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Sulphur(IV) oxide, SO₂, its Preparation and Properties

Sulphur(IV) oxide is also commonly known as sulphur dioxide. It is a chemical compound whose molecule consists of one sulphur atom and two oxygen atoms, resulting in the formula SO₂.

Note that sulphur is also spelt as sulfur, therefore, sulphur dioxide may also be written as sulfur dioxide.

Preparation of Sulphur(IV) oxide, SO₂

SO₂ is prepared in the laboratory by heating a mixture of copper and conc. H₂SO₄.

The gas (SO₂) is dried by passing it into a solution of conc. H₂SO₄.

Cu_(s) + 2H₂SO_4(l) → CuSO_4(aq) + 2H₂O_(l) + SO_2(g)

Test for SO₂

Here are some tests that can be carried out to distinguish SO₂ from other substances:

(1). It has an irritating or pungent or choking smell and taste metallic.

(2). It decolourises KMnO₄ due to the fact that it reduces the KMnO₄ to colourless compound, without the precipitation of sulphur (unlike H₂S).

(3). SO₂ turns acidified K₂Cr₂O₇ solution from golden yellow to green.

Properties of Sulphur(IV) oxide

Below are important properties which SO₂ exhibits:

(1). It is a colourless gas with an irritating smell.

(2). It has somewhat a sweet taste.

(3). It is fairly poisonous and is used for fumigation.

(4). It is readily soluble in water, and the solution turns moist blue litmus paper red, which indicates that it is acidic.

(5). As a reducing agent - in the presence of water, SO₂ acts as a powerful reducing agent.

This is so because it forms the trioxosulphate(IV) ion, SO₃^2- when it reacts with water. The SO₃^2- suppliers electrons to the oxidizing agent it reacts with.

H2O(l) + SO2(g)

H2SO3(aq)

2H+ + SO32-(aq)

SO₃^2-_(aq) + H₂O_(l) → SO₄^2-_(aq) + 2H⁺ + 2e^-

Examples:

Iron(III), Fe³⁺ is reduced to iron(II), Fe²⁺.

Chlorine is reduced to chloride ions, Cl₂ + 2e^- → 2Cl^-

(6). As a bleaching agent

The bleaching behaviour of sulphur(IV) oxide is actually due to its reducing properties.

Example, in the presence of water, SO₂ bleaches sponges, dyes and straw hats. The trioxosulphate(IV) acid formed between SO₂ and water takes up oxygen from colouring matter (i.e. sponges, dyes and straw hats), converting them to colourless (reduced) compounds.

The reduced compounds may be reconverted to the original compounds through oxidation process by exposing them to the oxygen of the air.

This is why bleached straw hats gradually become yellow with time (when exposed to air).

Note: bleaching action is a reduction process.

Uses of Sulphur(IV) oxide

SO₂ may be used in the following ways:

(1). As bleaching agent in the industry to bleach items such as straw hats, sponges and dyes.

(2). In the manufacturing of H₂SO₄.