The Contact Process (Cambridge (CIE) O Level Chemistry): Revision Note

The Contact Process

• Sulfuric acid is synthesised by the Contact process

• Concentrated sulfuric acid is used in car batteries, making fertilisers, soaps and detergents

Raw materials

• The raw materials for the process are:

  • Sulfur dioxide (SO₂)

    • Produced by burning sulfur in air (S + O₂ → SO₂) or by roasting sulfide ores

  • Oxygen (O₂)

    • Obtained from the air

The Key Stage: Oxidation of SO₂

• The most important stage in the Contact process is the reversible oxidation of sulfur dioxide to sulfur trioxide using a vanadium(V) oxide, V₂O₅, catalyst:

2SO₂ + O₂ ⇌ 2SO₃

• The conditions for this main stage of production are:

  • A temperature of 450 ºC

  • A pressure of 2 atm (200 kPa)

• Once sulfur trioxide is formed, it undergoes more processes to produce sulfuric acid

Explaining the Conditions in the Contact Process

• Similar to the Haber process, the pressure and temperature used need to be considered

• The equation for the main stage of the Contact process is:

2SO₂ + O₂  $\rightleftharpoons$  2SO₃

Temperature: 450ºC

• 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

• 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:

  • High pressures can be dangerous and very expensive equipment is needed 

  • A higher pressure causes the sulfur dioxide to liquefy