p - Block Elements Part - 8 | Class 12th

Ø  Sulphur Dioxide (SO₂):

Preparation:

·       Sulphur dioxide is formed when sulphur is burnt in air or oxygen: S(s) + O₂(g) → SO₂ (g)

·       In the laboratory it is obtained by treating a sulphite with dilute sulphuric acid. SO₃^2-(aq) + 2H⁺ (aq) → H₂O(l) + SO₂ (g)

·       Industrially, it is produced by roasting of sulphide ores.

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

 

Properties:

·       Sulphur dioxide is a colourless gas with pungent smell and is highly soluble in water.

·       With water, it forms a solution of sulphurous acid which is a dibasic acid and form two types of salts with alkalies – normal salt (sulphite) and acid salt (bisulphate or hydrogen sulphite).

SO₂(g) + H₂O(l) → H₂SO₃(aq)

·       Neither burns nor helps in burning but burning magnesium and potassium continue to burn in its atmosphere.

3Mg + SO₂ ¾® 2 MgO + MgS  

4K + 3SO₂ ¾®¾ K₂SO₃ + K₂S₂O₃

·       With sodium hydroxide solution, it forms sodium sulphite, which then reacts with more sulphur dioxide to form sodium hydrogen sulphite.

2NaOH + SO₂ → Na₂SO₃ + H₂O                                   

Na₂SO₃ + H₂O + SO₂ → 2NaHSO₃

·       SO₂ is oxidised to sulphur trioxide by oxygen in the presence of vanadium pentoxide (V₂O₅) catalyst.

2SO₂ + O₂ → 2SO₃

·       Moist sulphur dioxide behaves as a reducing agent. It converts iron(III) ions to iron(II) ions and decolourises acidified potassium permanganate(VII) solution (This used as a test for SO₂).

2Fe³⁺ + SO₂ + 2H₂O → 2Fe²⁺ + SO₄^2- + 4H⁺

5 SO₂ + 2MnO₄^- + 2H₂O → 5 SO₄^2- + 4H⁺ + 2Mn²⁺

Acidified K₂Cr₂O₇ ¾® Cr³⁺ (green coloured solution)

 

Bleaching Action: SO₂ + 2H₂O ¾® H₂SO₄ + 2H

This is due to the reducing nature of SO₂

Coloured matter + H ↔ Colourless matter. Therefore, bleaching is temporary.

 

Ø  Uses: Sulphur dioxide is used:

·     Used in manufacture of H₂SO₄ & paper from wood pulp.

·     As a bleaching agent for delicate articles like wool, silk and straw.

·     Used in refining of petroleum and sugar.

·  Liquid SO₂ is used as a solvent to dissolve a number of organic and inorganic chemicals.