Introduction

An oxoacid is an acid containing oxygen,hydrogen and at least one other element, with at least one hydrogen atom bound to oxygen that can dissociate to produce H+ cation and the anion of acid. 

Many of the oxoacids of sulphur do not exist as free acids, but are known as anions and salts. Acids ending in -ous have S in the oxidation number(+4), and form salts ending in -ite.Acids ending in – ic have S in the oxidation number (+6) and form salts ending in -ate. 

Due to sulphur having p𝚷-d𝚷 bonding,the oxyanions have strong 𝚷 bonds and so they have little tendency to polymerize compared with the phosphates and silicates. 

Oxo-acids having S-S linkage are called thio-acids. 

These acids may be grouped into following five series:

1. Sulphoxylic acids, H2SO2 -It is an unstable acid.  
2. Sulphurous acid series(H2SO3,S=+4 dibasic) 

PREPARATION

    SO2+H2O⟶H2SO3

STRUCTURE

• It is a dibasic acid. 
• Sulphur exists in + 4 oxidation state. 
• In sulphurous acid, sulphur has one 1S=O, 2 O-H & 1 lone pair. 
• The sulfite ion exists in crystals and has a pyramidal structure i.e. tetrahedral with one position occupied by a lone pair. 
• The bond angles 0-S-O are slightly distorted (106°) due to the lone pair, and the bond lengths are 1.51 Å. 
• The 𝚷 bond is delocalized, and hence the S-O bond have a bond order of 1.33) 

PROPERTIES

• Acidic nature:Dibasic nature. H₂SO3, ionises in two stages and hence is a dibasic acid

H₂SO3 ⟶ H++HSO3 –(Bisulfite ion). K₁ = 1.3 x 10–²

HSO3–⟶ H+SO32- (Sulfite ion), K₂ = 6.24 x 10–²

• Reducing property:H₂SO3(S = + 4) acts as a reducing agent. When it reduces a certain substance, it is oxidised to H₂SO4(S = + 6) and gives nascent hydrogen. 
• H₂SO3+ H₂O——→ H₂SO4+2H

ii)Thiosulphurous acid, H2S2O2 iii)Hyposulfurous/dithionous acid H2S2O4 (S=+3) 

      3.Sulphuric acid (oil of vitriol) H₂SO4 (S=+6,dibasic acid) 

PREPARATION

Sulphur is burnt in air so that it is oxidised to SO₂. SO₂ thus obtained is oxidised to SO3, which, when dissolved in H₂O, gives H₂SO4.

S+O2⟶ SO2

2SO2 + O2 ⟶2SO3

SO3 + H₂O⟶H₂SO4

In ancient days, it was called oil of vitriol, since it was prepared by distilling green vitriol (ferrous sulphate), FeSO4.7H₂O.

2[FeSO4.7H₂O]⟶Fe₂O3+SO₂+13H₂O+H₂SO4

STRUCTURE

• Sulphur has +6 oxidation state
• Sulphur has 2 S=O, & 2 S-OH
• It has tetrahedral geometry

PROPERTIES

Physical properties

Pure H₂SO, is a colourless viscous, heavy and syrupy oily liquid of density 1.84 at 15°C.

which does not fume. It forms a constant boiling mixture with water. This mixture contains 98℅ H₂SO4, and its boiling point 340°C which is usually given as the boiling point of H2SO4, 95℅ acid boils at 295°C. Pure H₂SO4, is prepared by mixing 98% acid with a calculated quantity of SO3 when cooled with ice, it solidifies to colourless crystals which melt at 10.38°C. High boiling point and high viscosity of H2SO4, is due to the fact that H₂SO4 molecules H-bonding.

USES & IMPORTANCE OF H2SO4

So important is the acid and so varied are its uses that it is often called the king of chemicals It is used: in the manufacture of various fertilisers like (NH4)2 SO4, ammoniated phosphate sulphate, superphosphate of lime, K2SO4, etc., (ii) in the manufacture of other acids such as HCI, H3PO4, HNO3, etc.. (i) in the manufacture of explosives like nitroglycerine, trinitrotoluene (TNT), picric acid. gun cotton, dynamite etc. (iv) in the manufacture of many important chemicals like Na2CO3, alums, metal sulphates, ethers, phosphorus etc., (v) in the refining of petroleum, (vi) in many metallurgical processes for the purification of metals, (vi) in the manufacture of coaltar, dyes and a number of drugs and disinfectants. (viii) in the textile, paper and dyeing industries, (i) in the leather industry for tanning. (x) for cleansing metals before electroplating enamelling, galvising etc., (pickling) (xi) in storage batteries, (xi) in the manufacture of paints and pigments, (xii) as an important laboratory reagent, (xiv) for the synthesis of various organic compounds and sulfonation reactions, (xv) as an oxidising and dehydrating agent.

     4.Peroxy-sulphuric acid series.

PROPERTIES

•  The name peroxo indicates that the compound contains an—O—O— linkage. 
• Two peroxoacids of sulphur are known: peroxymonosulfuric acid H₂SO5, and peroxodisulfuric acid H₂S₂O5
•  No peroxyacids of Se and Te are known. 
• H₂S₂O5, is a colourless solid, m.p. 65°C. It is obtained by electrolysis of sulphates at high current density. It is soluble in water, and is a powerful and useful oxidising agent. 
• Peroxymonosulfuric acid, H2SO5 (also called Caro’s acid) 
• Peroxydisulfuric acid H2S2O8(also called Marshall’s acid). 

PREPARATION

1. i) By Berthelot’s method. Berthelot (1878) obtained HS₂0, by passing electric discharge

through a mixture of SO2, and O2, and dissolving the product in H₂SO4, 

SO₂ + O₂ + H₂SO4⟶ H2S2O8

8. ii) By the electrolysis or anodic oxidation of 50-60% solution of H2SO4,is electrolysed in a large test tube in which a glass tube opened at both the end is placed, H₂SO4, gets ionised to give bisulphate ions (HSO3–) which unite together at anode pt wire) to form Marshall’s acid, H2S2O8.H2, gas is given out at cathode.

H₂SO4 ⇆ 2H+ + 2HSO4–(Ionisation)

2HSO4–⟶ H₂S₂O8 + 2e (At anode)

              Oxidation

Reduction H₂(g) (At cathode)

2H+ + 2e⟶H2

The anode is a platinum wire fused in a capillary tube and surrounded by a glass tube open

     5.Thionic acids-

• Dithionic acid H₂S₂O6, is known only in solution.
•  The acid is dibasic, and salts called dithionates are known. e.g. Na-S-O. 
• No acid salts exist. The acid and its salts contain S in the oxidation state (+V). 

These are of two types: (i) Dithionic acid, H₂S2O6, (ii) Polythionic acids H2SnO6, (n = 3 to 6). Different members of polythionic acids are (a) trithionic acid (H₂S3O6)(b) tetrathionic acid (H2S4O6) (c) pentathionic acid (H₂S5O6), and (d) hex thionic acid (H₂S6O6).

Conclusion:

Sulphur oxoacids are chemical compounds that contain sulphur, oxygen, and hydrogen. Sulfuric acid is one of the most important chemicals and is known as the “Chemical Master”. It is used in the production of dyes, chemicals and explosives.