Sulphuric acid

Industrial preparation of sulphuric acid

The Contact process

1. Sulphur is burnt in air to produce sulphur dioxide.
   S + O_{2 → SO2
   The gas is then purified and dried.}
2. Sulphur dioxide and excess oxygen are passed through vanadium (V) oxide catalyst. This occurs at 450⁰C -500⁰C.
   2SO₂ + O₂ ⇌2SO₃
   
3. Sulphur dioxide is dissolved in concentrated sulphuric acid to form oleum (H₂S₂O₇)
   SO₂ + H₂SO₄ → H₂S₂O₇
4. The oleum H2S2O4 is then diluted with water to produce concentrated sulphuric acid. H₂S₂O₇ + H₂O → 2H₂SO₄

NOTE

1. Although platinum, Pt (platinum asbestos) catalyst can be used in place of Vanadium pentoxide (V₂O₅) catalyst but it is not preferred because it is easily poisoned by impurities and it is very expensive.
2. It is possible to add sulphur trioxide directly to water to produce concentrated sulphuric acid, (H₂SO₄) as follows:
   SO₃ + H₂O → H₂SO_4.
   However this is not done in industry as this reaction is highly exothermic, meaning it releases a large amount of heat and the heat produced vaporizes the sulphuric acid to form a mist of concentrated sulphuric acid which is dangerous to health and difficult to condense.
3. The stage 2, of the contact process is the one that gives the process its name because the two gases react together in contact with a solid catalyst.

PROPERTIES OF CONCENTRATED SULPHURIC ACID

(a) Physical properties of concentrated sulphuric acid

1. It is a dense oily liquid. It is sometimes referred to as oil of vitriol.
2. It is colourless and odourless liquid with a specific gravity of 1.84g/cm3.
3. It has a boiling point of 333⁰C.
4. It is hygroscopic.
5. It has no effect on dry litmus paper.
6. It does not conduct electricity.

(b).Chemical properties of concentrated sulphuric acid

1. Acidic properties:
   It behaves as a typical acid when dissolved in water to form dilute sulphuric acid. It has all the chemical properties of acid at this time.
2. Dehydrating properties:
   It removes water or the elements of water (that is oxygen and hydrogen) from the substance.
   Dehydrating agent is a substance which can remove chemically combined water or elements of water from a compound.
   For example:
   • Hydrated copper (II) sulphate
     CuSO₄.5H₂O → CuSO₄(s) + 5H₂O (water molecules are removed from CuSO4.5H2O)
   • Sucrose(sugar)
     C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O(g) (the elements hydrogen and oxygen are removed and combined chemically to form vapour)
   • Glucose
     C₆H₁₂O₆ (s)→ 6C(s) + 6H₂O(g) (elements of water are removed from this substance and formed water vapour.)
   • Ethanol
     C₂H₅OH → CH₂CH₂ + H₂O (elements of water are removed and combined to form water)
3. As an oxidizing agent:
   
   • It oxidizes some metals like iron, copper and zinc metals to their metal sulphate and itself reduces to sulphur dioxide.
     Zn(s) + H₂SO₄ (l)→ ZnSO₄(aq) + SO₂(g) + 2H₂O (l)
     Cu + H₂SO₄ (l) → CuSO₄(aq) + SO₂(g) + 2H₂O(l)
     Fe + H₂SO₄ (l) → FeSO₄ (aq)+ SO₂(g) + 2H₂O(l)
   • . It oxidizes non-metals like carbon and sulphur to their oxides and the acid is reduced to sulphur dioxide.
     C + H₂SO₄ → 2H₂O + CO₂ + SO₂
     S + H₂SO₄ → H₂O + SO₂
     
   • Hydrogen sulphide gas when bubbled through hot concentrated sulphuric acid, the acid is reduced to sulphur dioxide and water and itself oxidized to sulphur.
     H₂S(g) + H₂SO₄(l) → S (s) + SO₂(g) + 2H₂O(l)
     

Uses of sulphuric acid

• Used in the manufacture of fertilizers eg. ammonium sulphate.
• Used in the manufacture of detergents.
• Used in car batteries or accumulators as electrolyte.
• Used as a drying agent.
• Used in the manufacture of paint and pigment of dyes.
• Used in extraction of metals.

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