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Sulphur and Its Inorganic Compounds

Lessons

Senior 3 · Chemistry

Sulphur and Its Inorganic Compounds

4 lessons

Lesson 1 of 4

Sulfur: Occurrence, Allotropes and Properties

25 min

The yellow element of the volcanoes

Around the steaming vents of active volcanoes, bright yellow crusts build up on the rocks. The ancient name for this yellow element was brimstone, "burning stone", because it burns with a blue flame and a choking smell. Sulfur is also hidden in less obvious places: in the proteins of eggs, onions and your own hair, in crude oil and natural gas, and in the ores of many metals. It is the starting point for sulfuric acid, the chemical that industry uses more of than any other.

A note on spelling: REB's books write sulphur; this course uses sulfur, the spelling chosen by IUPAC, the international body that sets chemical names. Both mean the same element, and you may meet both in exams.

Where sulfur is found

Free (native) sulfur occurs around volcanoes and in large underground beds in some countries. In the past, underground sulfur was mined by pumping superheated water down a pipe to melt it and forcing the liquid sulfur up with compressed air (the Frasch process).

Combined sulfur is far more common:

Type of compound	Examples
sulfide ores	galena, $\mathrm{PbS}$ ; zinc blende, $\mathrm{ZnS}$ ; iron pyrites ("fool's gold"), $\mathrm{FeS_2}$ ; copper pyrites, $\mathrm{CuFeS_2}$
sulfates	gypsum, $\mathrm{CaSO_4 \cdot 2H_2O}$ ; barytes, $\mathrm{BaSO_4}$
fossil fuels	hydrogen sulfide, $\mathrm{H_2S}$ , in natural gas; sulfur compounds in crude oil and coal
living things	proteins in eggs, hair, onions and garlic

Today most of the world's sulfur is recovered from natural gas and oil. The sulfur compounds must be removed anyway, or they would form sulfur dioxide when the fuel is burned. Hydrogen sulfide is separated and part of it is burned; the sulfur dioxide formed then reacts with the rest:

$\mathrm{2H_2S(g) + SO_2(g) \rightarrow 3S(s) + 2H_2O(l)}$

Definition

Transition temperature: the temperature at which one allotrope of an element changes into another, and at which both can exist together. Example: rhombic sulfur and monoclinic sulfur have a transition temperature of 95.5 °C.

Allotropes of sulfur

Like carbon (Unit 1), sulfur has allotropes. Both of its crystalline forms are made of the same molecule, $\mathrm{S_8}$ , a crown-shaped ring of eight atoms; they differ in how the rings are packed together (Figure 1.1).

Rhombic sulfur (also called α-sulfur or orthorhombic sulfur) is the stable form below 95.5 °C. Its bright yellow crystals are shaped like two pyramids joined base to base. Crystals form when a solution of sulfur in an organic solvent is left to evaporate at room temperature.
Monoclinic sulfur (β-sulfur) is stable between 95.5 °C and its melting point, 119 °C. It forms long, pale yellow needles when molten sulfur cools and crystallises above 95.5 °C. Left at room temperature, the needles slowly turn into tiny rhombic crystals over a few days.

Figure 1.1: The two crystalline allotropes of sulfur.

What happens when sulfur is heated

Table 1.1: Heating sulfur slowly in a test tube

Temperature	What you see	Explanation
about 119 °C	melts to a pale yellow, runny liquid	$\mathrm{S_8}$ rings roll over each other easily
about 160–200 °C	turns dark red-brown and so thick it hardly pours	the rings break open and join into very long chains, which tangle
above about 250 °C, up to 444 °C	becomes runnier again, then boils	the long chains break into shorter pieces
boiling sulfur poured into cold water	forms plastic sulfur, a brown, rubbery solid that can be stretched	the tangled chains are frozen before they can form rings; in a few days it turns back to brittle rhombic sulfur

Warning

Safety. Heat sulfur gently in a well-ventilated room or fume cupboard, holding the tube with a holder and pointing it away from everyone. If it catches fire it burns to sulfur dioxide, a choking, toxic gas: put the flame out by covering the mouth of the tube. Molten sulfur causes serious burns.

Properties of sulfur

Physical properties. Sulfur is a yellow, brittle solid with no smell (the bad smell of rotten eggs is hydrogen sulfide). It does not dissolve in water, but dissolves in some organic solvents. It is a non-metal: it does not conduct heat or electricity.

Chemical properties.

Burns in air or oxygen with a blue flame, forming sulfur dioxide: $\mathrm{S(s) + O_2(g) \rightarrow SO_2(g)}$ .
Combines with metals when heated, forming sulfides. In S1 you saw iron and sulfur glow as they formed iron(II) sulfide; zinc and copper react in the same way: $\mathrm{Zn(s) + S(s) \rightarrow ZnS(s)}$ and $\mathrm{2Cu(s) + S(s) \rightarrow Cu_2S(s)}$ .
Combines with hydrogen when hydrogen is passed over molten sulfur: $\mathrm{H_2(g) + S(l) \rightarrow H_2S(g)}$ .
Is oxidised by hot concentrated acids, but does not react with dilute acids: $\mathrm{S(s) + 2H_2SO_4(l) \rightarrow 3SO_2(g) + 2H_2O(l)}$ .

