Sulfur

Sulfur (S) is a yellow non-metal solid, and in 0620 it is the link between the atmosphere topic and industrial chemistry: it explains acid rain and it is the starting material for the Contact Process that makes sulfuric acid.

Where sulfur appears in 0620

• Air quality: sulfur dioxide from burning fossil fuels is a cause of acid rain.
• Manufacture of sulfuric acid: the Contact Process turns sulfur into H2SO4.
• Oxides: SO2 and SO3 are acidic oxides.
• Elements, compounds and mixtures: the iron + sulfur experiment shows the difference between a mixture and a compound.

Position in the periodic table

• Symbol: S
• Atomic number: 16
• Group 16 (VI), Period 3
• Electron configuration: 2, 8, 6
• Forms S2- ions in some ionic compounds (e.g. Na2S)

Physical properties

Property	Detail
Appearance	Bright yellow solid
Melting point	115 C
Structure	Simple molecular (S8 rings)
Conductivity	Non-conductor
Solubility	Insoluble in water

Sulfur exists as S8 molecules — rings of eight atoms. Its melting point is low because only weak intermolecular forces between whole S8 molecules need to be overcome, not the strong covalent bonds within them.

Combustion

S(s) + O2(g) -> SO2(g)

Sulfur burns with a blue flame to give sulfur dioxide, a colourless, toxic gas with a choking smell.

Sulfur dioxide and acid rain

SO2 is a major cause of acid rain. It is released by burning sulfur-containing fossil fuels (coal and oil), by volcanoes, and by some industry. In the atmosphere:

SO2(g) + H2O(l) -> H2SO3(aq) (sulfurous acid)

Further oxidation gives sulfuric acid:

2SO2(g) + O2(g) -> 2SO3(g)

SO3(g) + H2O(l) -> H2SO4(aq)

Acid rain corrodes limestone buildings and metals, acidifies lakes (killing fish), and damages trees and crops. See air quality and climate change.

The Contact Process

Sulfuric acid is manufactured industrially in three stages:

1. Burn sulfur: S(s) + O2(g) -> SO2(g)
2. Oxidise SO2 (reversible): 2SO2(g) + O2(g) ⇌ 2SO3(g)
3. Absorb SO3 and add water: SO3 -> H2SO4

Stage 2 uses a catalyst of vanadium(V) oxide (V2O5), a temperature of about 450 C, and a pressure of about 2 atmospheres. In practice SO3 is dissolved in concentrated H2SO4 first (to avoid a fog), then diluted. Sulfuric acid is one of the most important industrial chemicals.

Vulcanisation

Heating natural rubber with sulfur forms cross-links between the polymer chains. The rubber becomes harder, more elastic and more heat-resistant — suitable for vehicle tyres.

Iron and sulfur: mixture vs compound

Heating a grey mixture of iron filings and sulfur powder produces iron(II) sulfide:

Fe(s) + S(s) -> FeS(s)

Before heating, a magnet can pull the iron out of the mixture. After heating, FeS is a single compound with new properties and cannot be separated by a magnet — a classic illustration of the difference between a mixture and a compound.

Key facts

• Symbol: S — proton number: 16
• Position: Group VI, Period 3
• Electron configuration: 2, 8, 6 (forms S2-)
• Key property: yellow, simple molecular solid (S8 rings), insoluble in water
• On burning: blue flame -> SO2 (an acidic oxide and pollutant)
• Industrial role: raw material for sulfuric acid via the Contact Process

Common exam mistakes

• Spelling it “sulphur”. Cambridge 0620 uses sulfur, sulfuric acid and sulfate.
• Confusing SO2 and SO3. Sulfur dioxide (SO2) forms first and causes acid rain; sulfur trioxide (SO3) is the Contact Process intermediate.
• Dropping the reversible arrow. Stage 2 is 2SO2 + O2 ⇌ 2SO3 (equilibrium), and the conditions are V2O5, ~450 C, ~2 atm.
• Blaming acid rain on CO2. Carbon dioxide makes rain only weakly acidic; acid rain is caused by SO2 and oxides of nitrogen.

Worked exam questions

Explain how burning fossil fuels that contain sulfur leads to acid rain. Include one equation. (3 marks)

Mark scheme

• Burning the sulfur produces sulfur dioxide: S + O2 -> SO2 [1]
• SO2 is released into the atmosphere and dissolves in rainwater [1]
• It forms an acid, e.g. SO2 + H2O -> H2SO3 (sulfurous acid), lowering the pH of the rain [1]

Examiner note: the equation must involve sulfur, not carbon. A word answer with no equation caps at 2 marks.

State the catalyst and temperature used in stage 2 of the Contact Process, and explain why a catalyst is used. (3 marks)

Mark scheme

• Catalyst: vanadium(V) oxide / V2O5 [1]
• Temperature: about 450 C [1]
• A catalyst speeds up the reaction (higher rate) without being used up, so SO3 is made faster / more economically [1]

Examiner note: a catalyst changes the rate, not the position of equilibrium or the yield. Saying it “increases the yield” loses the explanation mark.