The Contact Process (CIE IGCSE Chemistry)

Extended tier only

• Sulfuric acid is synthesised by the Contact process
• Concentrated sulfuric acid is used in car batteries, making fertilisers, soaps and detergents

Stage 1

• The first stage is the production of sulfur dioxide
• The oxygen for this stage is obtained from the air
• The sulfur for this stage is obtained by:
  • Burning sulfur to oxidise the sulfur 

sulfur  +  oxygen  →  sulfur dioxide

S + O₂ → SO₂  

• • Roasting sulfide ores

metal sulfide + oxygen → metal oxide + sulfur dioxide

Stage 2

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

sulfur dioxide + oxygen sulfur trioxide 

2SO₂ + O₂ 2SO₃ 

• 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)

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

Extended tier only

• 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    2SO₃

Temperature: 450 ºC

• High temperature favours the reverse reaction as it is endothermic
  • So, a higher yield of reactants will be made
• Low temperature favours the forward reaction as it is exothermic
  • So, a higher yield of products will be made
• However, at low temperature the rate of reaction is very slow
• So, 450 ºC is a compromise temperature between having a lower yield of products but being made more quickly

Pressure: 200 kPa / 2 atm

• Low pressure favours the reverse reaction as there are more moles of gaseous reactant
  • So, a higher yield of reactants will be made
• High pressure favours the forward reaction as there are fewer moles of gaseous product
  • So, a higher yield of products will be made
• 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 liquify