Manufacturing Sulphuric Acid: The Contact Process

Sulphuric Acid is Manufactured in Industry

  1. Sulphuric acid, H2SO4 is manufactured in industry through Contact Process.
  2. The raw materials used are sulphur, air and water
  3. The Contact process consists of four stages.

Stage 1

  1. Molten sulphur is burnt in dry air to produce sulphur dioxide
  2. The gas produced is then purified and cooled.
    S+ O 2 S O 2
  3. Sulphur dioxide can also be produced by burning metal sulphide such as lead(II) sulphide or zinc sulphide in dry air.
    2PbS+3 O 2 2PbO+2S O 2

Stage 2

  1. In a converter, sulphur dioxide and excess oxygen are passed through vanadium(V) oxide.
  2. vanadium(V) oxide act as catalyst to expedite the process.
  3. The optimum condition for maximum amount of product are as follow:
    1. Temperature: 450 – 500 °C
    2. Pressure: 2 – 3 atm
  4. About 99.5% of the sulphur dioxide, SO2 is converted into sulphur trioxide, SO3 through this reversible reaction.

Stage 3

Sulphur trioxide is dissolved in concentrated sulphuric acid to form oleum (H2S2O7).
S O 3 + H 2 S O 4 H 2 S 2 O 7

Stage 4

The oleum, H2S2O7 is then diluted with water to produce concentrated sulphuric acid, H2SO4 in large quantities.
H 2 S 2 O 7 + H 2 O2 H 2 S O 4


  1. The two reactions in the third and fourth stages are equivalent to adding sulphur trioxide, SO3 directly to water.
    S O 3 + H 2 O H 2 S O 4
  2. However, this is not done in industry because sulphur trioxide, SO3 reacts too violently with water.
  3. This produces a lot of heat and a large cloud of sulphuric acid, H2SO4 mist.
  4. The mist is corrosive, pollutes the air and is difficult to condense.



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