Oxoacids are acids with oxygen in them. Sulphur forms different oxoacids like \(H_2SO_4\) \(H_2SO_3\). Oxoacids of Sulphur have at least one S=O bond and one S-OH bond. You must have probably heard that sulphuric acid is renowned as the “King of Chemicals” because of its numerous applications. 

Have you ever considered that sulphuric acid is an elemental acid? Sulphuric acid is one of the sulphur oxoacids. To know more about oxoacids of sulphur, read the below article.

Oxoacids of Sulphur Definition

Oxoacids are acids that contain oxygen. Oxoacids of sulphur are the chemical compounds that contain sulphur, oxygen and hydrogen. The most important industrially used oxoacid is sulphuric acid \(({{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_4}).\) Sulphur has a tetrahedral structure in oxoacids with respect to oxygen.

These oxoacids usually feature at least one \({\rm{S}}\,{\rm{ = }}\,{\rm{O}}\) bond and one \({\rm{S}}\,{\rm{ – }}\,{\rm{OH}}\) bond. Some of these oxoacids have terminal peroxide groups, terminal \({\rm{S}}\,{\rm{ = }}\,{\rm{S}}\) and terminal and bridging oxygen atoms. Let’s have a look at some of the most well-known oxoacids and their characteristics.

Sulphur has several oxoacids, some of which are only known via their salts. Some examples of oxoacids of sulphur are;

Name of the Oxoacids of Sulphur	Formula	Structural formula
Sulphurous acid	\({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}\)
Sulphuric acid	\({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)
Permonosulphuric acid	\({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_5}\)
Perdisulphuric acid	\({{\rm{H}}_2}{{\rm{S}}_2}{{\rm{O}}_8}\)
Thiosulphuric acid	\({{\rm{H}}_2}{{\rm{S}}_2}{{\rm{O}}_3}\)
Dithionic acid	\({{\rm{H}}_2}{{\rm{S}}_2}{{\rm{O}}_6}\)
Pyrosulphuric acid	\({{\rm{H}}_2}{{\rm{S}}_2}{{\rm{O}}_7}\)

Oxoacids of Sulphur

The oxoacids of Sulphur are:

Sulphurous Acid \(\left( {{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_3}} \right)\)

This acid is obtained by dissolving SO₂ gas in water; the reaction is represented as

This acid is only known in solution. It is a fairly strong acid that ionises into two stages due to its diprotic nature.

Structure of Sulphurous Acid

The structure of the unionised acids is highly speculative. It has been proposed that the molecule has a pyramidal structure, as depicted. In this case, sulphur uses three of the four \({\rm{s}}{{\rm{p}}^3}\) hybrid orbitals for sigma bonding, while a single pair of electrons occupy one \({\rm{s}}{{\rm{p}}^3}\) hybrid orbital. There is also a d-p bond between the \({\rm{S}}\) and ({\rm{O}}\).

Properties of Sulphurous Acid

1. Because it is easily oxidised into sulphuric acid, this acid works as a strong reducing agent even by ambient oxygen. As a result, halogens are reduced to hydracids, ferric salts are reduced to ferrous salts, and so on.
2. It also acts as acts a bleaching agent.

Sulphuric Acid \(\left( {{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}} \right)\)

Sulphuric acid is one of the most important chemicals and is known as the “King of Chemicals.” It is also known as the oil of vitriol because it was traditionally made from green vitriol. It is a mineral acid made up of elements such as sulphur, oxygen and hydrogen, with the molecular formula \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\). It is a colourless, odourless and viscous liquid that is miscible with water at all concentrations. Sulphuric acid is very important commercially and is used in almost all industries in some form or another.

Structure of Sulphuric Acid

Two hydrogen atoms are intimately connected to two oxygen atoms in sulphuric acid, generating two \({\rm{ – OH}}\) groups. The molecule has a tetrahedral structure and is a covalent compound.

Manufacture of Sulphuric Acid

On the commercial scale, sulphuric acid was earlier prepared by the lead chamber process, but it is mainly manufactured by contact process.

Contact Process

Manufacture of sulphuric acids involves the three steps;

1. Production of \({\rm{S}}{{\rm{O}}_2}\) by burning sulphur or roasting iron pyrites

2. Sulphur dioxide is converted into sulphur trioxide by the reaction with oxygen in the presence of a catalyst \(({{\rm{V}}_{\rm{2}}}{{\rm{O}}_5})\).

3. Sulphur trioxide is absorbed in 98 per cent sulphuric acid to produce oleum.

The reaction is exothermic, reversible and the forward reaction leads to a decrease in volume. As a result, low temperature and high pressure are ideal conditions for maximising yield. However, the temperature should not be too low; otherwise, the rate will slow.

In actuality, the plant is run at \(2\) bar of pressure and \(720\,{\rm{K}}\) of temperature. To make oleum, the \({\rm{S}}{{\rm{O}}_3}\) gas from the catalytic converter is absorbed in concentrated \({{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}.\) The appropriate concentration of \({{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}\) is obtained by diluting the oleum with water. In the industry, two stages are carried out simultaneously to keep the process running smoothly and cut costs. As a result, the sulphuric acid obtained by the contact process is \(96\, – \,98\,\% \) pure.

