Chemistry · Ch 7 — The p-Block Elements

Oxoacids of Sulphur

7.16

Oxoacids of Sulphur

Sulphur forms a remarkably wide range of oxoacids, including $H_2SO_3$ (sulphurous acid), $H_2S_2O_3$ (thiosulphuric acid), $H_2S_2O_4$ , $H_2S_2O_5$ , the family $H_2S_xO_6$ (with $x = 2$ to $5$ ), $H_2SO_4$ (sulphuric acid), $H_2S_2O_7$ (pyrosulphuric acid, i.e. oleum), $H_2SO_5$ (peroxomonosulphuric acid), and $H_2S_2O_8$ (peroxodisulphuric acid). Not all of these are stable enough to be isolated as pure compounds — several exist only in aqueous solution or are known through their salts.

The structures of four representative oxoacids are shown in the figure:

Sulphurous acid ( $H_2SO_3$ ) — the sulphur atom carries one lone pair, is doubly bonded to one terminal oxygen, and is singly bonded to two $-OH$ groups, giving an overall pyramidal arrangement around sulphur (three bonding groups plus one lone pair).
Sulphuric acid ( $H_2SO_4$ ) — the sulphur atom is doubly bonded to two terminal oxygens and singly bonded to two $-OH$ groups, with no lone pair on sulphur, giving a tetrahedral geometry.
Peroxodisulphuric acid ( $H_2S_2O_8$ ) — two tetrahedral sulphur centres are not bonded directly to each other but are instead linked through a central peroxide bridge, $-O-O-$ ; each sulphur additionally carries two terminal (doubly bonded) oxygens and one $-OH$ group.
Pyrosulphuric acid, or oleum ( $H_2S_2O_7$ ) — two tetrahedral sulphur centres are linked through a single bridging oxygen atom (an $S-O-S$ linkage, with no peroxide bond); each sulphur again carries two terminal oxygens and one $-OH$ group. …

Figure 7.6Fig. 7.6: Structures of some important oxoacids of sulphur

What this figure shows. Four ball-and-stick structures arranged left to right, each with a central purple/lavender S sphere bonded via navy-blue lines to purple O spheres and to OH groups (labelled 'HO' or 'OH' in text next to a purple O sphere), with the common name and formula printed below each structure. (1) Sulphurous acid (H2SO3): S atom bears an orange/peach teardrop-shaped lone-pair marker (with two dots) at the top, one O atom bonded to the upper-right (double bond, terminal, no H), and two OH groups bonded at the lower-left and lower-bottom positions (labelled 'HO' and 'HO') — an overall pyramidal (3 bonding groups + 1 lone pair) shape. (2) Sulphuric acid (H2SO4): S atom bonded to two terminal O atoms at the top (both double bonds, no lone pair shown on S) and two OH groups at the bottom (labelled 'HO' and 'HO') — tetrahedral shape. (3) Peroxodisulphuric acid (H2S2O8): two separate tetrahedral S centres NOT directly bonded to each other, instead linked through a central -O-O- peroxide bridge (two O atoms bonded to each other in the middle, each also bonded to one of the S atoms). Left S: one terminal O double-bonded at top, one terminal O double-bonded to the left, one OH group at the bottom-left (labelled 'OH'), plus the bond to the bridging O. Right S: one terminal O double-bonded at top, one terminal O double-bonded to the right, one OH group at the bottom-right (labelled 'HO'), plus the bond to the bridging O. (4) Pyrosulphuric acid / Oleum (H2S2O7): two tetrahedral S centres linked through a single bridging O atom (no peroxide, just one O connecting the two S atoms directly, S-O-S). Left S: one terminal O double-bonded at top, one terminal O double-bonded to the left, one OH group a …

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