The Contact Process (OCR GCSE Chemistry A (Gateway))

The Contact Process

Higher Tier Only

• 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)    ⇌    SO₃ (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