O Level Revision : Combined Science - Science in Industry - Industrial Processes

For each process, the candidate should know the raw materials, conditions for reactions and the products and their uses.

Summary of the industrial processes

 Sulphuric acid

Important stages in the manufacture of sulphuric acid (Contact process)

a) Production of sulphur dioxide can be produced in two ways: (i)  by heating iron pyrites (iron sulphide)

(ii)  by burning sulphur in air.

Sulphur + oxygen  →  sulphur dioxide

b) Producing sulphur trioxide

Sulphur dioxide is oxidised by air to sulphur trioxide.

450 – 500Oc

Sulphur dioxide + oxygen      →   sulphur trioxide + heat vanadium (V) oxidecatalyst)

The reaction is reversible and exothermic. The use of a catalyst vanadium (v) oxide and a temperature of 450OC favours the forward reaction.

c) Absorption

The sulphur trioxide is dissolved in the concentrated sulphuric acid to form oleum, a fuming liquid ideal for transportation.

NB: Sulphur trioxide is not dissolved in water because the heat of reaction would produce a mist

of sulphuric acid which would be difficult to condense.

d) Dilution of oleum

Oleum is added to water to form concentrated sulphuric acid. Oleum + water              concentrated sulphuric acid

 Industrial gases

a) Important stages in the manufacture of ammonia (Haber process)

Ammonia is obtained by reacting hydrogen and nitrogen in the ratio 3:1.

450OC – 200 atmospheres

Nitrogen + hydrogen  →   ammonia + heat iron catalyst

The reaction between nitrogen and hydrogen is exothermic and reversible. So to favour a

Z-1 forward reaction the following conditions have to be maintained. (i)   Nitrogen and hydrogen are passed on over an iron catalyst.

(ii)  Temperature of 450°C to 500°C is used.

(iii) Pressure of 200 – 300 atmospheres increases the yield of ammonia.

The haber process

b) Other industrial gases

The raw materials are basically gases that are referred to as industrial gases. These gases include nitrogen, hydrogen, oxygen and sulphur dioxide. The gases are obtained from two important industrial processes called:

(i)   fractional distillation of liquefied air

(ii)  electrolysis of dilute sulphuric acid/acidified water.

(i)   Fractional distillation of liquefied air

Air is separated at Sable chemicals into oxygen and nitrogen. Air from the atmosphere is compressed and cooled until it is liquid. The liquid is distilled and the different fractions are obtained at different boiling temperatures:

Fractional distillation of liquified air

(ii)  Electrolysis of water

• From the electrolytic chemical decomposition of water, two constituent gases hydrogen and oxygen are obtained.

• Pure water is not a conductor and so cannot be an electrolyte. The use of acidified water increases condition of the electrolyte as more ions are provided. An electric current is passed through the acidified water.
• This breaks water into hydrogen ions and the oxygen ions.
• Hydrogen irons gain electrons at the cathode to become hydrogen gas. Oxygen is released at the anode after the hydroxyl ions give up electrons forming oxygen and water.

• The electrodes used should be inert - they should not decompose during electrolysis of the acidified water.
• Examples of such electrodes are made from carbon (very cheap) and platinum (expensive).

 Water electrolysis

The two gases can be distinguished using special tests with glowing and burning splints respectively. Do you remember the positive test result for each gas?

• Oxygen is used for steel making and in welding.
• Hydrogen is used in the manufacture of ammonia and hydrogenation of oil.
• Nitrogen is used in its gaseous form for the manufacture of ammonia, in medicine and as refrigerant.

The interdependence of Zimbabwe Iron and Steel Company (ZISCO), Zimbabwe

Electricity Supply Authority (ZESA) and Sable Chemical Industries.

ZESA supply electricity to Sables for electrolysis of water for the production of hydrogen and oxygen. The Sable Chemicals then supply oxygen to ZISCO which is used to purify iron through the oxygen lance process. Sables and ZESA would get their steel from ZISCO.

Manufacture of nitric acid

Ammonia is oxidised to form Nitric acid in the following equation; Platinum rhodium

Ammonia + oxygen →  nitrogen dioxide +  water + heat 900OC

The reactants are passed over a catalyst platinum rodium at temperatures of 900OC. An absorption tower allows reaction of nitrogen dioxide + water to be absorbed

Nitrogen dioxide + water +  oxygen →   nitric acid (dilute)

 The catalytic oxidation of ammonia

Nitric acid uses

Nitric acid is used in the manufacture of fertilisers, explosives and dyes.

The manufacture of ammonium nitrate fertilisers

It is manufactured through the neutralisation of nitric acid with ammonia.

Nitric acid  +  ammonia  → ammonia nitrate

A product in solution is then evaporated and crystals are formed which is ammonium nitrate fertiliser.