sulphur and its compounds

This unit highlights the preparation of sulphur dioxide in the laboratory and on a large industrial scale. Discusses the properties of sulphur dioxide and its uses such asbleaching of straws and sponges, fumigation of our houses to kill insects (i.e. black ants), preservation of some liquids (i.e. orange juice). Sulphur dioxide is also used in the manufacture of sulphuric acid. Sulphuric acid is widely utilized to make paints, plastics, detergents (i.e. Omo, Nomi, Aerial, etc.) and fertilizers.

Sulphur dioxide could be obtained on a large scale from burning sulphur that is itself obtained from natural gas, a component of crude petroleum. In Uganda, the discovery of oil in the western part of the country will in the near future become a source of sulphur.

Sulphur dioxide is a major pollutant of atmospheric air. It is toxic, causes respiratory ailments, damages plant life and corrodes metals. The gas is soluble in water forming what is known as ‘acid rain’. Acid rain erodes marble and limestone. In industries where this gas is evolved it is recommended to bubble the gas through an alkali to form sulphites and thus control its polluting effect on the environment.

Sulphites are useful as mild oxidising agents but more often they act as reducing agents. They are oxidised to sulphates by substances such as potassium manganate (VII), potassium dichromate (VI), chlorine, bromine, hydrogen peroxide, oxygen and iron (III) chloride.

A mixture of sodium sulphite and sulphur when heated react to form sodium thiosulphate. A solution of a ‘fixer’ containing sodium thiosulphate is added to developed films to remove unreduced silver ions to form a solublecomplex, which is washed away. This prevents unreduced silver ions from becoming reduced and darkening slowly over time. This is useful in photography, which is a very vital practical application.

USES AND PROPERTIES OF SULPHUR DIOXIDE

This sub-topic deals with

• Methods of preparing sulphur dioxide in the laboratory and on a large scale.


• The physical and chemical properties /reactions of sulphur dioxide gas.


• Uses of sulphur dioxide


• Preparation and properties of sulphite salts.

Demonstration Experiments:

(a) Laboratory methods of preparing sulphur dioxide

Sulphur dioxide in the laboratory is prepared by:
(i) Reacting dilute hydrochloric acid or dilute sulphuric acid with sodium sulphite crystals
Diagram of the apparatus used

Equation of reaction taking place

(ii) Heating a mixture of copper turnings and concentrated sulphuric acid
Diagram of Apparatus used is the same as the one above except a source of heat must be applied.

Equation of reaction taking place

Uses of sulphur dioxide:

Sulphuric acid used to make soap, paints and pigments

(i) Soap and Detergents

(ii) Fertilizers

(iii) Bleaching agent for wool, straws and sponges.

(iv) Preservative for some liquids e.g. orange juice.

(v) Preparation of calcium hydrogen sulphite used for bleaching wood-pulp in the manufacture of papers.

(vi) Rrefrigerant.

(g) Preparation of sulphite salts for example sodium sulphite and zinc sulphite.

Physical properties

Sulphur dioxide is:

(i) a colourless gas with an irritating smell.

(ii) Two and half times denser than air.

(iii) soluble in water forming an acidic solution.

Confirmatory test for sulphur dioxide

To confirm the presence sulphur dioxide, we use:

(i) Acidified orange potassium dichromate (VII) solution that changes from orange to green.

(ii) A purple solution of potassium manganate (VII) that turns to colourless.

Chemical properties (Reactions)

As a reducing agent:

Sulphur dioxide acts as a reducing agent by supplying electrons or gaining oxygen from other substances. Examples include:

The bleaching action: This is due to sulphurous acid which removes oxygen from the dye.
Equations of reaction

With acidified dichromate(VI) solution

Equation of reaction:

Observation:

Solution turns from orange to green.

With potassium manganate(VII) solution

Equation of reaction

Observation:
Solution turns from purple/pink to colourless

With iron (III) sulphate solution

Iron (III) ions in solution are reduced by sulphur dioxide to iron (II) ions.

Equation of reaction

Observation:

Solution turns from brown to green

With bromine solution

Bromine in reduced to bromide ions.

Equation of reaction

Observation:

Solution turns from brown to colourless

With iodine solution

Iodine is reduced to iodide ions.

Equation of reaction

Observation:

Solution turns from brown to colourless.

With sodium hydroxide solution

With a little sulphur dioxide bubbled through an excess of the alkali, a normal salt is formed but equimular quantities react to form an acid salt.

Equation of reaction

(ii) Examples in which sulphur dioxide acts as an oxidising agent include:

With burning magnesium ribbon

Equation of reaction

Observation:

Mixture of white and yellow solids deposited at the bottom and sides of the jar.

Explanation:

The burning metal gives out a lot of heat that decomposes the gas into its elements sulphur and oxygen. The metal then continues to burn in the oxygen formed.

With hydrogen sulphide gas

Equation of reaction

Observation:

Yellow solid of sulphur formed

Large scale production of sulphur dioxide gas

By burning sulphur in air

By burning iron (II) sulphide or zinc sulphide in air