3 SO2 + O3 ==> SO3
Sulphur trioxide is prepared by passing oxygen and sulphur dioxide through concentrated sulphuric acid to dry the gases, which are then passed over a heated platinised asbestos catalyst at 450 degC.
SO2 + O2 ==> 2 SO3
Sulphur trioxide is prepared by heating concentrated sulphuric acid with a large excess of phosphorus pentoxide, P2O5.
H2SO4 + P2O5 ==> SO3 + 2 HPO3
Sulphur trioxide is more conveniently on the small scale by distilling fuming sulphuric acid in a retort and collecting the sulphur trioxide in a perfectly dry receiver cooled in a freezing mixture.
H2SO7 <==> H2SO4 + SO3
Sulphur trioxide reacts vigorously with water to form sulphuric acid.
SO3 + H2O ==> H2SO4
Sulphur trioxide reacts with water vapour in the air to form white fumes consisting of tiny droplets of sulphuric acid.
Sulphur trioxide readily reacts with basic oxides to form sulphates.
CaO + SO3 ==> CaSO4
Sulphur trioxide is a planar covalent molecule with one sulphur oxygen double bond and with a bond angle of 120 degrees between the other two oxygen atoms. Sulphur trioxide has a resonance structure.