Introduction to Le Chatelier's Principle
Introduced| English |
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| Le Chatelier's principle/lə ˈtʃeɪtlɪəz ˈprɪnsɪpl/ |
A system that pushes back
- Disturb a balanced reaction and it fights the change.
- Add a reactant, and it consumes some of it.
- Squeeze the volume, and it shifts to relieve the pressure.
- The balance always adjusts to oppose whatever you did.
Shifting to counteract
- Le Chatelier's principle 勒夏特列原理 says a system at equilibrium responds to a stress.
- It shifts in the direction that partly counteracts that stress.
- The result is a new equilibrium.
Concentration and pressure
- Add a substance and it shifts away from it; remove one and it shifts toward it.
- Increase the pressure (smaller volume) and it shifts toward fewer gas moles.
- Decrease the pressure and it shifts toward more gas moles.
Adding more reactant to an equilibrium shifts it toward...
The system shifts away from the added reactant, making products.
Increasing the pressure shifts a gas equilibrium toward the side with...
Fewer gas moles relieve the higher pressure.
Removing a product shifts the equilibrium ____ (toward products).
The system shifts toward the side you removed from.
Temperature shifts
- Treat heat as a reactant or a product.
- Heating an exothermic reaction shifts it backward.
- Heating an endothermic reaction shifts it forward.
Stress the equilibrium
Change temperature, pressure or a concentration and watch the equilibrium shift to oppose the change.
Heating an exothermic reaction shifts it...
Heat is a product of an exothermic reaction, so adding heat shifts it back.
You add more reactant to a system at equilibrium. What happens?
- The system shifts forward to use up the added reactant.
- More product forms until a new balance is reached.
A concentration change alters the value of $K$.
Only temperature changes $K$; concentration and pressure only shift position.
The shift only partly counteracts the stress, reaching a new equilibrium.
It never fully undoes the change; it settles at a new balance.
The system shifts to partly undo the stress, never fully -- it reaches a new equilibrium, not the old one. For pressure, only gas moles count (solids and liquids do not shift it). And only a temperature change actually changes $K$; concentration and pressure shifts leave $K$ the same.
Le Chatelier's principle says a stressed equilibrium shifts to partly counteract the stress. Adding a substance pushes away from it; raising the pressure favours fewer gas moles; heating favours the endothermic direction. Only a temperature change alters $K$ itself.