The Contact Process

The Contact Process

1. Burn sulphur in air to form sulphur dioxide SO₂

S + O₂ -> SO₂

2. Use an excess of air to react sulphur dioxide to more oxygen to form sulphur trioxide

2SO₂ + O₂   <->    2SO₃                             ∆H = -196 kJ mol^-1

3. Reacting sulphur trioxide with water will give an uncontrollable fog of concentrated sulphuric acid. Instead, sulphur trioxide is absorbed in concentrated sulphuric acid to give fuming sulphuric acid (oleum):

SO₃ + H₂SO₄ -> H₂S₂O₇

4. This is converted into twice as much concentrated sulphuric acid by careful addition of water:

H₂S₂O₇ + H₂O -> 2H₂SO₄

The reversible reaction here is:

2SO₂ + O₂   <->    2SO₃                             ∆H = -196 kJ mol^-1

• Because the forward reaction is exothermic, a low temperature has to be used. Again, if a low temperature is used, the rate of reaction would be too slow, so 450ºC is a compromise.

• As for the pressure, a low pressure is needed because the forward reaction contains fewer molecules than the back reaction.

• The catalyst, vanadium (V) oxide (V₂O₅) speeds up the rate of reaction but has no effect on the equilibrium. Again, without a catalyst, the rate of reaction would be extremely slow.

Uses of sulphuric acid include:

• Making fertilisers – including ammonium sulphate and other substances

• Detergents – including hand soaps and shampoos

• Paint Manufacture – used to extract titanium dioxide from titanium ores