Introduction to Le Chatelier's Principle
Introduced| English | Meaning |
|---|---|
| Le Chatelier's principle/lə ˈtʃeɪtlɪəz ˈprɪnsɪpl/ | A system at equilibrium shifts to partly oppose a change imposed on it. |
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...
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The system shifts away from the added reactant, making products.
Increasing the pressure shifts a gas equilibrium toward the side with...
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Fewer gas moles relieve the higher pressure.
Removing a product shifts the equilibrium ____ (toward products).
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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...
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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 $K$.
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Only temperature changes $K$; concentration and pressure only shift position.
The shift only partly counteracts the stress, reaching a new equilibrium.
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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.