AP® Chemistry · Unit 7 · Topic 7.9
Le Chatelier's Principle
Le Chatelier's Principle says a system at equilibrium responds to a stress by shifting in whatever direction partly cancels that stress, settling at a new position that still satisfies the same K.
The real engine behind the principle is Q versus K. At equilibrium Q equals K. Add more reactant, and because reactant concentration sits in Q's denominator, Q instantly drops below K. The system now has a reaction quotient smaller than its equilibrium constant, so it must run forward, consuming reactant and building product, until Q climbs back up to meet K. Remove a reactant and the opposite happens: Q jumps above K, and the reaction runs in reverse. Pressure and volume changes work the same way, through concentration, and temperature changes work by actually altering K itself.
Picture the cobalt chloride equilibrium: Co(H_2O)_62+ (pink) + 4 Cl^- <→ CoCl42- (blue) + 6 H_2O. Add chloride ions and Q drops below K, so the system shifts forward, turning the solution more blue as it rebuilds equilibrium. Add water or dilute the solution, and the shift reverses, pulling the color back toward pink. In every case the system is just restoring Q to K, not inventing new chemistry.
This connects directly to the ICE table work from earlier in the unit: a stress gives you a new starting point, and you solve a fresh ICE table to find where Q meets K again. It also sets up reaction quotient problems where you predict shift direction purely by comparing Q and K numerically, without needing any intuition about 'stress' at all.
Key terms
- Le Chatelier's Principle
- A system at equilibrium shifts in the direction that partly counteracts an applied stress.
- Reaction quotient (Q)
- The same ratio of products to reactants as K, but calculated at any moment, not just at equilibrium.
- Equilibrium constant (K)
- The fixed value Q equals once a reaction reaches equilibrium at a given temperature.
- Stress
- Any change imposed on an equilibrium system, such as added reactant, removed product, or a volume change.
- Shift
- The net forward or reverse reaction a system undergoes as it moves from one equilibrium position to another.
Watch it explained
- How it worksHow does a stressed system shift to partly undo whatever was done to it?
- How it worksWalk me through what happens to a cobalt chloride equilibrium as the color shifts from pink to blue.
- Common mistakeWhy isn't a shift toward products the same as an increase in the equilibrium constant?
- Going furtherWhat happens when you remove a product from an equilibrium mixture as fast as it forms?
Check yourself
End of topic.