Understanding the manufacture of sulphuric acid

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Malaysia SPM Form 4 Chemistry, Chapter 9: Manufactured Substances in Industries


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[edit] Uses of Sulphuric Acid

Sulphuric acid is very important industrially, and has many uses including:

  • the production of fertilisers such as ammonium sulphate, potassium sulphate, calcium superphosphate (Ca(H2PO4)2), etc.; these are straight fertilisers, as they supply one of the important elements of nitrogen, phosphorus, or potassium (NPK);
  • the manufacture of non-soapy detergents: modern detergents are organic compounds 'sulphonated' with concentrated sulphuric acid;
  • the making of artificial silks like rayon: here, the fine threads in the alkaline cellulose solution are neutralised by passing them through a bath of sulphuric acid;
  • the cleaning of metals by removing the surface oxide coating: this is called pickling and is important in preparing articles for electroplating.
  • its use as an electrolyte inside batteries for cars: most car batteries are made up of lead plates in a sulphuric acid electrolyte; occasionally, the electrolyte needs to be 'topped up' with distilled water ; this is because small amounts of hydrogen and oxygen gases are given off by the chemical changes inside the battery, and therefore the sulphuric acid loses water and becomes too concentrated ; in the manufacture of drugs, paints, dyes and many other chemicals .



[edit] Manufacture of Sulphuric Acid: The Contact Process

  • The large-scale manufacture of this acid is extremely important as it is the most common acid used in industry, with over 1 000 million metric tonnes being produced annually.

It is manufactured by the Contact Process.


Stage 1
Combustion of Sulphur sulphur + oxygen \to sulphur dioxide
S (s) + O_2 (g) \to SO_2 (g) or
Heating of metal sulphide such as lead(II) sulphide 2PbS(s) + 3O2(g) ¾¾® 2PbO(s) + 2SO2(g) or Combustion of hiydrogen sulphide 2H2S(g) + 3O2(g) ¾¾® 2SO2(g) + 2H2O(ce)

  • The raw materials are sulphur and air (oxygen). Sulphur dioxide is produced by burning either sulphur or ores which contain sulphur.
  • Purification of sulphur dioxide
  1. The sulphur dioxide is then purified, by removing impurities like arsenic compounds which would otherwise poison the catalyst.
  2. It is then passed through an electrostatic dust precipitator, which, as its name implies, charges dust particles which are then removed by being attracted to oppositely charged plates.

Stage 2
Formation of Sulphur trioxide sulphur dioxide + oxygen ¾¾® sulphur trioxide 2SO2 (g) + O2 (g) ¾¾® 2SO3 (g)
Catalyst: vanadium(V) oxide
Temperature: 450°C
Pressure: 2-3 atmospheres


  • Sulphur dioxide and air are then washed, dried and passed over a vanadium(V) oxide catalyst at 450°C and 2-3 atmospheres.
  • The reaction is reversible but at these temperatures and pressures, 98% conversion to sulphur trioxide is achieved:
  • This reaction is exothermic, which means it favours a low temperature for high conversion to sulphur trioxide.


Stage 3
Formation of oleum H2S2O7
sulphur trioxide + concentrated sulphuric acid ¾¾®oleum SO3(g) + H2SO4(aq) ¾¾® H2S2O7(l)

  • The next step is to dissolve the sulphur trioxide produced in concentrated sulphuric acid, to form oleum, or fuming sulphuric acid.

Stage 4
Formation of Sulphuric acid Oleum + water ¾¾® sulphuric acid H2S207 (1) + H2O (1) ¾¾® 2H2SO4(aq)



  • This oleum is then diluted with water to the required strength of acid:
  • Although this may seem a roundabout route to take to form the acid, it is necessary because sulphur trioxide cannot be dissolved directly in water as it reacts too violently, forming tiny droplets of sulphuric acid which are very difficult to remove.

[edit] Sulphur Dioxide as pollutant

  • Sulphur dioxide (SO2) is the pollutant primarily associated with acid rain.
  • Gaseous at normal temperature and pressure it dissolves in water and readily oxidises to sulphuric acid.
  • Levels of SO2 have reduced over recent years with a move away from widespread burning of coal in homes and factories.
  • It is one of the main pollutants that led to the introduction of legislation governing atmospheric pollution such as the 1956 Clean Air Act.

[edit] Sources of Sulphur Dioxide

  • The principal source of SO2 is from the combustion of fossil fuels in domestic premises and , more importantly, non-nuclear power stations.
  • Fossil fuel burning power stations account for around two thirds of total SO2 emissions in the UK.
  • Other industrial processes contribute a further 20%, with vehicles, primarily diesel, accounting for a mere 2%.

[edit] Health effects

  • SO2 is an irritant when it is inhaled and at high concentrations (over 1000ppb) may cause severe problems in asthmatics such as narrowing of the airways, known as bronchoconstriction.
  • Asthmatics are considerably more sensitive to the effects of SO2 than other individuals and an effect on lung function may be experienced at levels as low as 200ppb.
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