15. Sulphur dioxide

SULPHUR DIOXIDE

Perparation :

(i)  Sulphur dioxide is formed together with a little (6-8%) sulphur trioxide when sulphur is burnt in air or oxygen:

S(s) + O₂(g)  ®  SO₂(g)

(ii)   In the laboratory it is readily generated by treating a sulphite with dilute sulphuric acid.

SO₃^{2 –} (aq) + 2H⁺ (aq) ® H₂O(l) + SO₂(g)

(iii)  Industrially it is produced as a by- product of the roasting of sulphide ores.

4FeS₂(s) + 11O₂ (g) ®2Fe₂O₃(s) + 8SO₂(g)

The gas after drying is liquefied under pressure and stored in steel cylinders.

Properties :

(i) Sulphur dioxide is a colorless gas with pungent smell and is highly soluble in water. It liquefies at room temperature under a pressure of two atomsphere and boils at 263 K.

(ii) Sulphur dioxide, when passed through water, forms a solution of sulphurous acid.

 SO₂(g) + H₂O(l)  H₂ SO₃ (aq)

(iii)It reacts readily with sodium hydroxide solution, forming sodium sulphite which then reacts with more sulphur dioxide to form sodium hydrogen sulphite.

2NaOH + SO₂ — Na₂SO₃ + H₂O

  Na₂SO₃ + H₂O + SO₂ ® 2NaHSO₃ 

(iv) In it reaction with water and alkalies, the behaivour of sulphur dioxide is very similar to that of carbon dioxide.

Sulphur dioxide reacts with chlorine in the presence of charcoal (which acts as a catalyst) to give sulphuryl chloride, SO₂Cl₂ It is oxidised to sulphur trioxide by oxygen in the presence of vanadium (v) oxide catalyst.

SO₂(g) + Cl₂(g) ® SO₂Cl₂(l)

2SO₂ (g) + O₂ (g)  2SO₃(g)     Addition reactions

(v) When moist, sulphur dioxide behaves as a reducing agent. For example it converts iron (III) ions to irons (II) ions and decolourises acidified potasssium permanganate (VII) solution; the latter reaction is a convenient test for the gas.

2Fe³⁺ + SO₂ + 2H₂O ® 2Fe²⁺ + SO₄^2–  + 4H⁺ 

5SO₂ + 2MnO4^–  + 2H₂O ®  5SO₄^2–  + 4H⁺   + 2Mn²⁺

(vi)  Bleaching action : 

SO2 + 2H2O ® H2SO4 + 2H

 coloured matter  colourless matter

Bleaching is through reduction but it is temporary.

It is a more powerful reducing agent in alkaline medium than in acidic medium.

Uses :          

Sulphur dioxide is used (i) in refining petroleum and sugar (ii) in bleaching wool and silk and (iii) as an anti- chlor, disinfecatant and preservation. Sulphuric acid, sodium hydrogen sulphite and calcium hydrogen sulphite (industrial chemicals) are manufactured from sulphur dioxide. Liquid SO₂ is used as a solvent to dissolve a number of organic and inorganic chemicals.