The plant used in the process is consists of the following parts:

1. Sulphur burners: Here, sulphur dioxide is produced by roasting of pyrites or by burning sulphur.
2. Purification unit: It consists of the following parts;
   a. Dust remover: Dust is removed either by blowing steam or by using Cottrell electrical precipitator.
   b. Washer: Here, soluble impurities are removed by washing gases with water.
   c. Drier: A spray of concentrated sulphuric acid is used as a drying agent.
   d. Arsenic purifier: In this unit arsenic purifier containing gelatinous hydrated ferric oxide is used to absorb impurities.
3. Catalytic convertor: The pure gases from the testing box are preheated to \(720\,{\rm{K}}\) in a preheater. The gases are then fed via catalyst consisting of platinised asbestos or \({{\rm{V}}_{\rm{2}}}{{\rm{O}}_5}.\) \({\rm{S}}{{\rm{O}}_2}\) is oxidised to \({\rm{S}}{{\rm{O}}_3}\) at these temperatures, and the heat released in the reaction is used to warm the gases in the preheater.
4. Absorption tower: At the bottom of the absorption tower, \({\rm{S}}{{\rm{O}}_3}\) from the catalytic converter is inserted, from which \({{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}\) \((98\,\% )\) is sprayed. As \({\rm{S}}{{\rm{O}}_3}\) rises, it is absorbed by \({{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}\) resulting in the formation of oleum.

Property of Sulphuric Acid

The properties of Sulphuric acid are:

Physical Properties of Sulphuric Acid

1. Pure sulphuric acid is colourless, dense, oily liquid with a specific gravity of \(1.84{\mkern 1mu} \,{\rm{at}}\,{\rm{288}}{\mkern 1mu} \,{\rm{K}}{\mkern 1mu}.\)
2. Sulphuric acid is soluble in water. When sulphuric acid is mixed with water, a significant amount of heat is produced and a decrease in volume. The heat generated during the diluting of concentrated sulphuric acid has the potential to cause accidents. To avoid accidents, the concentrated acid should always be mixed with water and not water to the acid.
3. It forms a constant boiling mixture.
4. It fumes strongly in moist air.
5. It causes severe burns when brought in contact with the skin.

Chemical Properties of Sulphuric Acid

1. Dissociation: When water-sulphuric acid is boiled, it dissociates to produce sulphur trioxide and water.

2. Acidic Character: It is a dibasic acid and turns blue litmus to red. When it reacts with alkali, it forms two series of salts.

For, example,

3. Oxidising action: Sulphuric acid acts as a moderately strong oxidising agent because, on thermal decomposition, it produces nascent oxygen

For example, concentrated sulphuric acid oxidises metals like copper, mercury, etc., first to their oxides and then to sulphates.

4. Reaction with salts: It has moderate volatility and decomposes the salts of more volatile acids (e.g., chlorides, nitrates, sulphides, carbonates, etc.) to release the acids.

For example;

5. Sulphonating action: Many organic molecules, such as benzene, toluene, and others, react with concentrated sulphuric acid to form sulphonic acids.

For example,

6. Affinity towards water: Sulphuric acid has a high attraction to water. It releases heat and creates hydrates when dissolved in water. It can use this characteristic to extract water molecules from a variety of substances. As a result, sulphuric acid is an excellent dehydrating agent. The reactions that follow demonstrate how dehydrating it is.

For example,
(a) It removes water from ethyl alcohol and converts it to ethylene.

(b) The substances like sugar, wood, paper, etc., get charred by the action of concentrated sulphuric acid and a black mass is obtained because here concentrated sulphuric acid is acting as a strong dehydrating agent.

Uses of Sulphuric Acid

Sulphuric acid is one of the most significant compounds, with applications in practically every industry:

1. It is used in the production of fertilisers such as ammonium sulphate, superphosphate of lime, and others;
2. It is used in the production of dyes, explosives, and pharmaceuticals;
3. It is used in the production of acids such as \({\rm{HCl,}}\,{\rm{HN}}{{\rm{O}}_3}\) and others;
4. It is used in the production of a vast number of other chemicals.
5. It is used in galvanising, enamelling and metallurgical operations.
6. It is used in the leather industry, storage cells, petroleum industry.
7. It is used as a dehydrating agent and laboratory reagent.

Dithionic Acid, \({{\rm{H}}_{\rm{2}}}{{\rm{S}}_{\rm{2}}}{{\rm{O}}_{\rm{6}}}\)(Oxoacid of sulphur containing s-s bond)

The oxoacid of sulphur that contains the s-s bond is Dithionic acid. It is known only in solution. The concentrated aqueous solution of dithionic acid can be prepared by treatment of the barium salt with sulphuric acid. The salts of dithionic acid, viz., dithionates are thermally stable above room temperature and are readily soluble in water.

Structure

Dithionic acids have been assigned the structure, as shown, in which the bond angles are approximately equal to the normal tetrahedral angle.

Oxoacids of Phosphorus

Oxoacids of Phosphorous are chemical compounds that consist of atoms of phosphorous, oxygen and hydrogen.

Summary

Oxoacids of sulphur are the chemical compounds that contain sulphur, oxygen and hydrogen. Sulphuric acid \(\left( {{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}} \right)\) is one of the most important chemicals and is known as the “King of Chemicals”. It is used in the production of dyes, explosives, and pharmaceuticals.

FAQs

Q.1. Which Oxoacids of Sulphur has SOS link how it is prepared?
Ans: Pyrosulphuric acid has an \({\rm{SOS}}\) link. Pyrosulphuric acid is also known as oleum. It is prepared by dissolving sulphur trioxide in concentrated sulphuric acid.

Q.2. How many Oxoacids does Sulphur have?
Ans: Sulphur has a number of oxoacids, some of which are only known via their salts. 

Q.3. How do you name Oxoacids of Sulphur?
Ans: Name of the oxoacids of sulphur are Sulphurous acid, Sulphuric acid, Permonosulphuric acid, Perdisulphuric acid, etc.

Q.4. Which Oxoacid of Sulphur has Peroxy linkage?
Ans: Peroxy linkage is the linkage of the two oxygen atoms (O-O). Perdisulphuric acid has peroxy linkage.

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