The Contact Process (Cambridge International IGCSE Chemistry)

Spec point — 6.2.4 · State the symbol equation for the conversion of sulfur dioxide to sulfur trioxide in the Contact process, 2SO2(g) + O2(g) ⇌ 2SO3(g) State the sources of the sulfur dioxide (burning sulfur or roasting sulfide ores) and oxygen (air) in the Contact process State the typical conditions for the conversion of sulfur dioxide to sulfur trioxide in the Contact process as 450°C, 200kPa /2atm and a vanadium(V) oxide catalyst

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$\rightleftharpoons$sulfur trioxide

2SO₂ + O₂$\rightleftharpoons$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

Exam Hint You need to recall the temperature, pressure and catalyst needed for the Contact process and the equation for stage 2 only.

Explaining the conditions in the Contact process

Spec point — 6.2.4 · Explain, in terms of rate of reaction and position of equilibrium, why the typical conditions stated are used in the Contact process, including safety considerations and economics

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 $\rightleftharpoons$ 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

Exam Hint Remember: These conditions are a compromise between yield, rate, safety and cost.