Sulphuric acid in industry. Sulphuric acid is used in batteries, in cars and trucks, in paints, detergents, for making fibres and plastics and in the cleaning of newly made metals. Sulphur dioxide, air and water are the raw materials for the manufacture of sulphuric acid. The Production. Sulphur dioxide can be produced; -by burning sulphur which is obtained from underground deposits. The chemical equation for this reaction is; S(s) + O2 (g) ~ SO2(g). The reaction is exothermic and the heat released is used to melt the solid sulphur. Sulphur dioxide; SO2 is a colourless, dense gas with a choking smell. It is very soluble in water, 1 volume of water at 0° Centigrade dissolves about 80 volumes of the gas. It boils at - 10° C, and liquefies under three atmospheres at 20° C. It has a freezing point of 10.5° C, it decomposes on boiling to form a sulphur trioxide and steam. Pure sulphuric acid is covalent; its molecule having an approximately tetrahedral structure. The Contact process. Air and sulphur dioxide are heated to 450° Centigrade and passed over a vanadium (V) oxide catalyst. The reaction between oxygen in the air and the sulphur dioxide is shown in the following symbol equation: 2SO2(g) + O2(g) ~ 2SO3(g). The reaction is exothermic and reversible. The temperature and the pressure of the reactants is controlled to move the equilibrium position to the right so that a very large amount of sulphur trioxide is produced. Formation of sulphuric acid. As the reaction between sulphur trioxide and water is very violent the gas is dissolved in concentrated sulphuric acid. The product of this reaction is oleum, as the following equations show; sulphuric acid + sulphur trioxide ~ oleum. H2SO4(aq) + SO3(g) ~ H2S2O7 (l). The oleum is then added to the water to make sulphuric acid, as the following symbol equation shows;
H2S2O7(l) + H2O(l) ~ 2H2SO4(l). The sulphuric acid has a concentration of 98%. Acid production and pollution. In recent years the pollution of the air due to the release of sulphur dioxide not used in the reaction has been greatly reduced by making the manufacturing process more efficient. Properties of sulphuric acid. Sulphuric acid is a mineral acid. An acid is a substance which dissolves in water and releases hydrogen ions. In equation symbols, the H+ ion is called the ‘hydrated hydrogen ion’. H3O+ ion is called the ‘hydroxonium ion’. It is the presence of hydrogen ions which give an acid its properties. It is important to realise that the strength of the acid depends initially on its degree of ‘ionisation’ and not its concentration, although it is possible to have a concentrated weak acid with the same number of hydrogen ions in it as a very dilute strong acid. The amount of hydrogen ions present in an acid is measured on the ‘pH scale’. Universal indicator paper is used to identify the pH of an acid. The pH is less than 7, the weakest acid has a pH of 6 and the strongest a pH of 0. When sulphuric acid and water are mixed an exothermic reaction takes place. Water is less dense than sulphuric acid and if it is added to the acid it forms a layer on top. The heat from the reaction makes the water layer boil which causes drops of acid to rise into the air. To avoid this danger the acid should be added to the water. The mixture is heated as the acid sinks through the water but it does not boil. Sulphuric acid is manufactured by the contact process and can be used as a dehydrating agent such as demoisturising nitrogen, oxygen and chlorine gases, and removing water from a molecule of sucrose (a sugar) leaving carbon behind.