Q. What are Trioxides?

The selenium and tellurium trioxides are not stable, SO₂, is the only important trioxide in the group. Its preparation has been described above.

Sulphur trioxide exists in several polymorphic forms. The solid form, m .p. 290 K has a trimetric cyclic structure, in which four oxygen atoms are arranged approximately tetrahedrally around each sulphur atom, Fig. (c). This form gradually changes into a linear polymerised structure in the presence of moisture. The latter has a fibrous needle-like appearance, Fig.(b). On heating, the polymeric form dissociates into discrete SO₂, molecules present in the vapour. These have a triangular, symmetrical and planar structure, Fig. (a).

SO₂, is a powerful acidic oxide, it fumes in moist air and reacts explosively with water to form sulphuric acid.

SO₃+ H₂O------------> H₂SO₄

With excess SO₃ H₂S0₄ gives pyrosulphuric acid or oleum

H₂SO₄+ SO₃-----------------> H₂S₂O₇ (oleum)

In some reactions it acts as an oxidising agent, e.g., it oxidises HBr to free bromine.

2HBr+ SO₃------------------> H₂O + Br₂+ SO₂