Sulphuric Acid

The mineral acid composed of the elements sulphur, oxygen, and hydrogen that has a molecular formula of H₂SO₄. It is also known as the oil of vitriol.

Manufacture of sulphuric acid by contact process:

Principle:

1. Production of sulphur dioxide: Sulphur dioxide gas can be prepared either by burning sulphur or by the roasting of iron pyrite.
   S + O₂ → SO₂
   4FeS₂(iron pyrite) + 11O₂ → 2Fe₂O₃ + 8SO₂

2. Oxidation of sulphur dioxide: Sulphur dioxide is oxidized to sulphur trioxide in presence of vanadium pentoxide as a catalyst at about 450°C and 2 atm pressure.

3. Absorption of sulphur trioxide:: Sulphur trioxide is absorbed by conc. sulphuric acid to form pyrosulphuric acid called oleum.
   SO₃ + H₂SO₄ → H₂S₂O₇ (Oleum)

4. Dilution of oleum: Oleum is added with a calculated amount of water to produce sulphuric acid.
   H₂S₂O₇ + H₂O → 2H₂SO₄ (sulphuric acid)

Condition of high yield of sulphuric acid:

Production of SO₂ to SO₃ is one of the most important steps in the manufacture of sulphuric acid. This is a reversible reaction. A higher yield of SO₃ leads to a high yield of H₂SO₄. Hence, according to Le-Chatlier’s principle, it is favoured by:

1. Low temperature (optimum): Since the reaction is exothermic, the lower the temperature, the greater is the yield. But too low temperature makes the reaction too slow to attain equilibrium. That is why an optimum temperature of 450°C is supplied.

2. High pressure: Since the reaction takes place by the decrease in volume, high pressure of 2 atm favors the forward direction.

3. High concentration: High concentrations of SO₂ and O₂ are used for more production of SO₃.

4. Use of catalyst: The rate of reaction is increased by the use of vanadium pentoxide as a catalyst.

5. Purity of gas: SO₂ and O₂ should be free from impurities.

Working process:

1. Sulphur or pyrite burner: SO₂ gas is obtained by burning sulphur or by roasting iron pyrite .

2. Purification unit: SO₂ gas may be contaminated with impurities that are removed in a purification unit.
   
   i. Dust remover: Steam is sprayed from the top of tower to settle dust particles.
   ii. Scrubber: Coldwater is sprayed to remove water-soluble impurities.
   iii Drier: Conc. H₂SO₄ is sprayed to remove moisture.
   iv Arsenic purifier: It is used to remove arsenic impurities with the help of moist Fe(OH)₃.

3. Testing box: It test whether the SO₂ gas is pure or dry or not. If the gas is impure, it is recirculated in a purification unit.

4. Pre-heater: The pure and dry SO₂ gas is heated in a pre-heater at 450°C.

5. Contact tower: SO₂ gas obtained from pre-heater is passed towards contact tower which contains vanadium pentoxide as a catalyst and converted into SO₃ gas. Since reaction takes place in the contact of catalyst, the name of the process is called contact process.

6. Absorption tower: SO₂ gas obtained from the contact tower is passed towards the absorption tower which is packed with acid-proof stones. Here conc.H₂SO₄ is sprayed from the top of the tower which absorbs SO₃ to form an oleum.
   Oleum is diluted to a calculated amount of water to form sulphuric acid of the desired concentration.

*Water is not directly used to absorb sulphur trioxide as the reaction between SO₃ and H₂O is exothermic and they form a dense fog which is difficult to condense.

*Oleum is called fuming sulphuric acid.

Physical properties:

1. It is a colorless viscous liquid.
2. It is highly soluble in water.
3. Its melting point is 10°C and boiling point is 338°C.

Chemical properties:

1. Acidic Nature:

It is strong diprotic acid and ionizes in two steps.

H₂SO₄ → H⁺ + HSO4^– (Bisulphate ion)
HSO4^– ⇌ H⁺ + SO4^– (Sulphate ion)

It gives two types of salts with base.

