Contact Process

Manufacture of Sulphuric Acid

Sulphuric acid is made by the Contact process. The anhydride of sulphuric acid is sulphur(VI) oxide, SO3. It is made by the oxidation of sulphur(IV) oxide (sulphur dioxide), which is obtained from the following sources.

Sources of Sulphur(IV) Oxide

1. Sulphur is burned in air.2. Hydrogen sulphide from crude oil or

natural gas, is burned in air.3. Sulphide metal ores are roasted in air.4. CaSO4 is heated with coke to

manufacture cement, sulphur (IV) oxide is a by-product.

5. Flue-gas desulphurisation in coal-fired power stations.

FraschProcess

Contact ProcessWhen sulphur(IV) oxide and oxygen combine;

2SO2(g) + O2(g) 2SO3(g) H° = -197 kJ mol-1

Reaction ConditionsLe Chatelier’s Principle would favour a low temperature and a high pressure.However, at high pressures the sulphur(IV) oxide liquifies and at low temperatures the rate of reaction is too slow.

Addition of CatalystThe vanadium(V) oxide is so effective that a 95 % conversion is achieved at 450 °C and 2 atm.

Addition of CatalystThe catalyst is easily ‘poisoned’ by absorbing impurities. These block the active sites on the surface of the catalyst and impair its efficiency.A constant temperature of 450 °C is maintained by using heat exchangers to heat the incoming gases.

Conversion to Sulphuric Acid

The sulphur(VI) oxide is not absorbed in water because the reaction is so intensely exothermic as to vaporise the sulphuric acid formed.It can be safely absorbed in 98 % sulphuric acid to form fuming sulphuric acid, oleum, H2S2O7, which is then reacted with water to form the sulphuric acid.

EquationsSO3(g) + H2SO4(l) H2S2O7(l)H2S2O7(l) + H2O(l) 2H2SO4(l)

Uses of Sulphuric Acid

other usessteel picklingdyestuffs and intermediatesfibresplasticsdetergents and soapspaints and pigmentsgeneral chemicalsfertilisersmetal salts

Uses of Sulphur1. Used to vulcanise rubber (harden

the rubber).2. Used in gunpowder.3. Used to make matches.4. Used as a fungicide.

Uses of Sulphur(IV) oxide(Sulphur dioxide)

1. The manufacture of sulphuric acid.2. Used to makes sulphites SO3

2-, which are used to break down wood pulp and to bleach the wood pulp.

3. Used as a food preservative.4. Used to bleach straw, silk and wool.5. Used as a fungicide.