CHEMISTRY

TOPIC:SULPHUR (IV) OXIDE

By Kingsley Idiagbor,B.Sc(Hons),NCE,PGDCs,MNSM

Occurrence:Sulphur(IV) oxide is frequently produced as a by-product of smelting sulphide ores such as zinc sulphide and is often emitted to the atmosphere by way of industrial effluents

Laboratory Preparation: It is prepared suitably in the fume cupboard of the laboratory by the action of hydrochloric acid on sodium trioxosulphate(IV), Na₂SO₃. The gas liberated, this way, is passed through H₂SO₄acid to dry it and thereafter collected by downward delivery
The equation for the reaction: Na₂SO₃+ 2HCl--->2NaCl + H₂O + SO₂

Test for the gas

The gas can be identified as SO₂if it satisfies the following conditions:

Smell:It should have an irritating, suffocating odour similar to that of burning matches

Action on litmus paper:The gas prepared should bleach litmus paper but the colour reappears on exposure to the atmosphere;

Action on acidified KMnO₄: When the gas is passed through acidified potassium tetraoxomanganate(VII),KMnO₄, it should turn the colour of the reagent from purple to colourless.

Physical properties

1. The gas is colourless with irritating smell like that of burning matches
2. It bleaches litmus paper but colour reappears on exposure to air
3. It is very soluble in water
4. The gas is about 2.5 times denser than air

Chemical Properties

1. As an acid: The gas undergoes neutralization reaction with sodium hydroxide to form sodium trioxosulphate(IV) salt and water
   2NaOH + SO₂---->Na₂SO₃+ H₂O
2. Reaction with HNO₃acid:When it is passed through trioxonitrate(V)acid, thick reddish brown fumes of nitrogen (IV) acid can be readily observed
   2HNO₃+ SO₂---->H₂SO₄+ 2NO₂
3. As a reducing agent: It reduces purple reagent of acidified KMnO4 to the colourless Manganese ions as shown in the equation that follows:
   2KMnO₄+ 5SO₂+ 2H₂O-->K₂SO₄+ 2MnSO₄+ 2H₂SO₄
4. As bleaching agent: SO₂gas is a good bleaching agent because of its strong reducing power. The gas dissolves in the moist litmus or coloured substance to form H₂SO₃acid which then exerts the bleaching action on the dye. However, when compared to chlorine gas, the bleaching action is temporary in that the bleached dye reverts to the original gas when exposed to the atmosphere. While chlorine gas bleaches by oxidation, this gas (SO₂) does so by reduction process. Thus:
   H₂SO₃+ Dye(coloured)--->H₂SO₄+ Bleached Dye (colourless)

Uses of SO₂gas

1. The gas is used in the production of other chemicals, for example, it is an intermediate compound in the manufacture of H₂SO₄acid and sodium trioxosulphate (IV) that is used in turn for the making of papers.
2. It is a useful bleaching agent that is often used in the bleaching of oils, foods and textiles
3. SO₂gas is used as a food preservative
4. It is also a fumigant for the control of storage pests
5. SO₂is a refrigerant, just likeammonia

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Copyright ©Kingsley Idiagbor   | 5 September 2005-

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