Q. Describe about Dioxides?

These are generally obtained by heating the element in air. The dioxides of Se and Te can also be prepared by treating the element with conc. HNO, followed by heating the oxoacid formed e.g.

Se+ 4HNO₃------------> H₂SeO₃+ 4NO₂ + H₂O

H₂SeO₃---------------> SeO₂ + H₂O

SO₂, is a gas, SeO₂, is a white volatile solid while TeO₂, is a non-volatile white solid.  Gaseous SO₂, and SeO₂, have discrete symmetrical molecules which are bent or angular: Fig. (a). On solidification SeO₂, forms  long polymeric chains, Fig.(b).

So₂ and SeO₂ are acidic oxides as they form acids when dissolved in water whereas TeO₂ is slightly soluble in water and is an amphoteric oxide.

SO₂+ H₂O ---------------> H₂SO₃ (sulphurous acid)

SeO₂ + H₂O-------------> H₂SeO₃ (selenous acid)

SO₂ combines reversibly with oxygen in the presence of platinised asbestos or vanadium pentoxide to give sulphur trioxide. This reaction forms the basis of the Contact Process for manufacturing sulphuric acid as you will study later.

Volatile sulphur compounds mainly SO₂ are released into atmosphere as a result of combustion of sulphur containing fossil fuels. SO₂ released in densely populated area does great damage to the respiratory organs of human beings and animals to buildings and perhaps most seriously to plants and aquatic life as a result of acid rain.

SO₂ is a strong reducing agent in aqueous solution. The following reactions show this

2FeCl₃+SO₂ + 2H₂O-------------> 2FeCl₂+ H₂So₄+ 2HCl

Cl₂+SO₂ + 2H₂O--------------> 2HCl + H₂SO₄

2KMnO₄ + 2H₂O + 5SO₂--------> K₂So₄ + 2MnSO₄ + 2HeSo₄

Presumably the bleaching effect of SO₂ also depends upon its reducing properties.

SO₂ oxidises hydrogen sulphide to s provided moisture is present. At 1273 K it oxidises carbon to carbon dioxide.

2So₂ + H₂S-----------------> H₂So₄+ 3S

So₂+C-------------------------> CO₂+ S