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Group 16: Sulphur - Oxoacids

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Group 16: Sulphur - Oxoacids - Lesson Summary

Sulphur forms large number of oxoacids. Many of these do not exist as free acids, but are known as anions and salts. Oxoacids of sulphur, depending on their structural similarities, are divided into four series..

They are:

  • Sulphurous acid series
  • Sulphuric acid series
  • Thionic acid series
  • Peroxo acid series

Oxoacids with S-S linkages are called thio acids, while those with Peroxo linkages are called Peroxo acids.

Sulphurous acid series:
Sulphurous acid, di or pyrosulphurous acid and dithionous acid are categorised under the sulphurous acid series.

Sulphuric acid series:
Sulphuric acid, thiosulphuric acid and di or pyrosulphuric acid are grouped under the sulphuric acid series.

Thionic acid series:
It includes dithionic acid and polythionic acid.

Peroxo acid series:
It includes peroxo monosulphuric acid and Peroxo disulphuric acid.

Among these oxoacids, sulphuric acid (H2SO4) is the most important acid used in the chemical industry. 
sulphuric acid is manufactured by the contact process.

The contact process involves three steps.

Step -I: 
Production of sulphur dioxide: 
This can be made by burning sulphur or sulphide ores such as iron pyrites in excess of air.

 S (Sulphur)  +  O(Oxygen)  + Δ (Heating)       →     SO(Sulphur dioxide)
 4FeS (Iron pyrites) +  7O2 (Oxygen)  + Δ (heating)    →    2Fe2O3(Ferric Oxide) +   4SO2(Sulphur dioxide)

Step -II:

Formation of sulphur trioxide:

Sulphur dioxide is oxidised to sulphur trioxide with atmospheric oxygen in the presence of V2O5 as catalyst.

2SO2 (Sulphur dioxide) +  O2 (Oxygen)    + V2O(Catalyst)      →    SO(Sulphur triioxide)

Step -III

Conversion of sulphur trioxide into sulphuric acid:
The sulphur trioxide formed in the second step is dissolved in 98% sulphuric acid to give pyrosulphuric acid or oleum. Oleum is then diluted with water to give sulphuric acid of the desired concentration.

 SO(Sulphur triioxide)   +       H2SO4  (Sulphuric acid-98%)      →    H2S2O(Pyrosulphuric acid -Oleum)
 H2S2O7 (Pyrosulphuric acid -Oleum) +  H2O (Dilution )              →    2H2SO4(Sulphuric acid)


Physical properties of sulphuric acid:

Sulphuric acid is a colourless, dense, oily liquid with a specific gravity of 1.84 at 298 k.

It has a boiling point of 611 k. The high boiling point and viscous nature is due to hydrogen bonding.

Concentrated sulphuric acid has a strong affinity for water. A large amount of heat is produced when the acid is mixed with water.

The acid is always diluted by adding it to water slowly with constant stirring. Remember - never add water to sulphuric acid.

Chemical properties of sulphuric acid:
Sulphuric acid is a strong dibasic acid. In an aqueous solution, it ionises in two steps

Step 1:
           H2SO4(aq) + H2O(I)    →  H3O+(aq) + HSO4-(aq);
           Ka1 = very large (Ka1 > 10)

Step 2:
           H2SO4(aq) + H2O(I)    →  H3O+(aq) + SO42-(aq);
           Ka= 1.2 x 10-2

The acid neutralises alkalis and forms two series of salts bisulphates and sulphates.

Concentrated sulphuric acid reacts with sodium hydroxide and forms sodium bisulphate and sodium sulphate.

H2SO4 (Conc.Sulphuric acid) + 2NaOH (Sodium hydroxide)→  2NaHSO4 (Sodium bisulphate) + H2O (Water)
H2SO(Conc.Sulphuric acid)  + 2NaOH (Sodium hydroxide) →  Na2SO4 (Sodium sulphate) + 2H2O (Water)


Sulphuric acid, being less volatile, is used in the manufacture of more volatile acids from their corresponding salts.

Concentrated sulphuric acid liberates hydrogen fluoride from calcium fluoride and hydrogen chloride from sodium chloride.

CaF(Calcium fluoride)  + H2SO4 (Conc. Sulphuric acid) →  CaSO4(Calcium sulphate)  +  2HF (Hydrogen floride)

Concentrated sulphuric acid is a powerful dehydrating agent. The dehydrating property is used in drying many wet gases that do not react with the acid.

Sulphuric acid also removes water from organic compounds like carbohydrates.

It chars cane sugar, glucose and starch to carbon.

C12H22O11(Cane sugar) + (H2SO4)     →   12C (Carbon)  +   11H2O (Water)

Hot concentrated sulphuric acid is a good oxidising agent. It oxidises both metals and non-metals and itself gets reduced to sulphur dioxide.

Hot concentrated sulphuric acid oxidises copper to copper sulphate .Similarly, it oxidises non-metallic carbon to carbon dioxide.
Cu (Copper) + 2H2SO4 (Hot suphuric acid) → CuSO4(Copper sulphate) + SO(Sulphur dioxide) + H2O (Water)
C (Carbon)  +  2H2SO4(Hot suphuric acid)  →   CO2 (Carbon dioxide) +  2SO2 (Sulphur dioxide) + 2H2O (Water)

Uses of sulphuric acid:

  • Sulphuric acid, because of its wide applications, is referred as the king of chemicals.
  • It is used in the manufacture of:
  • Fertilisers like ammonium sulphate and superphosphate.
  • Dyes, pigments and paints.
  • Explosives such as TNT.
  • Other important chemicals like hydrochloric acid, nitric acid, phosphoric acid and sodium carbonate.
  • It is used in the refining of petroleum.
  • As a pickling agent.
  • As a laboratory agent, and a dehydrating and oxidising agent.


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