Cambridge IGCSE CHEMISTRY 0620 - Sulfur notes

Sulfur: Sources and Uses

Core:

Name some sources of sulfur

Name the use of sulfur in the manufacture of sulfuric acid

State the uses of sulfur dioxide as a bleach in the manufacture of wood pulp for paper and as a food preservative (by killing bacteria)

Sources of sulfur

• Sulfur is found in its elemental state underground in the USA, Mexico and Poland.
• It is also a by-product from the removal of sulfur from petroleum and natural gas.
• Sulfur can also be obtained from sulfide ores.

Uses of sulfur

• The main use of sulfur is in making sulphuric acid which is a very important chemical used in many industries.
• It is also used extensively in making rubber tyres more flexible (vulcanising), where the rubber is heated with sulfur.

Sulfur dioxide

• Sulfur dioxide can be made by the direct combination of sulphur with oxygen.
• This is the method used in the first stage of the manufacture of sulfuric acid:
  • S + O₂ → SO₂

Uses of sulfur dioxide

• As a bleach in the manufacture of wood pulp for paper.
• As a preservative for foods and drinks by killing bacteria.
• Sulfites are often added to foods and these release sulfur dioxide in acidic conditions.

Extended Subject Content

Sulfuric Acid: Manufacture, Properties and Uses

Supplement:

• Describe the manufacture of sulfuric acid by the Contact process, including essential conditions and reactions

• Describe the properties and uses of dilute and concentrated sulfuric acid

Manufacture of Sulfuric Acid
Sulfuric acid is synthesised by the Contact process which uses sulfur and oxygen from air and is done in three distinct stages.
Stage 1

• The first stage is the oxidation of sulfur:
  • S + O₂ → SO₂

Stage 2

• The main stage is the oxidation of sulfur dioxide to sulfur trioxide using a V₂O₅ catalyst:
  • 2SO₂ + O₂ ⇌2SO₃

• The conditions for the main stage of production of sulfur trioxide need to be considered:

Conditions during Stage 2

Temperature: 450ºC

• The reaction is exothermic, so increasing the temperature shifts the position of equilibrium to the left in the direction of the reactants.
• Therefore the higher the temperature, the lower the yield of sulfur trioxide.
• The optimum temperature is a compromise between a higher rate of reaction at a higher temperature and a lower equilibrium yield at a higher temperature.

Pressure: 2 atm

• An increase in pressure shifts the position of equilibrium to the right in the direction of a smaller number of gaseous molecules.
• However the position of equilibrium lies far to the right (the equilibrium mixture contains about 96% sulfur trioxide).
• So the reaction is carried out at just above atmospheric pressure because:
  • a) it is not worth spending the extra energy or money required to produce high pressures.
  • b) a higher pressure would increase the problems of dealing with the corrosive mixture of gases.

Stage 3

• Once stage 2 is completed, the sulfur trioxide is absorbed into a solution of 98% sulphuric acid to produce a thick liquid called oleum:
  • SO₃ + H₂SO₄ → H₂S₂O₇

• It is not absorbed into water because a fine mist of sulfuric acid would be produced and this would be difficult to condense and is also highly dangerous.
• Oleum is added to water to form concentrated sulfuric acid:
  • H₂S₂O₇ + H₂O → 2H₂SO₄

Properties of Sulfuric Acid

• Sulfuric acid is a strong dibasic acid as two of its hydrogen atoms can be replaced by a metal.
• It reacts in a similar way to other acids with metal carbonates, oxides, hydroxides (and ammonia) and metals, e.g:
  • ZnO + H₂SO₄ → ZnSO₄ + H₂O
  • Mg + H₂SO₄ → MgSO₄ + H₂
  • Na₂CO₃ + H₂SO₄ → Na₂SO₄ + CO₂ + H₂O

• Concentrated sulphuric acid is corrosive and a powerful oxidising agent.
• Concentrated sulphuric acid is also a very powerful dehydrating agent and is very good at removing water from other substances.
• For example, if mixed with sugar (C₆H₁₂O₆), concentrated H₂SO₄ will remove water molecules and leave behind carbon in a spectacular looking reaction that produces a tower of pure carbon.

The reaction of concentrated H₂SO₄ and sugar, which dehydrates the sugar leaving behind a tower of carbon

Uses of Sulfuric Acid

Dilute

• Used as a catalyst in many organic reactions.
• Also used as to clean the surface of metals.

Concentrated

• Used in car batteries, making phosphate fertilisers, soaps and detergents.
• It is also used to make acid drain cleaners and in the production of paints and dyes.

Exam Tip

 

You need to know the conditions used in both the Haber process and the Contact process (see above) and be able to explain the reasons why the conditions of Stage 2 of sulfuric acid manufacture are chosen in terms of the equilibrium reactions.