CIE IGCSE Chemistry

Revision Notes

12.1.1 Sulfur & Sulfuric Acid

Sulfur: Sources & Uses

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 Only

Sulfuric Acid: Manufacture, Properties & Uses

Manufacture of sulphuric acid

  • Sulfuric acid is synthesised by the Contact process which use 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


Sulfuric-Acid-Dehydrating-Sugar, IGCSE & GCSE Chemistry revision notesThe reaction of concentrated H2SO4 and sugar, which dehydrates the sugar leaving behind a tower of carbon


Uses of sulfuric acid


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


  • 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 and be able to explain the reasons why those conditions are chosen in terms of the equilibrium reactions.

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.

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