Sulphur (yellow) is burnt in oxygen to form sulphur dioxide gas.
S + O2 → SO2
The newly formed sulphur dioxide is washed and dried to remove all impurities.
The impurities would poison the catalyst in the next step, and render the catalyst ineefective.
Sulphur dioxide and oxygen gases are reacted in a converter.
The catalyst used is vanadium pentoxide, also called vanadium (V) oxide.
The reaction to produce sulphur trioxide takes place on the surface of the catalyst under high pressure, hence the process is called the Contact Process.
2SO2 + O2 ↔ 2SO3
The sulpur trioxide is then dissolved in sulphuric acid.
This forms fuming sulphuric acid (oleum).
Sulpur trioxide does not dissolve easily in water; is highly exothermic, forming a mist.
SO3 + H2SO4 ↔ H2S2O7
The oleum is diluted with water to eventually form sulphuric acid.
H2S2O7 + H2O ↔ 2H2SO4