What does Le Chatelier's principle actually mean?
Le Chatelier's principle says that when a system at equilibrium is disturbed, it shifts in the direction that partly undoes the disturbance until a new equilibrium is reached. Add a reactant and it makes more product; remove heat from an exothermic reaction and it makes more heat by running forward.
What each change does
- Concentration: adding a reactant (or removing a product) shifts the equilibrium toward products. Adding a product shifts it toward reactants. Adding a pure solid or liquid does not shift it.
- Pressure by changing volume: compressing a gas mixture shifts it toward the side with fewer moles of gas. If both sides have the same number of gas moles, there is no shift.
- Adding an inert gas at constant volume: no shift, because the partial pressures of the reacting gases do not change.
- Temperature: treat heat as a reactant in an endothermic reaction and as a product in an exothermic one. Heating shifts toward the side that absorbs heat.
- Catalyst: no shift. It speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but in the same place.
Why it works: Q and K
The principle is a shortcut for a calculation. The reaction quotient Q has the same form as the equilibrium constant K but uses the current concentrations. Adding a reactant makes Q smaller than K, so the forward reaction runs until Q equals K again. Concentration and pressure changes move Q; only temperature changes K itself.
Worked example: making ammonia
N2(g) + 3 H2(g) ⇌ 2 NH3(g), ΔH ≈ −92 kJ per 2 mol NH3. The forward reaction is exothermic and goes from 4 moles of gas to 2.
- Raising the pressure favors ammonia (fewer gas moles).
- Removing ammonia as it forms pulls the reaction forward.
- Lowering the temperature favors ammonia and increases K, but it also slows the reaction. That is why the industrial Haber process uses a moderate temperature with an iron catalyst: a compromise between yield and rate.
A frequent exam trap is saying a catalyst increases yield, or that a pressure change alters K. Neither is true.
See the shift
Equilibrium is easier to trust once you watch both reactions running at once and see one outpace the other after a change. Type "What does Le Chatelier's principle actually mean?" into Isotoon for a short animated video of that. Related: what a mole is, the chemistry page, and how to study for exams without forgetting.
Questions
- Does the system fully undo the change?
- No. It only partly offsets it. If you add a reactant, its concentration at the new equilibrium is still higher than before, just lower than right after you added it.
- Why does temperature change K but concentration does not?
- K depends on the energy difference between reactants and products relative to temperature. Changing concentrations moves the system away from equilibrium without changing that balance, so the system returns to the same K.