Unit 12: Sulphur and its Compounds
From elemental allotropes to the industrial production of one of the world's most important chemicals.
12.22 Sulphur (Allotropes, Uses)
Sulfur is a non-metal with a simple molecular structure. It is known for existing in several different structural forms called allotropes. The most common and stable allotrope at room temperature is rhombic sulfur, which consists of crown-shaped $S_8$ molecules.
Because the $S_8$ molecules are large, the van der Waals forces between them are relatively strong, making sulfur a yellow, brittle solid at room temperature. Its main uses are in the vulcanization of rubber (making it harder and more durable) and, most importantly, as the primary raw material for the manufacture of sulfuric acid.
Solved Examples:
-
What is an allotrope?
Solution: Allotropes are different structural forms of the same element in the same physical state. For example, rhombic sulfur and monoclinic sulfur are both solid allotropes made of $S_8$ molecules packed in different crystal structures. -
Write the equation for the first step in manufacturing sulfuric acid from
elemental sulfur.
Solution: The first step is burning sulfur in air (oxygen) to produce sulfur dioxide.
$S(s) + O_2(g) \rightarrow SO_2(g)$
12.23 Sulphur Dioxide (SO₂) & Sulphate(IV) Ions (SO₃²⁻)
Sulfur dioxide ($SO_2$) is a colorless, toxic gas with a sharp, choking
smell. It is an acidic oxide, dissolving in water to form the weak acid,
sulfurous acid ($H_2SO_3$).
$SO_2(g) + H_2O(l) \rightleftharpoons H_2SO_3(aq)$
In sulfur dioxide, sulfur has an oxidation state of +4. It can be readily oxidized to the
more stable +6 state. This makes $SO_2$ and its related sulfite ion ($SO_3^{2-}$) effective
reducing agents. A key chemical test for $SO_2$ is its ability to reduce
the orange dichromate(VI) ion to the green chromium(III) ion.
$Cr_2O_7^{2-}(aq) + 14H^+(aq) + 3SO_2(g) \rightarrow 2Cr^{3+}(aq) + 3SO_4^{2-}(aq) +
7H_2O(l)$
This reducing property makes it useful as a food preservative (antioxidant) and for bleaching paper pulp.
Solved Examples:
-
What would you observe if sulfur dioxide gas is bubbled through a solution
of acidified potassium dichromate(VI)?
Solution: The color of the solution would change from orange to green. -
Why is sulfur dioxide used as a food preservative?
Solution: It acts as an antioxidant. It is more readily oxidized than the food components, so it reacts with oxygen first, preventing the food from spoiling.
12.24 The Contact Process (Manufacture of SO₃)
The Contact Process is the industrial method for producing sulfuric acid.
The key stage is the catalytic oxidation of sulfur dioxide to sulfur trioxide.
$$ 2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) \quad \Delta H = -197 \, kJ/mol $$
This is a reversible, exothermic reaction. According to Le Chatelier's principle, a high yield of $SO_3$ is favored by high pressure and low temperature. However, a compromise must be reached for economic viability.
Conditions Used:
- Temperature: A compromise temperature of 450 °C is used. This is low enough to give a good yield (~98%) but high enough for a fast reaction rate.
- Pressure: A slightly elevated pressure of 1-2 atmospheres is used. Although high pressure would favor the products, the yield is already very high at low pressure, so the high cost of high-pressure equipment is not justified.
- Catalyst: A vanadium(V) oxide ($V_2O_5$) catalyst is used to ensure the reaction reaches equilibrium quickly at the chosen temperature.
Solved Examples:
-
Why is a compromise temperature used in the Contact Process?
Solution: A low temperature would give a higher yield because the forward reaction is exothermic, but the reaction rate would be too slow. A high temperature would give a fast rate but a low yield. 450 °C provides the best economic balance between a high yield and a fast rate. -
What is the role of the vanadium(V) oxide catalyst?
Solution: It increases the rate at which the reaction reaches equilibrium, allowing a lower, more favorable temperature to be used. It does not affect the position of the equilibrium itself.
12.25 Sulphur Trioxide (SO₃) & Sulphuric Acid (H₂SO₄)
Sulfur trioxide ($SO_3$) is an acidic oxide that reacts extremely vigorously and exothermically with water to form sulfuric acid. This reaction is too dangerous to be performed directly in industry. Instead, the $SO_3$ is dissolved in concentrated sulfuric acid to form a substance called oleum ($H_2S_2O_7$), which is then safely diluted with water to produce more sulfuric acid.
Sulfuric acid ($H_2SO_4$) is one of the most important industrial chemicals in the world. It is a strong acid, a powerful dehydrating agent, and a strong oxidising agent (when concentrated).
- As an Acid: Used to make fertilizers (like ammonium sulfate), detergents, and paints.
- As a Dehydrating Agent: It has a strong affinity for water and can remove it from other substances, famously charring sugar by removing the elements of water: $C_{12}H_{22}O_{11}(s) \rightarrow 12C(s) + 11H_2O(l)$.
- As an Oxidising Agent: Concentrated H₂SO₄ can oxidize metals like copper and non-metals like carbon.
Solved Examples:
-
What is the largest single use of sulfuric acid?
Solution: The manufacture of fertilizers, such as ammonium sulfate ($(NH_4)_2SO_4$). -
What would you observe if concentrated sulfuric acid is added to a beaker of
sugar?
Solution: The white sugar would turn brown and then black as it is dehydrated to carbon. The reaction is highly exothermic, producing a large volume of steam, causing a "carbon snake" to rise from the beaker.
12.26 Sulphides (S²⁻)
Sulfides are compounds containing the sulfide ion ($S^{2-}$), where sulfur has an oxidation state of -2. Many metal sulfides are found naturally as ores (e.g., ZnS, PbS).
Most metal sulfides are insoluble in water. When a soluble sulfide or an acid is added to a
solution containing certain metal ions, a colored sulfide precipitate can form. The sulfide
ion is basic and reacts with acids to produce hydrogen sulfide ($H_2S$)
gas, which is toxic and has a characteristic smell of rotten eggs.
$$ S^{2-}(aq) + 2H^+(aq) \rightarrow H_2S(g) $$
Solved Examples:
-
How can hydrogen sulfide gas be identified?
Solution: By its highly unpleasant smell of rotten eggs. It also turns moist lead(II) ethanoate paper black. - Metal sulfide ores are often roasted in air as the first step of extraction.
What is the purpose of this?
Solution: Roasting in air (oxygen) converts the metal sulfide into a metal oxide (e.g., $2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$). It is generally easier to reduce a metal oxide to the metal than it is to reduce a metal sulfide.