Overview of Le Châtelier’s Principle

Le Châtelier’s Principle describes how a system at chemical equilibrium responds when subjected to an external “stress,” such as changes in concentration, temperature, or pressure. The system will shift its position of equilibrium to counteract the change and partially restore balance.

Formal Statement

“If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the disturbance.”

Factors That Disturb Equilibrium

• Concentration Changes: Adding or removing reactants or products.
• Temperature Changes: Heating or cooling an exothermic or endothermic reaction.
• Pressure Changes: Altering total pressure for reactions involving gases.
• Catalysts: Increase rate but do not shift equilibrium position.

Rules for Predicting Shifts

• Concentration Increase: Equilibrium shifts away from the added species.
• Concentration Decrease: Equilibrium shifts toward the removed species.
• Temperature Increase: Equilibrium favors the endothermic direction.
• Temperature Decrease: Equilibrium favors the exothermic direction.
• Pressure Increase (gases): Equilibrium shifts toward side with fewer moles of gas.
• Pressure Decrease: Equilibrium shifts toward side with more moles of gas.

Illustrative Examples

Haber Process (Ammonia Synthesis)

N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g) ΔH = –92 kJ/mol
• Increasing pressure shifts toward ammonia
• Increasing temperature shifts toward reactants (endothermic reverse)

Formation of Nitric Oxide

N₂O₄(g) ⇌ 2 NO₂(g) ΔH = +58 kJ/mol
• Heating shifts toward products (endothermic forward)
• Removing NO₂ shifts equilibrium toward NO₂ formation

Industrial and Biological Applications

• Chemical Manufacturing: Optimizing yield in processes like methanol synthesis and sulfuric acid production.
• Pharmaceuticals: Controlling reaction conditions to favor desired product formation.
• Physiology: Acid–base balance in blood where carbonate equilibrium shifts with CO₂ levels.

Limitations and Considerations

Le Châtelier’s Principle provides qualitative predictions but does not quantify the extent of shift. Thermodynamic parameters (K_eq, ΔG) and kinetic factors influence actual concentrations at the new equilibrium.

Conclusion

Le Châtelier’s Principle is a powerful tool for understanding and predicting how equilibria respond to changes. By applying its rules, chemists can fine‑tune reaction conditions to improve yields and control process outcomes in both laboratory and industrial settings.