Uses of sulfur

Making sulfuric acid (Lesson 3): by far the largest use.
Vulcanising rubber: heating natural rubber with sulfur links its long molecules together, making it hard-wearing enough for tyres and shoe soles.
A fungicide: fine sulfur dust protects crops such as grapes and vegetables against fungal diseases.
Matches, fireworks and gunpowder, because it burns easily.
Medicines: sulfur ointments for some skin diseases.

Worked example

Problem. Iron pyrites, $\mathrm{FeS_2}$ , is heated in air to make sulfur dioxide for a sulfuric acid plant. (a) Calculate the percentage of sulfur by mass in iron pyrites. (b) What mass of sulfur is contained in 1.2 tonnes of pure pyrites? (Fe = 56, S = 32)

Solution. (a) $M_r(\mathrm{FeS_2}) = 56 + 2 \times 32 = 120$ .

$\%\,\mathrm{S} = \frac{2 \times 32}{120} \times 100 = 53.3\%$

(b) Mass of sulfur $= \frac{64}{120} \times 1.2\,\mathrm{t} = 0.64\,\mathrm{t}$ (640 kg).

Worked example

Problem. Mugisha melts some sulfur in a crucible, lets it cool until a crust forms, pierces the crust and pours away the liquid underneath. He finds long, shiny needle-shaped crystals inside. A week later the needles have lost their shine and look powdery. Explain both observations.

Solution. Sulfur crystallising from the melt, above the transition temperature of 95.5 °C, forms monoclinic sulfur, which grows as needles. At room temperature, below 95.5 °C, the stable allotrope is rhombic sulfur, so the needles slowly change into many tiny rhombic crystals. The outline of each needle stays, but it is now made of small crystals, which look dull and powdery.

Tip

Rhombic below, monoclinic above: the transition temperature is 95.5 °C, just below the boiling point of water. Both allotropes are built from $\mathrm{S_8}$ rings; plastic sulfur is made of long tangled chains.

Periodic Videos: Sulfur (7 min), from the University of Nottingham. Watch for the colour of sulfur and of its flame, and list any compounds of sulfur that the presenters mention.

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Practice

Name two sulfide ores and give their formulae.
Why is most sulfur today obtained from natural gas and oil refineries? Write the equation for the reaction that produces it.
State two differences and one similarity between rhombic and monoclinic sulfur.
What is the transition temperature of sulfur, and what does it mean?
Describe what you see when sulfur is heated slowly from room temperature to its boiling point, and explain the change at about 180 °C.
Write equations for the reactions of sulfur with (a) oxygen, (b) zinc, (c) hydrogen.
Calculate the percentage of sulfur in zinc blende, ZnS. (Zn = 65, S = 32)
Explain why a tyre made of vulcanised rubber lasts longer than one made of untreated rubber.

Show answersHide answers

Any two: galena, PbS; zinc blende, ZnS; iron pyrites, $\mathrm{FeS_2}$ ; copper pyrites, $\mathrm{CuFeS_2}$ .
Sulfur compounds must be removed from these fuels anyway, to prevent sulfur dioxide pollution when they are burned, so the sulfur is a cheap by-product: $\mathrm{2H_2S(g) + SO_2(g) \rightarrow 3S(s) + 2H_2O(l)}$ .
Differences: rhombic is stable below 95.5 °C, monoclinic above; rhombic crystals are bipyramids, monoclinic are needles; rhombic is denser. Similarity: both are made of $\mathrm{S_8}$ rings (both yellow, both pure sulfur).
95.5 °C: the temperature at which rhombic and monoclinic sulfur change into each other and can exist together.
It melts to a runny pale yellow liquid (about 119 °C), becomes dark brown and very thick (about 160–200 °C), then runnier again, and boils at 444 °C. At about 180 °C the $\mathrm{S_8}$ rings open and join into long chains that tangle, so the liquid becomes thick.
(a) $\mathrm{S(s) + O_2(g) \rightarrow SO_2(g)}$ ; (b) $\mathrm{Zn(s) + S(s) \rightarrow ZnS(s)}$ ; (c) $\mathrm{H_2(g) + S(l) \rightarrow H_2S(g)}$ .
$M_{r} = 97$ : $\frac{32}{97} \times 100 = 33.0\%$ .
Sulfur atoms form cross-links between the long rubber molecules, so they cannot slide past each other; the rubber becomes harder, stronger and more elastic, and wears away more slowly.

Sources and credits

Chemistry 2e, 18.10: Occurrence, Preparation, and Properties of Sulfur (where sulfur is found, its recovery from natural gas, its S₈ molecules and its allotropes), Paul Flowers, Klaus Theopold, Richard Langley and William R. Robinson, OpenStax, via LibreTexts, licensed under CC BY 4.0. Adapted by Ganzaa.
Photo: Sulphur-crystals-vulcano.JPG, Heidi Soosalu, licensed under CC BY-SA 3.0, via Wikimedia Commons.
Video: Sulfur – Periodic Table of Videos, Periodic Videos (University of Nottingham), embedded from YouTube.
The diagram, tables, worked examples and Practice items were written and drawn by Ganzaa.

1The yellow element of the volcanoes

2Where sulfur is found

3Allotropes of sulfur

4What happens when sulfur is heated

5Properties of sulfur

6Uses of sulfur