8.7.2 Synthesis of Sulphuric Acid by Contact Process

The contact process is an industrial method used to produce sulphuric acid. The manufacture of sulphuric acid is represented by the following chemical reactions.

i. Formation of Sulphur Dioxide

Sulphur dioxide is produced by burning sulphur.

S_(s) + O_2(g) → SO_2(g)

ii. Oxidation of Sulphur Dioxide

SO₂ is oxidized into SO₃ using vanadium pentaoxide (V₂O₅) .

2SO_2(g) + O_2(g) ⇌ 2SO_3(g) Δ H^° = -198 kJ

iii. Conversion of Sulphur Trioxide into Sulphuric Acid

The oxidation of sulphur dioxide to sulphur trioxide is the most important step in the production of sulphuric acid. This reaction provides the following information:

a) The reaction is exothermic. b) The reaction occurs with a decrease in number of moles.

Le Chatelier's principle suggests three ways to maximize the yield of sulphuric acid.

a) Low Temperature

• Since the forward reaction is exothermic, the low temperature favours the formation of sulphur trioxide.
• However, too low temperature slows down the reaction speed.
• So, a compromised temperature of 450–500°C is used.

b) High Pressure

• Since forward reaction produces fewer moles, high pressure shifts the equilibrium to the right.
• But maintaining high pressure is very expensive.
• So, the compromised pressure of 2 atm is used.

c) Adding Excess Oxygen

• According to Le Chatelier's principle, an increase in O₂ concentration at equilibrium shifts the equilibrium to the right.
• However, adding too much O₂ decreases the concentration of SO₃, which slows down the reaction rate.
• Thus, a degraded 1:1 ratio of SO₂ and O₂ is used to obtain high yields.

Intermediate Level

Haber's Process

Contact Process

Key Points

• Haber: Low T, high P, catalyst, remove NH₃.
• Contact: Low T, high P, catalyst, excess O₂.

Important Points

• 1. Haber's process: N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92.46 kJ.
• 2. Haber conditions: 400°C, 200–300 atm, Fe catalyst, remove NH₃.
• 3. Contact process: 2SO₂ + O₂ ⇌ 2SO₃, ΔH = −198 kJ.
• 4. Contact conditions: 450–500°C, 2 atm, V₂O₅ catalyst, excess O₂.
• 5. Le Chatelier's principle used to optimize conditions.
• 6. Practically 100% conversion possible in Haber process.

Remember

• - Haber's Process: N₂ + 3H₂ ⇌ 2NH₃
• - Haber conditions: 400°C, 200–300 atm, Fe catalyst, remove NH₃
• - Contact Process: 2SO₂ + O₂ ⇌ 2SO₃
• - Contact conditions: 450–500°C, 2 atm, V₂O₅ catalyst, excess O₂
• - Le Chatelier's Principle: Used to optimize conditions

Common Mistakes

• 1. Thinking high temperature increases yield in exothermic reactions: It decreases yield.
• 2. Forgetting the role of catalyst: Catalyst increases rate, not yield.
• 3. Ignoring economic considerations: Conditions are a compromise.
• 4. Confusing Haber and Contact processes: Haber = NH₃; Contact = H₂SO₄.

Quick Revision

• Haber's process: N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92.46 kJ.
• Haber conditions: 400°C, 200–300 atm, Fe catalyst, remove NH₃.
• Contact process: 2SO₂ + O₂ ⇌ 2SO₃, ΔH = −198 kJ.
• Contact conditions: 450–500°C, 2 atm, V₂O₅ catalyst, excess O₂.
• Le Chatelier's principle used to optimize conditions.
• Practically 100% conversion possible in Haber process.

60-Second Revision

• Haber's Process: N₂ + 3H₂ ⇌ 2NH₃
• Haber conditions: 400°C, 200–300 atm, Fe catalyst, remove NH₃
• Contact Process: 2SO₂ + O₂ ⇌ 2SO₃
• Contact conditions: 450–500°C, 2 atm, V₂O₅ catalyst, excess O₂
• Le Chatelier's Principle: Used to optimize conditions

QUALITY VALIDATION

• Topic: 8.7 Industrial Application of Le Chatelier's Principle
• Chapter: 8 (Chemical Equilibrium)

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