OCR Gateway GCSE Chemistry

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6.1.6 The Contact Process

The Contact Process

  • The contact process is used to manufacture sulfuric acid  
  • Sulfuric acid is important in the manufacture of fertiliser 
  • It is also used in metal extraction, and making paints and dyes
  • The raw materials required are sulfur, water and air.
  • Stage 1: Sulfur is burned in oxygen:

S (s) + O2 (g)   →   SO2 (g)

  • Stage 2 (Contact Process): Sulfur dioxide and oxygen react to form sulfur trioxide using vanadium (IV) oxide as a catalyst:

2SO2 (g) + O2 (g)    ⇌    SO(g)

  • Stage 3: Sulfur trioxide is converted to sulfuric acid 

H2O (l) + SO3 (g)   →      H2SO4 (aq)

Explaining Reaction Conditions in Stage 2

Temperature: 450 ºC

  • The forward 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
  • 450ºC is a compromise low enough to achieve a reasonable yield but high enough to achieve a reasonable rate of reaction

Pressure: 2 atmospheres (200 kPa) 

  • 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 already (the equilibrium mixture contains about 96% sulfur trioxide)
  • The reaction is carried out just above atmospheric pressure because:
    • It is not worth spending the extra energy or money required to produce high pressures
    • A higher pressure would increase the problems of dealing with the corrosive mixture of gases

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