7.9 Introduction to Le Châtelier's Principle

What Le Châtelier's Principle Says

Le Châtelier's Principle states that if a system at equilibrium is disturbed by a stress (change in concentration, pressure/volume, or temperature), the system shifts in the direction that partially counteracts the stress, establishing a new equilibrium. This shift changes the amounts of reactants and products but the equilibrium constant K stays the same unless temperature changes.

Concentration and Volume/Pressure Stresses

Adding a reactant or product shifts equilibrium away from the added species (consuming it); removing one shifts toward replacing it. Decreasing volume (increasing pressure) shifts equilibrium toward the side with fewer moles of gas; increasing volume shifts toward the side with more moles of gas. If moles of gas are equal on both sides, pressure/volume changes have no effect on position.

Temperature Stress and K

Temperature changes are unique: they actually change the value of K. For an exothermic reaction, increasing temperature shifts equilibrium toward reactants (K decreases); for endothermic reactions, increasing temperature shifts toward products (K increases). Adding a catalyst speeds up both forward and reverse rates equally, so it does not shift equilibrium or change K—it just gets the system to equilibrium faster.

Key terms

Le Châtelier's Principle

If a system at equilibrium is stressed, it shifts in the direction that partially counteracts the stress to reach a new equilibrium.

Effect of adding reactant/product

Shifts equilibrium away from the added species (toward consuming it) to restore equilibrium.

Effect of decreasing volume (increasing pressure)

Shifts equilibrium toward the side with fewer moles of gas.

Effect of temperature change on K

Unlike other stresses, temperature changes actually alter K: increasing T increases K for endothermic reactions and decreases K for exothermic reactions.

Effect of a catalyst on equilibrium

No shift in position, no change in K—just speeds up reaching equilibrium by increasing both forward and reverse rates equally.

Try an AP-style question

For N₂ + 3H₂ ⇌ 2NH₃, what happens when the volume is decreased?

1. It doesn't shift, since only volume changed
2. K increases, since the gas is compressed
3. The equilibrium shifts right, toward fewer gas moles
4. It shifts left, toward the side with more gas moles