The p-Block Elements

Oxoacids of Sulphur

What do you know about sulphur? In the previous chapters, we have looked at its configuration and properties. However, are you aware of what oxyacids of sulphur are? Okay, don’t be nervous! It’s nothing that is complicated. Well, in this chapter we will help you with the basics of oxyacids of sulphur. It is an interesting concept and we are sure you are going to enjoy looking at the various properties of these oxoacids.

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Oxoacids of Sulphur

As we have already mentioned, oxoacids are the acids that contain oxygen. A lot of experiments have shown sulphur to form many oxoacids. For example, these oxoacids are H2SO4, H2SO3, etc. You already knew about these acids, didn’t you? Yes! See, we told you it wasn’t that difficult!

In oxoacids, sulphur shows a tetrahedral structure with respect to oxygen. Generally, these oxoacids have a minimum of one S=O bond and one S-OH bond. We also observe terminal peroxide groups, terminal S=S, terminal and bridging oxygen atoms in these oxoacids. Let us now look at some of the most popular oxoacids and their properties.

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1) Sulphuric Acid, H2SO4

Sulphuric acid is possibly the most common oxoacid. We have seen a lot of applications of this acid, haven’t we? It is a diprotic acid. It signifies its property of ionising into two protons. In sulphuric acid, one atom of sulphur bonds to two hydroxyl groups. The other two oxygen atoms form pie bonds with the atom.

Thus, sulphuric acid exhibits tetrahedral geometry. As the bond length of the sulphur-oxygen bond (S=O) is quite less as compared to the bond length of S-OH, the oxygen atoms repel the OH groups. Hence, the bond angle of O=S=O bond is greater than the HO-S-OH bond angle. We produce it industrially by the contact process.

2) Sulphurous Acid, H2SO3

Sulphurous acid is a diprotic acid and thus, gives rise to two protons. In sulphurous acid, one atom of sulphur bonds with two hydroxyl groups. Also, one oxygen atom forms a pie bond with the sulphur atom. We prepare this by dissolving sulfur dioxide in water. As of this date, we do not have any evidence of the existence of sulphurous acid in solution phase. However, the molecule is capable to be isolated in its gaseous phase.

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3) Peroxodisulphuric Acid, H2S2O8

Peroxodisulphuric acid contains sulphur in +6 oxidation state. Thus, it acts as a very strong oxidizing agent. It is, also, very explosive in nature. We know it by the common name, Marshall’s acid.

It contains one peroxide group that acts as a bridge between the two sulphur atoms. Each atom connects to one hydroxyl group (S-OH bond) and two oxygen atoms (S=O bond) other than the peroxide group. We can prepare it by the reaction of the chlorosulfuric acid with hydrogen peroxide. The reaction is given as follows:

2ClSO3H    +    H2O2   →    H2S2O8    +   2HCl

4) Pyrosulphuric Acid, H2S2O7

Pyrosulphuric acid is also known as oleum. Its molar mass is 178.13 g/mol. It is an anhydride of sulphuric acid, is a colourless, crystalline solid and has a melting point of 36°C. It can be prepared by reacting excess sulphur trioxide with sulphuric acid. The reaction goes like this:

H2SO   +     SO3    →    H2S2O7

It reacts with bases to form salts which are called pyrosulphates. We use it in the manufacturing of explosives and dyes. it is also used in petroleum refining. It is a strong dehydrating agent and is corrosive in nature. It can cause burns on the skin and irritation to the eyes. Long exposure can prove to be fatal.

Solved Example for You

Q: What are thioacids and peroxy acids?

Ans: Oxoacids having S-S linkages are thioacids, while those with Peroxy linkages (O-O) are Peroxo Acids.

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