Sulphur occurs widely in nature as sulphates, sulphides, and from natural wastes as hydrogen sulphide, H2S, and sulphur dioxide, SO2. Selenium and tellurium are present in small amounts; often as impurities in metal sulphide ores. Polonium is a product of the radioactive decay of uranium and thorium. So the chemistry discussed here will be essential that for sulphur.
Sulphur, selenium and tellurium will not react with acids. Polonium will dissolve in sulphuric acid and hydrochloric acid in accord with its metallic behaviour.
Since the Group is very much to the right of the Periodic Table we would expect the elements to readily accept electrons to form elemental ions, like the halides. But this is not so as their electronegativities are not high enough. The only notable elemental ions known are the chalconide, S‾� and Se‾� (the equivalent to an oxide), and only with the more electropositive metals. The M2Te is unstable.
These metal chalconides can react with acids to give the simple hydrides. Example; hydrogen sulphide, H2S (the equivalent to water). The hydrides are all gases with nasty odour and are poisonous. The hydrides when dissolve in water give a weak acid. Example,
Their low electronegativities also do not allow them to take part in hydrogen bonding, like oxygen.
For these elements covalent bonding is the standard form of interaction. The maximum coordination number is not limited to two or four, since the d orbitals are available for bonding.
Another interesting chemistry is the formation of polysulphide bonds. Elemental sulphur is actually present as a S8 cyclic molecule. For example, beside hydrogen sulphide there are also other members like H-(S)n-H, where n is not greater than 6. These are known as sulfanes. Such catenation is not important for the other congeners.
OXIDES & ACIDS
Sulphur, selenium and tellurium burn in air to give the dioxides; SO2, SeO2 and TeO2. Sulphur dioxide is a gas, selenium dioxide a volatile solid and tellurium dioxide a non-volatile solid.
Sulphur dioxide dissolves in water to give sulphurous acid, often represented as H2SO3. In reality it is a mixture of H3O+ and HSO3‾. The alkali bisulphites, HSO3‾ and suphites, SO3‾� are well known.
Although sulphur dioxide dissolves in water to give sulphurous acid, by itself it can behave as a Lewis base via its lone electron pair.
Sulphur dioxide and the sulphites are good reducing agents in basic solutions.
Sulphur dioxide can further be oxidised to sulphur trioxide, SO3. Sulphur trioxide dissolves in water to give sulphuric acid. When concentration sulphuric acid is saturated with sulphur trioxide it gives oleum, or fuming sulphuric acid.
Oleum is then diluted to generate more sulphuric acid. This is used in the commercial preparation of sulphuric acid; the most important commercial acid.
Concentrated sulphuric acid is a strong dehydrating agent. It will char most carbohydrates by the removal of its water content. So when handling concentrated sulphuric acid you must certainly wear a pair of rubber gloves and a plastic apron.
Sulphuric acid is a strong bibasic acid. Sulphates and bisulphates of all electropositive elements are known. Most are soluble in water except for the Group IIA elements of Ca, Sr, Ba, and Ra and a few other divalent cations.
Sulphur trioxide itself is a Lewis acid. This is because the electronegative oxygen partners "sponged" off the electrons in the covalent bonds leaving the sulphur atom a little electron deficient. So commercially sulphur trioxide is available as sulphur trioxide pyridine, [C5H5N.SO3] complex or sulphur trioxide triethylamine [(CH3CH2)3N.SO3] complex.
Selenium dioxide dissolves readily in water to give selenous acid, OSe(OH)2. Tellurium dioxide is insoluble in water. It can dissolve in strong bases to give the tellurites.
Selenium trioxide, SeO3, reacts readily with water to give the selenic acid. Tellurium dioxide, TeO3, reacts slowly with water to give telluric acid, Te(OH)6. However it dissolves in strong base to give the tellurate ion.
PEROXO ACIDS
Two peroxo acids of sulphur - peroxomonosulphuric acid, H2SO5, and peroxodisulphuric acid, H2S2O8 - are better known. The other congeners do not form peroxo acids.
The peroxodisulphate is an oxidising agent and a free radical generator.
Peroxomonosulphuric acid, also known as Caro's acid, is prepared from the hydrolysis of the peroxodisulphuric acid.
THIO ACIDS
Sulphur compound with a -S-S- unit is named with the prefix thio. Sulphur in Greek is theion.
Thiosulphuric acid, S-S(O)(OH)2, is not very stable, so the thiosulphate is prepared by the reaction of the sulphite with a dithionite. The dithionite is prepared by reacting the sulphite with an excess of sulphur.
Sodium thiosulphate, Na+ S-S(O)(OH)O‾, can react with dissolve silver bromide from an emulsion to form the [Ag(S-SO3)]‾ complex. This reaction is used in the development of photographic films.
Sodium thiosulphate is also used for the quantitative analysis of hydrogen peroxide and iodine. The reactions are
The S4O6‾� is a trithionate ion with structural formula of [O(O)2S-S-S-S(O)2O]‾�.
Sodium dithionite, Na2+[OS(O)-S(O)O]‾� is a good reducing agent in an alkaline solution.
Summary of important oxo acids of sulphur
| ACID | Molecular formula | Structural formula | Salts |
|---|---|---|---|
| Sulphuric acid | H2SO4 | O2S(OH)2 | Sulphate |
| Thiosulphuric acid | H2S2O3 | OS(OH)2-S | Thiosulphate |
| Dithionous acid | H2S2O4 | OS(OH)-S(O)(OH) | Dithionite |
| Peroxodisulphuric acid | H2S2O8 | O2S(OH)-O-O-S(O)2(OH) | Peroxodisulphate |
THIONYL CHLORIDE
Thionyl chloride, SOCl2, is both a weak Lewis base (via its lone electron pair) and a weak Lewis acid (via its vacant d orbital). It decomposes on heating to give the sulphur dioxide and atomic chlorine. So it is an important chlorination reagent, especially in organic chemistry.
