The Contact Process (Cambridge (CIE) IGCSE Chemistry)

The Contact process

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

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 

Explaining the conditions in the Contact process

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