Contact Process

• 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 OR 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 OR 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
• You need to recall the temperature, pressure and catalyst needed for the Contact process and the equation for stage 2 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
• 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
• 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
• Remember: These conditions are a compromise between yield, rate, safety and cost.
• The raw materials in the contact process include:

  • sulfur
  • oxygen from the air
  • water
• Sulfur comes from fossil fuels containing sulfur
• The stages in the process include:

  1. S(g) + O2(g) → SO2 - sulfur burning in oxygen
  2. 2SO2 + O2 ⇌ 2SO3
  3. SO3 + H2SO4 → H2S2O7 (oleum)
• 2SO2 + O2 ⇌ 2SO3

  • enthalpy change is negative
  • forward reaction is exothermic
  • backwards reaction is endothermic
• Temperature 450 C

  • low temp used - compromised temp used
  • the rate of reaction is too slow
  • high temp = high cost
• Pressure 2 atm

  • high pressure
  • high rate of reaction
  • high pressure = higher cost
• Catalyst vanadium (V) oxide

  • oxidation state of vanadium means that it is effective at 450 C
  • no effect on equilibrium
  • higher rate of reaction for the forward and backward reaction
  • some costs