Chapter 15 · Chemical Equilibrium
15.7Disturbing a Reaction at Equilibrium: Le Châtelier's Principle
9 min · two checks
Predict
If you disturb an equilibrium, what does the system do?
The idea
- State Le Châtelier’s principle.
- Predict the shift for a concentration change, in words.
Le Châtelier’s principle is a direction tool. When a system at equilibrium is disturbed, it responds to reduce the disturbance. Add reactant: shift toward products, using up some of what you added. Remove product: shift toward products, replacing some of what you removed. The new equilibrium is not the old set of concentrations. K is the same if the temperature is the same. Only the position along the reaction changes.
The principle is not a calculation. It will not tell you the new concentration; Q versus K and an ICE table will. It also gets misquoted. Adding a pure solid that is already present does nothing. Adding a catalyst does nothing to the position. Those are not “disturbances” of the concentration ratio. The next sections take the real disturbances one at a time: concentration, volume, temperature.
Keep these
- The system shifts to offset the stress.
- K stays fixed unless temperature changes.
- Catalysts and extra pure solid (already present) are not shifts.