CHEMISTRY

TOPIC: TETRAOXOSULPHATE (VI) ACID, H₂SO₄

Kingsley Idiagbor, B.Sc.(Hons), NCE, PGDCs, MNSM

H₂SO₄ acid is a heavy chemical. By this, we mean it is used in large amounts and most frequently in the industries. It is sometimes called oil of vitriol. In fact, most manufacturing processes use this acid, either directly or indirectly at some stage in the process

INDUSTRIAL PREPARATION

Two major processes are used in the industrial preparation of this acid namely, lead chamber process and the more popular contact process

CONTACT PROCESS

The major reactions that constitute the contact process are outlined below:

First stage: Sulphur is roasted in dry air to obtain sulphur (IV) oxide:

S + O₂ --------------->SO₂

Second stage: Next, sulphur oxide is mixed thoroughly with excess air to form sulphur (VI) oxide in a reversible reaction that is carried out in an electric chamber. Thus:

The following conditions are considered optimum for the favorable production of the SO₃ gas

1. low temperature, because of the exothermic nature of the reaction (However, the operating temperature of the process is about 450 - 500^oC)
2. high pressure since the reaction proceeds with a reduction in volume (Again, in this process atmospheric pressure suffices)
3. use of excess air (by increased oxygen, the equilibrium shifts to the right in accordance with the Le Chatelier's principle)
4. vanadium pentoxide, V₂O₅, serves as the catalyst for the optimum production of SO₃

Third Stage: The sulphur trioxide is made to react with concentrated tetraoxosulphate (VI) acid (or sulphuric acid), H₂SO₄ to form oleum (or heptaoxodisulphate(VI) acid, H₂S₂O₇.

SO₃ + H₂SO₄--->H₂S₂O₇

At the final stage of the contact process, the oleum is diluted with the right amount of water to produce about 98% of H₂SO₄ acid.
H₂S₂O₇ + H₂O----->2H₂SO₄

Physical properties

1. Conc H₂SO₄ acid is a thick colourless viscous liquid, which used to be called "oil of vitriol"
2. It has specific gravity of about 1.84 g cm^-3
3. In concentrated form, it is very corrosive and causes serious burns when it gets in contact with the skin
4. The acid has a great affinity for water

Chemical properties

1. As an acid: It reacts with metals found about hydrogen in the activity series to liberate the gas:
   Zn + H₂SO₄------->ZnSO₄ + H₂
   With bases: It undergoes neutralization reaction with most alkalis to form the corresponding salts and water
   H₂SO₄ + MgO--->MgSO₄ + H₂O
   With trioxocarbonates: The acid releases carbon (IV) oxide from metallic trioxocarbonates:
   CuCO₃ + H₂SO₄ ------->CuSO₄ + H₂O + CO₂
2. As an oxidizing agent: The acid oxidizes metals to form the corresponding tetraoxosulphates(VI) and sulphur (IV) oxide
   Zn + H₂SO₄--->ZnSO₄ + H₂O + SO₂
3. As dehydrating agent: Due to the strong affinity that H₂SO₄ has for water, it has the ability of removing water molecule from chemical compounds. For example, when the acid reacts with sugar, a charred substance called sugar charcoal is formed. Thus:
   C₁₂H₂₂O₁₁ ----->12C + 11H₂O
4. Displacing acids from their salts: The acid is capable of displacing some acids from their salts:
   E.g. H₂SO₄ + NaCl------>Na₂SO₄ + HCl
   H₂SO₄ + KNO₃---------->K₂SO₄ + HNO₃

USES OF H₂SO₄ acid

1. Catalyst and dehydrating agent in many organic chemical manufacturing and petrochemical processes
2. It is used in the manufacture of detergents
3. The acid is used in the manufacture of titanium dioxide (a widely used white pigment), and other dyes and pigments
4. The acid is useful in fertiliser production, mainly phosphates but also ammonium and potassium sulphate
5. It is important in the manufacturing processes for a range of plastics
6. Tetraoxosulphate(vi) acid is often used in metal processing such as pickling and de-scaling of steel, and non-ferrous metal plating and purification

7. It is used in drug production through sulphonation
8. Manufacture of food acids (citric and lactic acid) and edible oils
9. The acid is used during the manufacture of Adhesives, explosives and synthetic rubber
10. It is important in Water and effluent treatment
11. The acid is sometimes used as drying agent during the preparation of Chlorine gas
12. In Wood pulping
13. It is used in leather tanning
14. It serves as electrolyte in accumulators (that is-car batteries)

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Copyright © Kingsley Idiagbor | 29 June 2006  - 

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