Oxoacids of sulfur

Sulfur forms many oxoacids, though few of them can be isolated in pure form. Most are prepared in aqueous solution or as crystalline salts of the corresponding oxoacid anions. Table 16.6.1 lists the common oxoacids of sulfur. 

Molecular structure of oxides of cyclic poly-sulfur (a) SgO, (b) S7O, (c) SgO, (d)S702,(e)Si202. 

Formula Name Oxidation Schematic structure Salt 

The hydrogen sulfate (or bisulfate) anion HSOJ exists in crystalline salts such as (H3OXHSO4), K(HS04) and Na(HS04). The bond lengths of HSO in (H30)(HS04) are S-O = 145.6 pm and S-OH = 155.8 pm. 

Disulfuric acid (also known as pyrosulfuric acid), H2S2O7, which is the major constituent of fuming sulfuric acid , is formed from sulfur trioxide and sulfuric acid  

Many of the sulfur oxoaeids and their salts are eonneeted by oxidation-reduetion equilibria some of the more important standard reduetion potentials are summarized in Table 15.19 and displayed in graphie form as a volt-equivalent diagram (p. 435) in Fig. 15.28. By use of the eouples in Table 15.19 data for many other oxidation-reduetion equilibria ean readily be ealeulated. (Indeed, it is an instruetive exereise to eheek the derivation of the numerieal data 

The formal interrelationship between the various oxoacids of sulfur can also be illustrated in a. scheme which places less emphasis on oxidation-reduction reactions but which is useful in suggesting possible alternative synthetic routes 

Likewise addition of SO3 to FI202, H2S and H2S generates the formulae of the other oxoacids as follows  

It should be emphasized that not all the processes in these schemes represent viable syntheses, and other routes are frequently preferred. The following sections give a fuller di.scussion of the individual oxoacids and their salts. 

The modem process uses a potassium-sulfate-promoted vanadium(V) oxide catalyst on a silica or kie,selguhr support. The SO2 is obtained either by burning pure sulfur or by roasting sulfide minerals (p. 651) notably iron pyrite, or ores of Cu, Ni and Zn during the production of these metals. On a worldwide basis about 65% of the SO2 comes from the burning of sulfur and some 35% by the roasting of sulfide ores but in some countries (e.g, the UK) over 95% conies from the former. 

Tkble 15.19 Some standard reduction potentials of sulfur species (25°, pH 0) 

We considered oxoacids in some detail in Section 11.4. This Section will now give us the opportunity to revise those rules and apply them to the sulfur oxoacids. With sulfur, remember, we have the added complication that an oxygen atom can be substituted by sulfur, giving a range of thioacids, for example thiosulfuric acid (12.18), dithionic acid (12.19) and polythionic acid (12.20). 

Sulfuric acid is manufactured commercially by the contact process (see also the Case Study Industrial Inorganic Chemistry). Sulfur dioxide mixed with air is passed over a catalyst (usually vanadium pentoxide, V2O5) whereupon it is oxidized to sulfur trioxide  

Because sulfur trioxide cannot be satisfactorily absorbed by water, it is treated with 98% concentrated sulfuric acid to form a fuming liquid called oleum, H2S2O7, which is then carefully diluted with water to give sulfuric acid  

The 19th century claim that there is no better barometer to show the state of an industrial nation than the consumption of sulfuric acid per head of population still holds true today. The chemical finds numerous applications, including the production of fertilizers, petrochemicals, dyestuffs and detergents. 

The dehydration of sucrose to elemental carbon by concentrated sulfuric acid (a) before and (b) after addition of acid. 

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Reactions with sulfides, polysulfides, sulfur oxides and the oxoacids of sulfur are complex and the products depend markedly on reaction conditions (see also p. 745 for blue crystals in chamber acid). Some examples are ... 

Sulfur forms several oxides that in atmospheric chemistry are referred to collectively as SOx (read sox ). The most important oxides and oxoacids of sulfur are the dioxide and trioxide and the corresponding sulfurous and sulfuric acids. Sulfur burns in air to form sulfur dioxide, S02 (11), a colorless, choking, poisonous gas (recall Fig. C.1). About 7 X 1010 kg of sulfur dioxide is produced annually from the decomposition of vegetation and from volcanic emissions. In addition, approximately 1 X 1011 kg of naturally occurring hydrogen sulfide is oxidized each year to the dioxide by atmospheric oxygen ... 

Oxides and Oxoacids of Sulfur Sulfur dioxide (S02) and sulfur trioxide (S03) are the most important of the various oxides of sulfur. Sulfur dioxide, a colorless,... 

Other lower oxoacids of sulfur including sulfane monosulfonic acids must be intermediates which then undergo reactions according to Eqs. (7)-(12). The freshly prepared sol is purified by precipitation with saturated aqueous NaCl, isolated by centrifugation, peptized in water and precipitated again. This procedure is repeated several times until no sulfate ions can be detected in the aqueous phase any longer. 

There are many other oxoacids of sulfur, of which the most important are peroxodisulfate j which has a peroxo (O—O) bond, and compounds with S-S bonds including thiosulfate dithionite and tetrathionate. The reaction... 

Write a brief account of the oxoacids of sulfur, paying particular attention to which species are isolable. 

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