In the preparation of H2SO4 by Contact Process, why is SO3 not absorbed directly in water to form H2SO4?


Dissolution of SO3 in water is highly exothermic in nature. This leads to the formation of mist of tiny droplets which are highly corrosive and they can even attack the lead pipelines (lead pipelines are used to cover the tower in contact process).

Thus the acid fog which is formed by the direct reaction of water with sulphur trioxide is very difficult to condense. Therefore the following reaction takes place.


Absoption of SO3 in 98 percent of H2SO4 to form oleum


SO3 + H2SO4H2S2O7


Then dilution of oleum takes place to get H2SO4.


H2S2O7 + H2O 2H2SO4


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