Le Chatelier’s Principle

Rucete ✏ Chemistry In a Nutshell

1. What Is Le Chatelier’s Principle?

• When a system at equilibrium is disturbed, it will shift in a direction that minimizes the disturbance and re-establishes equilibrium.

2. Types of Disturbances & System Responses

A. Change in Concentration

• Add reactant → shifts right (toward products)
• Remove reactant → shifts left (toward reactants)
• Add product → shifts left
• Remove product → shifts right

🔹 Example:
A + 2B ⇌ C + D
Adding A → shift right to use up A and form more C & D.

B. Change in Temperature

• Exothermic Reaction (ΔH < 0): Heat is a product
• Endothermic Reaction (ΔH > 0): Heat is a reactant

Change	Exothermic (ΔH < 0)	Endothermic (ΔH > 0)
Increase T	Shifts left	Shifts right
Decrease T	Shifts right	Shifts left

🔹 Example:
If ΔH = –250 kJ/mol (exothermic), increasing temperature shifts equilibrium left.

C. Change in Pressure / Volume (for gases only)

• Increase pressure (by decreasing volume) → shifts toward the side with fewer gas molecules
• Decrease pressure (by increasing volume) → shifts toward the side with more gas molecules

🔹 Example:
A(g) + 2B(g) ⇌ C(g) + D(g)
Left side: 3 mol gas
Right side: 2 mol gas
Increase pressure → shifts right

3. Common Ion Effect

• Occurs when a common ion is added to a solution at equilibrium.
• Suppresses ionization and shifts equilibrium to the left.

🔹 Example:
CH₃COOH ⇌ H⁺ + CH₃COO⁻
Adding CH₃COONa (which provides CH₃COO⁻) shifts equilibrium left, reducing H⁺ ion concentration.

In a nutshell

Add Goes Away, Remove Comes Back!

• Add something → shift away
• Remove something → shift toward it
• Heat acts like a reactant or product
• More pressure → fewer gas moles