Sulphur dioxide, SO2, is a dense, colourless gas which is formed when many sulphur compounds are burnt in air. It is very toxic, and has a suffocating odour. It occurs naturally as one of the decay products of plant and animal matter, and also as one of the gases which are emitted from volcanoes (along with CO2 and H2S).
Since SO2 is produced when some of the sulphur-containing components in oil and coal are burnt, our power stations and factories release huge volumes of this toxic gas into the atmosphere. It is estimated that about 200 million tonnes of sulphur are released each year in this way, compared to 300 million tonnes from natural sources. This has serious environmental consequences, since the SO2 can dissolve in the water vapour in clouds to form dilute sulphurous acid (H2SO3) and sulphuric acid (H2SO4). This, along with nitric and nitrous acids which are produced in the same way from nitrogen oxides, forms the basis of 'acid rain', which then falls back to Earth killing and damaging forests and forests, polluting lakes, and corroding buildings and statues.
SO2 is not all bad. It has a number of beneficial uses as well. One of these is as a preservative in various types of foodstuffs. Under the correct conditions, SO2 can react with oxygen to produce, amongst other products, sulphur trioxide (SO3). This means that SO2 is quite effective at scavenging reactive oxygen-containing compounds that would otherwise oxidise the foodstuff, causing it to go stale prematurely. The colours and flavours of dried fruit and juices are preserved in this way by addition of small quantities of SO2. Similarly, SO2 is also used to prevent oxidation in winemaking and to prevent the growth of wild yeasts and bacteria which would sour the wine.
Most SO2, however, is used in the production of sulphuric acid, H2SO4. This is such an important acid that over 20 million tons of sulphuric acid is used each year in the US, mostly by the steel, fertiliser and petroleum industries. The preparation first requires the burning of sulphur to SO2, and this is then converted to SO3 by reaction with O2. This reaction has a very high activation energy and is quite slow even at high temperatures. However, the reaction has been made economically worthwhile by the introduction of a catalyst, such as finely divided platinum, which greatly increases the reaction rate. SO3 is also a gas, but it very reactive indeed, and so is dissolved in water almost immediately to form sulphuric acid.
Sulphuric acid is manufactured in larger quantities than any other acid. Almost every manufactured item in the modern world comes into contact with H2SO4 at some stage in its history. It is so important, that at one time the annual production of sulphuric acid was taken as a measure of the degree of industrialisation of a country, and earned it its nickname of the 'king of chemicals'. Sulphuric acid is familiar to us as the electrolyte in the lead acid batteries in automobiles.
Sulphuric acid is normally sold as a 98% mixture with water (18 Molar). The pure substance is obtained as an oily, viscous, colourless liquid by addition of sufficient SO3 to react with the remaining H2O. Addition of further SO3 to 100% H2SO4 gives fuming sulphuric acid, or oleum. This is a very strong acid which contains polysulphuric acids, such as H2S2O7 and H2S3O10 and H2S4O13.
The structure of sulphuric acid is considered to be a mixture of several rapidly interconverting equivalent forms:
Sulphuric acid is a strong dibasic acid, which means that both of its two hydrogens can be used to react with other chemicals, liberating the tetrahedral sulphate ion (SO42-) in the process.
