(VII).

Properties:

• colourless gas with a choking, irritating smell; poisonous
• denser than air; very soluble in water
• easily liquefied
• acidic oxide: dissolves in water to form sulphurous acid (sulphuric(IV) acid), which turns blue litmus red

• reducing agent (in the presence of water): it reduces dichromate (orange → green), manganate(VII) (purple → colourless), iron(III) to iron(II), and chlorine to chloride
• with hydrogen sulphide it acts as an oxidising agent: 2H₂S + SO₂ → 3S + 2H₂O

How SO₂ bleaches

Sulphur(IV) oxide bleaches by reduction, in the presence of water. It removes oxygen from the coloured dye, leaving a colourless product. The bleaching is temporary: air slowly re-oxidises the dye, so the colour returns. This is why paper and straw bleached with SO₂ turn yellow with age. Chlorine, which bleaches by oxidation, gives permanent bleaching.

Uses of SO₂:

• making sulphuric acid (Contact process)
• bleaching wool, silk, straw and wood pulp, which chlorine would damage
• preserving fruit juices, dried fruit and wine (it kills microbes)
• fumigating stores and killing pests

Key points

• Sulphur occurs native, in sulphide and sulphate ores and in crude oil and gas; the Frasch process uses superheated water and hot air.
• Allotropes: rhombic (octahedral, stable below 95.5 °C) and monoclinic (needles, 95.5–119 °C); plastic sulphur forms on quenching.
• Heating sulphur: runny amber → viscous dark brown (chains) → runny again → boils at 444 °C.
• Sulphur burns with a blue flame to SO₂; used for H₂SO₄, vulcanising rubber, matches, fungicides.
• SO₂: choking, acidic, a reducing agent; turns acidified K₂Cr₂O₇ green; bleaches by reduction (temporary); preserves food.

WASSCE examination focus

Quick check

1. Define allotropy and name two allotropes of sulphur.
2. What is the transition temperature of sulphur?
3. Give the test for sulphur(IV) oxide.
4. Why does paper bleached with SO₂ turn yellow with time?

Answers:

1. The existence of an element in two or more forms in the same physical state; rhombic and monoclinic sulphur.
2. 95.5 °C.
3. It turns orange acidified potassium dichromate(VI) green.
4. SO₂ bleaches by reduction; oxygen in the air slowly re-oxidises the dye, so the colour returns.