Equilibria in Industrial Processes (Cambridge International A-Level Chemistry)

Haber & Contact Processes

• Equilibrium reactions are involved in some stages of large-scale production of certain chemicals
• An understanding of equilibrium and Le Chatelier’s principle is therefore very important in the chemical industry

Haber process

• The Haber process involves the synthesis of ammonia according to:

N­₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g) ΔH _r = -92 kJ mol^-1

• Le Chatelier’s principle is used to get the best yield of ammonia

Maximising the ammonia yield

Pressure

• The forward reaction produces fewer moles of gas

  • 4 moles on the left and 2 on the right

• Increasing pressure shifts equilibrium to the right, increasing ammonia yield

• Higher pressure also increases collision frequency, enhancing the reaction rate

• However, very high pressures are costly and require strong containment

• Compromise pressure used

  • ≈ 200 atm

Temperature

• The forward reaction is exothermic

• Lowering temperature shifts equilibrium to the right, favouring ammonia formation

• But too low a temperature would slow the reaction rate, delaying equilibrium

• Compromise temperature:

  • 400–450 °C

Removing ammonia

• Ammonia is removed by cooling and condensing it to a liquid
• This shifts the equilibrium further to the right, producing more ammonia
• Stored ammonia is kept at low temperatures where decomposition is very slow, especially in the absence of a catalyst

Catalysts

• An iron catalyst is used to increase the rate of reaction without affecting equilibrium position
• Without it, the reaction would be too slow to be commercially viable

Contact process

• The Contact process involves the synthesis of sulfuric acid according to:

2SO­₂ (g) + O₂ (g) ⇌ 2SO₃ (g) ΔH _r = -197 kJ mol^-1

• Le Chatelier’s principle is used to get the best yield of sulfuric acid

Maximising the sulfuric acid yield

Pressure

• Fewer moles of gas on the right-hand side

  • 3 moles on the left and 2 on the right

• Increasing pressure shifts equilibrium to the right, favouring SO₃ formation

• However, the equilibrium constant (K_p)is already very large at low pressures - Industrial pressure used:

  • ~1 atm to save cost, as higher pressure gives little extra benefit

Temperature

• Reaction is exothermic

• Lower temperatures would favour SO₃ production, but also reduce the rate

• Compromise temperature:

  • ≈ 450 °C

Removing sulfuric acid

• SO₃ is removed by absorbing it into 98% H₂SO₄, forming oleum:

SO₃ + H₂SO₄ → H₂S₂O₇

• This removal shifts the equilibrium to the right, driving the reaction forward

Catalysts

• The Contact process uses vanadium(V) oxide as a catalyst to increase the rate of reaction