Sulphur dioxide

Preparation: Sulphur dioxide can be obtained on a large industrial scale by burning elemental sulphur in air, where about 6-8% is further oxidised to sulphur trioxide (S + O2 -> SO2; 2S + 3O2 -> 2SO3). It is also produced by roasting metal sulphide ores such as galena (PbS) and zinc blende (ZnS) in air (2ZnS + 3O2 --heat--> 2ZnO + 2SO2; 4FeS2 + 11O2 --heat--> 2Fe2O3 + 8SO2), the method by which most of the sulphur dioxide needed for sulphuric acid manufacture and other industrial purposes is generated. In the laboratory, sulphur dioxide is prepared by treating a metal or a metal sulphite with sulphuric acid (Cu + 2H2SO4 -> CuSO4 + SO2 + 2H2O; SO3^2- + 2H+ -> H2O + SO2).

Properties: sulphur dioxide is found in volcanic gases, and large quantities are released into the atmosphere from coal- and oil-burning power plants and copper-smelting operations, making it a significant air pollutant. It is a colourless gas with a sharp, suffocating odour, highly soluble in water, and about 2.2 times heavier than air; it can be liquefied (boiling point 263 K) under about 2.5 atmospheres pressure at 288 K.

Chemical properties: sulphur dioxide is an acidic oxide and dissolves in water to give the weak, unstable sulphurous acid (SO2 + H2O <=> H2SO3 <=> 2H+ + SO3^2-). It reacts with sodium hydroxide and sodium carbonate to give sodium bisulphite and sodium sulphite respectively (SO2 + NaOH -> NaHSO3; 2SO2 + Na2CO3 + H2O -> 2NaHSO3 + CO2, which with excess base gives Na2SO3 + H2O + SO2 on further reaction with NaHSO3). Sulphur dioxide can act as either an oxidising or a reducing agent depending on the partner reagent. As an oxidising agent it oxidises hydrogen sulphide to sulphur, being itself reduced (2H2S + SO2 -> 3S + 2H2O), and oxidises magnesium metal to magnesium oxide, itself being reduced to sulphur (2Mg + SO2 -> 2MgO + S). More commonly, since sulphur in SO2 (+4) can readily be oxidised further to +6, it acts as a reducing agent: it reduces chlorine to hydrochloric acid while itself being oxidised to sulphuric acid (SO2 + 2H2O + Cl2 -> H2SO4 + 2HCl), and it reduces potassium permanganate to Mn2+ and potassium dichromate to Cr3+ (2KMnO4 + 5SO2 + 2H2O -> K2SO4 + 2MnSO4 + 2H2SO4; K2Cr2O7 + 3SO2 + H2SO4 -> K2SO4 + Cr2(SO4)3 + H2O). On heating with oxygen in the presence of a vanadium pentoxide catalyst at about 450 °C, sulphur dioxide is oxidised to sulphur trioxide, the key step of the contact process for sulphuric acid manufacture (2SO2(g) + O2(g) --V2O5, 450°C--> 2SO3(g)). In the presence of moisture, sulphur dioxide has a temporary bleaching action on coloured substances such as wool, silk, sponges and straw: it forms sulphurous acid, which supplies nascent hydrogen that reduces (bleaches) the coloured compound to a colourless one (SO2 + 2H2O -> H2SO4 + 2[H]; coloured-X + 2[H] -> colourless-XH2); because the bleaching works by reduction rather than by destroying the chromophore outright, the bleached product slowly regains its original colour on standing in air as atmospheric oxyge …