Le Chatelier’s Principle explains what happens to a reversible reaction at equilibrium when conditions are changed. The system responds to oppose the change and restore balance.

What is equilibrium?

• In a reversible reaction, equilibrium is reached when the forward and backward reactions occur at the same rate
• The amounts of reactants and products stay constant (but not necessarily equal)
• This happens in a closed system

Le Chatelier’s Principle

“If the conditions of a reversible reaction at equilibrium are changed, the system will adjust to counteract the change and restore equilibrium.”

Changing concentration

• Increase concentration of reactants → system makes more products
• Increase concentration of products → system makes more reactants

💡 The system shifts away from what is added

Changing temperature

• Increase temperature → favours the endothermic direction
• Decrease temperature → favours the exothermic direction

💡 The system shifts to absorb or release energy to oppose the change

Changing pressure (gases only)

• Increase pressure → favours the side with fewer gas molecules
• Decrease pressure → favours the side with more gas molecules

💡 Count the number of moles of gas on each side to predict the shift

No effect on catalysts

• A catalyst speeds up the forward and backward reactions equally
• It does not affect equilibrium position, but helps it reach equilibrium faster

Example: N₂ + 3H₂ ⇌ 2NH₃ (Haber Process)