CHEMISTRY OF SULPHUR.

(Inc. SO_2, SO_3, and H₂SO₄)

Sulphur is a yellow solid; it is a typical non-metal and is found in Group VII of the Periodic Table. Like carbon it can exist as allotropes. The three main allotropes of sulphur are rhombic, monoclinic and plastic.

Sulphur is extracted from the ground by the frasch process. The sulphur is heated under ground using super heated water. Hot compressed air is then used to force the molten sulphur to the surface.

Combustion of sulphur to form SO₂

S_(s) + O_2(g) SO_2(g)

The making of sulphuric acid in the contact process starts off with the burning of sulphur to give sulphur dioxide.

Reaction of sulphur dioxide with water

SO_2(g) + H₂O_(l) H₂SO₃ H₂SO₄ (if there is enough oxygen)

Sulphuric acid (H₂SO₄)

Sulphuric acid is a strong acid. This means that in solution it is fully ionised giving a high concentration of hydrogen ions.

Reaction 1:       Metal + Acid = Salt + Hydrogen

Ca + H₂SO₄ CaSO₄ + H₂

Reaction 2:       Acid + Base = Salt + Water

H₂SO₄ + CuO CuSO₄ + H₂O

Manufacture of sulphuric acid

Sulphuric acid is made by the contact process. Sulphur dioxide and oxygen combine in contact, with the catalyst vanadium (v) oxide.

1. The air and sulphur dioxide is mixed and cleaned by electrostatic precipitation, then dried, heated and compressed.
2. The mixture is then passed over the catalyst and sulphur trioxide is formed.

2SO₂ + O₂ 2SO₃

3. Sulphur trioxide reacts with water to give sulphuric acid.

SO₃ + H₂O H₂SO₄

Diluting concentrated sulphuric acid

When diluting concentrated sulphuric acid never add water directly to the acid because a lot of heat will be generated in the small volume of acid making water boil and splash out of the container bringing a shower of concentrated acid. Instead, add concentrated acid to a large volume of water so as heat is generated it will spread through the large volume of water.

N.B. Safety glasses should be worn at all times.

The Contact Process

The importance of sulphuric acid

Sulphuric acid is manufactured on a huge scale. The acid is used at some point in the manufacture of almost all products.

Sources of sulphur dioxide

Sulphur dioxide is needed to make sulphuric acid. In the UK, sulphur dioxide is made by burning sulphur. Liquid sulphur is sprayed into a furnace where it burns in dry air. The hot sulphur dioxide is cooled by being passed through a heat exchanger. The heat turns water into steam, which is then used to generate electricity. Saving energy in this way makes an important contribution to the economics of the process.

Sulphur is imported from Poland, Mexico and the USA, where it is extracted by the frasch process. Another source of sulphur is impure natural gas. Natural gas in France and Canada may be contaminated with up to 25% hydrogen sulphide. The hydrogen sulphide must be removed before the gas is used as a fuel.

Metals such as zinc, copper and lead are extracted from sulphide ores. These ores are converted to oxides by roasting them in air, e.g.

            2ZnS + 3O₂ 2ZnO + 2SO₂

Worldwide, about 40% of the sulphur dioxide used in the contact process comes from the roasting of sulphide ores.

Properties of sulphur dioxide

Sulphur dioxide:

            is a colourless gas

            has a pungent smell

            is much denser than air

            is very soluble in water

            is acidic

            does not burn or support combustion

            turns aqueous potassium dichromate from orange to green.

In the contact process, a mixture of sulphur dioxide and air is passed into a converter in which there are four layers of the catalyst. The catalyst, which is vanadium (v) oxide, is in the form of pellets to increase its surface area. The process gets its name from the contact between the gases and the catalyst.

The sulphur dioxide combines with oxygen in the air to form sulphur trioxide:

            2SO₂ + O₂ 2SO₃

This reaction is reversible and approaches a state of equilibrium.

Sulphur trioxide reacts with water to form sulphuric acid. This reaction is violent and gives out a lot of heat. In practice, sulphur trioxide cannot be combined directly with water because it reacts to form a mist of sulphuric acid droplets, which cannot be collected. They would escape from the plant and pollute the atmosphere. To get round this problem, the sulphur trioxide is absorbed in 98% sulphuric acid:

            SO₃ + H₂O (in 98% sulphuric acid) H₂SO₄

This happens in towers packed with ceramic rings. The gases pass up the tower and come into contact with a large surface area of 98 % sulphuric acid trickling down the tower. The acid concentration increases to about 99.5%. Water is then added to dilute the acid back to 98.5%.

            * SO₃ + H₂SO₄ H₂S₂O₇ (oleum)

            * H₂S₂O₇ + H₂O 2H₂SO₄

_{By Paul Farren: Loreto College, Coleraine.}