Sulfuric Acid Production Using Contact Process - WorldOfChemicals

Manufacturing of sulfuric acid by Contact process

Production of Sulfuric Acid using Contact Process

Sulfuric acid (H2SO4) is a highly corrosive strong mineral acid. Sulfuric acid has a wide range of applications including domestic acidic drain cleaner, electrolyte in lead-acid batteries and various cleaning agents. It is also a central substance in the chemical industry. It is widely produced by contact process.

Production of Sulfuric Acid by Contact Process: 

Nowadays, sulphuric acid is prepared by the contact process all over the world.   Preparation of sulphuric acid by contact process is based upon the catalytic oxidation of sulfur dioxide (SO2) to sulfur trioxide (SO3).

The Contact Process involves the following steps:


  • To make sulfur dioxide
  • converting sulfur dioxide into sulfur trioxide (the reversible reaction at the heart of the process)
  • Converting sulfur trioxide into concentrated sulfuric acid.


To make sulfur dioxide

Sulfur dioxide can be obtained by burning sulfur in an excess of air -


S (s) + O2 (g)  → SO2 (g)


Sulfur dioxide can also be obtained by heating sulfide ores like pyrite in an excess of air:


4FeS2(s) + 11O2(g)  → 2Fe2O3(s )+ 8SO2(g)


In either case, an excess of air is used so that the sulfur dioxide produced is already mixed with oxygen for the next stage.


Converting sulfur dioxide into sulfur trioxide

This is a reversible reaction, and the formation of the sulfur trioxide is exothermic.


2SO2(g) + O2(g)    2SO3 (g)      ΔH = -196 KJ mol-1


The process can be shown as below –

Converting sulfur trioxide into sulfuric acid

Converting sulfur trioxide to sulfuric acid cannot be done just by adding water, as the reaction is so uncontrollable that it creates a fog of sulfuric acid. Instead, the sulfur trioxide is first dissolved in concentrated sulfuric acid:

 H2SO4 (l) + SO3(g)  → H2S2O7 (l)


The product is known as fuming sulfuric acid or oleum. This can then be reacted safely with water to produce concentrated sulfuric acid - twice as much as you originally used to make the fuming sulfuric acid.

H2S2O7 (l) + H2O →  2H2SO4 (l)

                                  (Sulfuric acid)

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