Equilibria & Le Chatelier's Principle
Dynamic equilibrium, Kc, and shifting equilibrium position
A reversible reaction reaches dynamic equilibrium when the rate of forward reaction equals the rate of backward reaction. Concentrations remain constant but reactions continue.
Le Chatelier's Principle: If a system at equilibrium is disturbed, it shifts to counteract the change.
Equilibrium constant Kc: For aA + bB ⇌ cC + dD: Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ
Large Kc → products favoured. Small Kc → reactants favoured. Kc only changes with temperature.
The Haber Process: N₂ + 3H₂ ⇌ 2NH₃ (ΔH = −92 kJ/mol). Compromise conditions: 450°C (faster, despite shifting left), 200 atm (shifts right), iron catalyst.
Key Points to Remember
- 1Dynamic equilibrium: forward rate = backward rate
- 2Le Chatelier: system shifts to counteract change
- 3Kc = products/reactants (only changes with temperature)
- 4Haber process: compromise between yield and rate
Pakistan Example
The Haber Process at Fauji Fertilizer — Feeding Pakistan
Pakistan imports much of its ammonia but also produces it domestically. The Haber process at facilities like Fauji and Engro uses Le Chatelier's principle: high pressure (200 atm) pushes equilibrium towards ammonia (fewer gas moles), but the 450°C temperature is a compromise — lower would give better yield but unacceptably slow rate.