Sulphuric acid is an extremely important industrial chemical. It is called the ‘King of Chemicals’ because it is used in the manufacture of a large number of substances, e.g., fertilizers such as ammonium sulphate, paints and pigments, synthetic fibres, soaps and detergents, etc.
Sulphuric acid is manufactured by the Contact process. In this process, sulphur dioxide is produced by burning sulphur in air. Sulphur dioxide is then mixed with more air and passed over a catalyst, vanadium(V) oxide (V2O5), heated to 723 K. This converts sulphur dioxide to sulphur trioxide (SO3). The SO3 is then dissolved in concentrated sulphuric acid, instead of water, to form a thick fuming liquid called oleum (H2S2O7). If SO3 is dissolved in water, a highly corrosive mist of sulphuric acid will be formed. Oleum is then diluted with water to get sulphuric acid of desired strength. The following reactions take place during the manufacture of sulphuric acid by Contact process.
1. Properties of Sulphuric Acid
Sulphuric acid is used in two forms: concentrated sulphuric acid and dilute sulphuric acid.
Concentrated sulphuric acid is a colourless oily liquid. It is about 98% H2SO4 and 2% water. It should be handled very carefully because it is a powerful dehydrating agent. It can dehydrate, i.e., can remove water or elements of water from many substances such as sugar, paper, wood, copper sulphate, etc.
Dilute sulphuric acid is about 10% H2SO4 and 90% water. It must always be prepared by adding conc. H2SO4 to water. Water should never be added to concentrated H2SO4. Why? Because when water and concentrated H2SO4 are mixed, a large amount of heat is evolved. But when water is added to the acid, the heat produced is so large that it could throw out the drops of acid and burn you. The dilute acid shows typical properties of acids. For example: It turns blue litmus red; it reacts with metals above hydrogen in the activity series to give hydrogen and salts called sulphates.
Sulphuric acid (H2SO4) is a dibasic acid, i.e., it contains two replaceable hydrogen atoms. It reacts with alkalis to form acid salts called hydorgensulphates, and normal salts called sulphates.
2. Uses of Sulphuric Acid
Every year, about 130 million tonnes of sulphuric acid are produced. In fact, it is the cheapest acid and is used by nearly every industry. It is used in the manufacture of fertilizers, plastics and fibres, paints and pigments, dyestuffs, soaps and detergents.