Oxides, Oxoacids and Peracids of Sulphur

Sulphur forms two industrially crucial gaseous oxides. Sulphur dioxide, $\text{SO}_2$, is

obtained by burning sulphur (or iron pyrites, $\text{FeS}_2$) in air or oxygen, or in the

laboratory by treating a sulphite with dilute sulphuric acid:

Structurally, $\text{SO}_2$ is a bent (angular) molecule, sulphur being $sp^2$ hybridised

with one lone pair, giving a bond angle of about $119.5^\circ$; as with ozone, the two S-O bonds

are found to be of equal, intermediate length, described by resonance between two canonical

structures with the formal S=O double bond alternating between the two oxygens. $\text{SO}_2$ is

an acidic oxide (dissolving in water to give sulphurous acid, $\text{H}_2\text{SO}_3$) and,

because sulphur here is in the intermediate $+4$ oxidation state, it can act as either an oxidising or a reducing agent depending on what it reacts with; its reducing/bleaching action

(e.g. on flowers or straw) is reversible, since the colour returns on exposure to air/oxidation

-- in contrast with the permanent, destructive bleaching action of chlorine.

Catalytic oxidation of $\text{SO}_2$ with more oxygen, over a vanadium(V) oxide catalyst, gives

sulphur trioxide:

In the gas phase, $\text{SO}_3$ is a planar, trigonal molecule (sulphur $sp^2$ hybridised, no

lone pair, bond angle $120^\circ$, all three S-O bonds equivalent by resonance). This reaction is

the central step of the Contact process, the industrial manufacture of sulphuric acid:

the $\text{SO}_3$ produced is absorbed in concentrated sulphuric acid (rather than water

directly, which would form a hard-to-condense acid mist) to give oleum

(disulphuric/pyrosulphuric acid, $\text{H}_2\text{S}_2\text{O}_7$), which is then carefully

diluted with the correct amount of water to yield sulphuric acid of the desired concentration:

The $\text{H}_2\text{SO}_4$ molecule itself has sulphur $sp^3$ hybridised at the centre of a

roughly tetrahedral arrangement of two S=O and two S-OH groups.

Beyond ordinary sulphuric and sulphurous acids, sulphur also forms two important peroxoacids,

containing an unusual $-\text{O-O-}$ peroxide linkage in place of an ordinary S-O-S or S-OH …