H₂SO₄ + NaOH → NaHSO₄ (Sodium bisulphate) + H₂O
H₂SO₄ + 2NaOH → Na₂SO₄ (Sodium sulphate) + 2H₂O

It neutralizes alkalis, basic oxides to form salt and water. It evolves CO₂ from carbonates and bicarbonates and SO₂ from sulphites and bisulphites.

2NH₃ + H₂SO₄ → (NH₄)2SO₄
2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2CO₂ + H₂O
Na₂CO₃ + H₂SO₄ → Na₂SO₄ + CO₂ + H₂O
Na₂SO₃ + H₂SO₄ → Na₂SO₄ + SO₂ + H₂O
2NaHSO₃ + H₂SO₄ → Na₂SO₄ + 2SO₂ + H₂O

Dilute sulphuric acid gives hydrogen gas with active metals.
Zn + H₂SO₄ → ZnSO₄ + H₂
Mg + H₂SO₄ → MgSO₄ + H₂

2. Concentrated sulphuric acid as an oxidizing agent:

Sulphur is in its highest oxidation state (+6) in sulphuric acid. So, it acts as an oxidizing agent.

i. Action with metals
Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O
Zn + 2H₂SO₄ → ZnSO₄ + SO₂ + 2H₂O

ii. Action with non metals (C, P, S):

– With carbon
H₂SO₄ → SO₂ + H₂O + [O] ] x 2
2[O] + C → CO₂

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C + 2H₂SO₄ → CO₂ + 2SO₂ + 2H₂O

– With phosphorous
H₂SO₄ → SO₂ + H₂O + [O] ] x 10
10[O] + P₄ + 6H₂O → 4H₃PO₄

---

P₄ + 10H₂SO₄ → 4H₃PO₄ + 10SO₂ + 4H₂O

– With sulphur
H₂SO₄ → SO₂ + H₂O + [O] ] x 2
2[O] + S → SO₂

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S + 2H₂SO₄ → 2H₂O + 3SO₂

iii. Action with reducing agents:

– With H₂S
H₂SO₄ → SO₂ + H₂O + [O]
[O] + H₂S → H₂O + S

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H₂SO₄ + H₂S → 2H₂O + SO₂ + S

– With HI
H₂SO₄ → SO₂ + H₂O + [O]
[O] + 2HI → H₂O + I₂

---

H₂SO₄ + HI → 2H₂O + SO₂ + I₂

3. Concentrated sulphuric acid as dehydrating agent:

Conc. sulphuric acid has a high affinity for water. When it is mixed with water, a large amount of heat is released. If water is added into conc. sulphuric acid, a large amount of heat is released which damages the vessel and causes injury. So, water is not added into conc. sulphuric acid.
A small amount of conc. sulphuric acid is added to the large volume of water with constant shaking and cooling.

i. It removes water of crystallization of hydrates salts.

CuSO₄.5H₂O (blue vitriol) + H₂SO₄(conc.) → CuSO₄ + H₂SO₄.5H₂O

ii. It removes water from glucose, fructose, or sucrose and form carbon (charring action).
C₆H₁₂O₆(glucose or fructose) + H₂SO₄(conc) → 6C + H₂SO₄.6H₂O
C₁₂H₂₂O₁₁(sucrose) + H₂SO₄(conc.) → 12C + H₂SO₄.11H₂O

iii. It removes water from formic acid, oxalic acid, and ethyl alcohol.

Uses of sulphuric acid:

• As a dehydrating agent, oxidizing agent, and precipitant.
• Manufacture of chemical fertilizers, dyes, drugs, explosives, etc.
• Manufacture of paints and pigments.

Sodium thiosulphate or Hypo (Na₂S₂O₃):

Uses:

• Estimation of iodine
• Photography
• Antichlor in bleaching industry to remove chlorine.

Some Important Questions

1. What happens when conc. H₂SO₄ is reacted with copper, sugar, blue vitriol, formic acid, oxalic acid.
2. Write the formula of aqua fortis, oil of vitriol, hypo, oleum.