© 1997 Richard McLeod

SULPHUR AND ITS COMPOUNDS

Sulphur

Sulphur occurs as the free element chiefly in Texas and Louisiana, U.S.A. It is extracted by pumping super-heated water into the underground sulphur beds, which forces molten sulphur to the surface.

Sulphur occurs as the element in many volcanic areas, it also occurs naturally as metal sulphides (eg. iron pyrites) and sulphates (eg. gypsum, hydrated calcium sulphate). Crude oil and natural gas contain sulphur as an impurity. Since the sulphur must be removed anyway to reduce pollution, these are important sources of sulphur.

Sulphur is a yellow solid which can be easily melted. When heated in air or oxygen, it burns with a blue flame giving sulphur dioxide.

S + O₂ ® SO₂

Most sulphur is used to make sulphur dioxide which is then used mainly to make sulphuric acid. Some sulphur is used in the vulcanisation (hardening) of rubber for car tyres, as a fungicide, and in making gunpowder and matches.

Uses of Sulphur Dioxide

Sulphur dioxide is used mainly in the manufacture of sulphuric acid (see next page). Sulphur dioxide is also used to bleach the yellow dye in paper, straw and wool. It does this by reducing the natural yellow dye to a colourless compound. It is used in paper-making (sulphites are used to separate the cellulose from the rest of the wood), as a fumigant (gas for killing pests), and as a preservative for some foodstuffs.

Sulphur Dioxide as a Pollutant

Sulphur dioxide occurs in the waste gases from the burning of oil, coal and other substances containing sulphur. It is therefore often found as an air pollutant; especially around factories and cities. Sulphur dioxide forms tiny drops of sulphurous acid in moist air and some of this is oxidised to sulphuric acid. These compounds irritate the respiratory system, cause "acid rain", increase the weathering of buildings and the rusting of iron and steel, and kill fish in lakes.

SULPHURIC ACID

Sulphuric acid is manufactured by the Contact Process in the following steps:-

1. Sulphur dioxide is produced. This can be done in a number of ways eg. by burning sulphur in

air.

S + O₂ ® SO₂

also i) by burning hydrogen sulphide from crude oil and natural gas.

ii) by roasting sulphide ores such as zinc blende, ZnS, in air.

2. The sulphur dioxide is dried and purified (to protect the catalyst).

3. The sulphur dioxide is oxidised to sulphur trioxide in the following way:-

Sulphur dioxide mixed with excess air is passed over a vanadium (V) oxide catalyst at about

450^oC. The reaction is exothermic and no external heat is needed once it has started (this is

important economically because it greatly reduces fuel costs).

4. The sulphur trioxide is then converted to sulphuric acid.

H₂O + SO₃ ® H₂SO₄

This is NOT done directly by adding the sulphur trioxide to water, because the reaction is

violent and forms a great deal of acid mist. Instead the sulphur trioxide is dissolved in 98%

sulphuric acid forming the 100% acid (or with further absorbtion, a liquid called "oleum",

H₂S₂O₇). The 100% acid or the oleum may then be carefully diluted with water to give the

ordinary 98% concentrated acid.

Reactions of Sulphuric acid

Sulphuric acid is a strong acid, i.e. it is completly ionised in dilute solution (weak acids are only slightly ionised)

H₂SO₄ 2H⁺ + SO₄^2-

Dilute sulphuric acid has the following typical acid reactions:-

a) It turns litmus red.

b) It reacts with many metals forming a sulphate and hydrogen.

eg. Mg + H₂SO₄ ® MgSO₄ + H₂

c) It reacts with bases forming a salt and water.

eg. 2NaOH + H₂SO₄ ® Na₂SO₄ + 2H₂O

(When half-neutralised it forms the acid salt, sodium hydrogen sulphate, NaHSO₄)

d) It reacts with carbonates forming a sulphate, water and carbon dioxide.

eg. CuCO₃ + H₂SO₄ ® CuSO₄ + H₂O + CO₂

Uses of Sulphuric Acid

Sulphuric acid is mainly used in the manufacture of "superphosphate" fertilizer. The second biggest use is in the manufacture of ammonium sulphate fertilizer. The acid has many other uses such as making pigments (for paints), rayon, detergents and metal sulphates. It is the electrolyte in car batteries.

Economics of Sulphuric Acid Manufacture

Sulphuric acid is manufactured on a very large scale, and it is a relatively cheap chemical. To be competitive it must be produced as cheaply as possible. Manufacturing plants are therefore sited so that the raw materials:- air, water and sulphur dioxide are obtained very cheaply or free. To lower transport costs, the plants are sited close to major users of the acid (eg. fertilizer plants). To lower fuel costs, heat given out during the reaction is used to keep the catalyst at the right temperature.

Test for a Sulphate

To a solution of the substance add barium chloride solution. If there is a white precipitate, add dilute hydrochloric acid. If the white ppt. does not dissolve, the original substance is a sulphate.

Preparation of Sulphates

Most sulphates are prepared by the usual methods for preparing salts i.e. action of sulphuric acid on an insoluble base, metal or carbonate, filtering off the excess insoluble substance, then evaporating the filtrate to obtain crystals.

Sodium and potassium sulphates are prepared by titration. Lead and barium sulphates are insoluble. They are prepared by precipitation.

END OF BOOKLET

REVISE FOR SULPHUR TEST