Chapter 5th:- Oxyacid of Boron, Phosphorus, and Sulphur 1st year Book
(Page 3)

Oxyacid of Sulphur

Sulphur forms a number of Oxyacid. Oxyacid with S-S links are called thio acids. Acids having Sulphur in the lower oxidation state have their names ending in -ous while those having Sulphur in higher oxidation state have names ending in -ic. Whereas their corresponding salts are ending in -ite and -ate respectively. Oxy acids having peroxy linkage are known as per acids or peroxo acids e.g., H2SO5, peroxomono Sulphuric acid, commonly known as Caro’s acid. In short, the oxy acids of Sulphur can be divided into following categories on the basis of their structures.

Classification of Oxyacids of Sulphur

  1. Sulphurous acid: H2SO3: It has no separate existence. It is the aqueous solution of SO2, which is known as Sulphurous acid.


  • When SO2 is dissolved in water gives H2SO3

Sulphurous Acid

  • H2SO3can also be prepared by the treatment of water with thionyl chloride SOCI2

Sulphurous acid


  • It is a dibasic acid and it forms two series of salts via bisulphites HSO3 (acid salts) and sulphites, SO3 (normal salts).
  • It acts as a strong reducing agent.

Bleaching Agent

Due to its reducing properties, Sulphurous acid acts as a bleaching agent.

It acts as a strong oxidizing agent, particularly when treated with strong reducing agents It itself is reduced to Sulphur e.g.

Bleaching Agent


Sulphurous acid is believed to exist in two Interconvertible forms, which are always in equilibrium with each other (see figure 7.12). The presence of S-H bond in unsymmetrical formula explains the reducing character of Sulphurous acid (like PH bonds in phosphorus acids which are also reducing agents.)

HdroxySufane dioxide

Sulphurous Acid                                HdroxySufane dioxide

(Symmetrical)                                    (Unsymmetrical)

Fig. 7.12: Structures of Sulphurous acid

Structure of Sulphite Ion, S032-:

It has a pyramidal structure having three oxygen atoms in a triangle and alone pair of electrons on one corner of a distorted tetrahedron. This shows that the Sulphur atom is sp2- hybridized. Since all the S-O bond in SO3 2- Ion are some (1.39Ǻ), so it is a resonance hybrid of the following three resonating structures:

Resonating Structures of Sulphite ion

Fig.7.13: Resonating structures of Sulphite Ion

  1. Sulphuric acid or oil of vitriol: H2SO4:


  • From green vitriol. FeSO4.7H2O: It is prepared by distilling green vitriol FeSO4.7H2O 

Sulphuric acid or oil of vitriol

  • Industrial methods: It is manufactured by two processes viz (a) lead chamber process and (b) contact process. The lead chamber process produces Sulphuric acid of about 98% strength while the Sulphuric acid produced by contact process is 100% pure.
  • Lead Chamber process in this process a mixture of SO2, and air is passed with steam (H2O) in presence of NO as a catalyst in lead chamber where H2SO4 is generated.

Lead Chamber


It involves the following steps:

Step 1: Generation of SO2, by burning S or FeS in excess of air:

Oxidation of SO2

Nitric oxide (NO) produced in the reaction takes up oxygen from the atmosphere and is converted into NO2, which is again used in the oxidation of SO2. This cycle goes on and the oxides of Nitrogen acts as a carrier of O2

By contact process (Direct method):

In this process SO2 is oxidized to SO3 by atmospheric O2 in the presence of some suitable catalyst (V2O5, F2O3 or pt.-asbestos) When SO3 is dissolved in 98% H2SO4. It gives oleum H2S2O7 (fuming Sulphuric acid) from which acid of desired concentration can be obtained by adding water.


It involves the following steps:

Generation of SO2


  • Anhydrous Sulphuric acid H2SO4 is an oily, thick liquid due to extensive intermolecular hydrogen bonding. It is colorless and if it is cooled to 0 °C. It gradually forms crystals which melt at 10.371 °C. Its density is 1.8269 and boiling point is 279.6 °C
  • A large amount of heat is produced when conc. H2SO4 is mixed with water. The evolution of heat is due to the formation of hydrates such as H2SO4.H2O; H2SO4.2H2`O and H2SO4.4H2O
  • Sulphuric acid is a strong dibasic acid it ionises as

Dibasic Acid

  • It is a powerful dehydrating agent and its corrosive action on skin is also due to dehydration of skin which then bums and produces itching sensation.
  • It can remove hydrogen and-OH group as HO molecule from several substances:

powerful dehydrating agent

  • Oxidising properties: H2SO4 is a powerful oxidising agent since it can lose nascent oxygen quite readily, when H2SO4 oxidises any substance. It is reduced to SO2, S, S2- or H2S

powerful oxidizing agent

Powerful Oxidizing Agent


  • It is regarded as the king of chemicals
  • It is used in the production of DOT by the action of chlorobenzene with trichloro acetaldehyde.

Chlorobenzene With Trichloro Acetaldehyde

  • It is used in refining petroleum
  • As a picking agent in industries for removing layers of basic oxides from metals like Fe and Cu before electroplating, enameling, galvanizing and soldering

Structure: H2SO4 has the following structure in which two oxygen atoms are found to be linked by means of double bonds in addition to two-OH groups

Structure of H2SO4

Fig. 7.14: Structures of H2SO4

The high boiling point (low volatility) and viscous nature of conc. H2SO4 is due to Hydrogen bonding in which a number of H2SO4 molecules are associated to form a clusture.

Hydrogen bonding in H2SO4

Fig. 7.15: Hydrogen bonding in H2SO4

  1. Caro’s acid or Peroxo mono Sulphuric acid: H2SO5:

Preparation: It is formed by the reaction of potassium per disulphate with Sulphuric acid.

potassium per disulphate

It is also prepared by the reaction of anhydrous H2O2 and chloro-sulphonic acid.

chloro sulphonic acid

It is clear from the above reaction that the structure of H2SO5 should be as:

Structure of Caro’s acid H2SO5

Fig. 7.16: Structure of Caro’s acid H2SO5

  1. Marshall’s acid or Peroxidisulphuric acid: H2S2O8 :

It is prepared by the reaction of two molecules of chloro sulphonic acid HSO3Cl and one molecule of H2O2.

Chlorosulphonic Acid

So, structure of this acid should be as follows:

Structures of Marshall's acid. H2S2O8

Fig. 7.17 Structures of Marshall’s acid. H2S2O8

  1. Thio Sulphuric acid: H2S2O3:

It is very unstable; it is stable only in solution. It is obtained by replacing an -OH group from H2SO4 by a–SH group.

Thio Sulphuric Acid

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