Oxoacids are acids that contain oxygen. Sulphur is known to form many oxoacids such as H2SO4, H2SO3, etc. In oxoacids of sulphur, sulphur exhibits a tetrahedral structure when coordinated to oxygen. Generally, oxoacids of sulphur contain at least one S=O bond and one S-OH bond. Terminal peroxide groups, terminal S=S, terminal and bridging oxygen atoms and chains of (-S-)n are also observed in addition to S=S and S-OH. Some popular oxoacids of sulphur and their properties are listed below:
Sulphuric acid, H2SO4
Sulphuric acid is one of the most popular oxoacids of sulphur. It is a diprotic acid (it ionizes to give two protons). In sulphuric acid, one atom of sulphur is bonded to two hydroxyl groups and the rest two oxygen atoms form pie bonds with the sulphur atom. Thus, sulphuric acid exhibits tetrahedral geometry. As the bond length of sulphur oxygen bond (S=O) is smaller in comparison to the bond length of S-OH, OH groups are repelled by the oxygen atoms. Hence, the bond angle of O=S=O bond is larger than the HO-S-OH bond angle. Industrially, it is produced by the contact process.
Sulphurous acid, H2SO3
Sulphurous acid is a diprotic acid that is, it ionizes two protons. In sulphurous acid, one atom of sulphur is bonded to two hydroxyl groups and one oxygen atom forms a pie bond with the sulphur atom. It is prepared by dissolving sulphur dioxide in water. Although, we don’t have any evidence of the existence of sulphurous acid in solution phase, the molecule can be isolated in the gaseous phase.
Peroxodisulphuric acid H2S2O8
Peroxodisulphuric acid contains sulphur in +6 oxidation state. Thus, it is a strong oxidizing agent and highly explosive in nature. It is popularly known as Marshall’s acid. It contains one peroxide group forming a bridge between the two sulphur atoms. Each sulphur atom is connected to one hydroxyl group (S-OH bond) and two oxygen atoms (S=O bond)other than the peroxide group.
It is prepared by the reaction of the chloro sulphuric acid with hydrogen peroxide:
2ClSO3H + H2O2 → H2S2O8 + 2 HCl
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