The Contact Process (WJEC GCSE Chemistry)

• Sulfuric acid is synthesised by the Contact process

Stage 1

• The first stage is the oxidation of sulfur to form sulfur dioxide:

S + O₂ → SO₂  

sulfur  +  oxygen  →  sulfur dioxide

Stage 2

• The main stage is the oxidation of sulfur dioxide to sulfur trioxide using a vandium(V) oxide, V₂O₅, catalyst:

2SO₂ + O₂    2SO₃

sulfur dioxide  +  oxygen    sulfur trioxide

• The oxygen used in this stage is obtained from air
• The conditions for this main stage of production are:
  • A temperature of 450 ºC
  • A pressure of 2 atm (200 kPa)

Stage 3

• The sulfur trioxide is absorbed into a solution of 98% sulphuric acid to produce a thick liquid called oleum (disulfuric acid):

SO₃ + H₂SO₄ → H₂S₂O₇

sulfur trioxide  + sulfuric acid  →  disulfuric acid 

• 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:

H₂O + H₂S₂O₇ → 2H₂SO₄

water  +   disulfuric acid  →  sulfuric acid 

Higher Tier

• The conditions for the second stage of the Contact process have been chosen carefully

2SO₂ + O₂    2SO₃

Temperature: 450ºC

• The forward reaction is exothermic, so increasing the temperature shifts the position of the equilibrium to the left 
  • Therefore the higher the temperature, the lower the yield of sulfur trioxide
• A greater yield of sulfur trioxide would be achieved by using a lower temperature
  • The position of the equilibrium would shift to the right but at low temperatures the rate of reaction is low
• 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 because there are fewer molecules of product formed
• 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:
  • It is not worth spending the extra energy or money required to produce high pressures
  • A higher pressure also causes the sulfur dioxide to liquefy

What are the uses of sulfuric acid?

• Sulfuric acid produced from the Contact process has a broad range of uses, including:
  • The production of fertilisers
  • Paints
  • Dyes
  • Detergents
  • Fibres
  • Plastics 

Image by Anna Kolosyuk on Unsplash

One use of sulfuric acid is in the production of paints