Study of Sulphuric Acid

Sulphuric acid is manufactured by the Contact process. First sulphur (or a sulphide ore) is burnt in air to give sulphur dioxide: S + O2 gives SO2. This SO2 is purified and then oxidised to sulphur trioxide over a catalyst of vanadium pentoxide (V2O5) at about 450 degrees Celsius and 2 atmospheres: 2 SO2 + O2 gives 2 SO3. This step is reversible, so moderate temperature and the catalyst are chosen to get a good yield. The SO3 is not passed into water directly, because that produces a thick acid mist that is hard to condense. Instead SO3 is absorbed in concentrated sulphuric acid to form oleum (fuming sulphuric acid, H2S2O7), which is then diluted carefully with water to give sulphuric acid of any required strength.

Dilute sulphuric acid is a typical strong, dibasic acid. It turns blue litmus red and reacts with active metals to give hydrogen, with bases and metal oxides to give sulphates and water, with carbonates and bicarbonates to release carbon dioxide, and with metal sulphides to release hydrogen sulphide. Because it is dibasic it can form two kinds of salts: normal sulphates (such as Na2SO4) and acid sulphates or bisulphates (such as NaHSO4). Dilute H2SO4 does NOT react with copper, because copper is below hydrogen in the reactivity series and dilute acid cannot oxidise it.

Concentrated sulphuric acid has a powerful affinity for water, so it can pull the elements hydrogen and oxygen out of a compound in the ratio of water (2:1), leaving the rest behind. This is dehydration, and it works even when there is no actual water present. Dropped onto sugar (C12H22O11) it removes all the hydrogen and oxygen as water and leaves a spongy black mass of carbon. It chars blue copper sulphate crystals (CuSO4.5H2O) to white anhydrous CuSO4 by removing the water of crystallisation, and turns moist blue litmus and paper black. It is also why concentrated H2SO4 is used as a drying agent for gases that do not react with it.

Hot concentrated sulphuric acid is an oxidising agent because on heating it gives up nascent oxygen: H2SO4 gives H2O + SO2 + [O]. This nascent oxygen oxidises metals and non-metals. With copper, hot conc. acid gives copper sulphate, sulphur dioxide and water: Cu + 2 H2SO4 (conc., hot) gives CuSO4 + SO2 + 2 H2O. With carbon it gives carbon dioxide, sulphur dioxide and water: C + 2 H2SO4 (conc., hot) gives CO2 + 2 SO2 + 2 H2O. With sulphur it gives sulphur dioxide and water. In every case the acid is reduced to SO2, which is the tell-tale sign of its oxidising action.