Le Chatelier's principle
Le Chatelier's principle
- Le Chatelier's principle: if you change a system at equilibrium, it shifts to oppose the change.
- This predicts which way the equilibrium moves.
- A catalyst is the exception — it shifts nothing.
Practice
Le Chatelier's principle says that a change to a system at equilibrium causes it to:
The equilibrium position always moves in the direction that lessens the change you made.
The changes
| Change | Equilibrium moves |
|---|---|
| increase a reactant's concentration | towards products |
| increase pressure | towards the side with fewer gas molecules |
| increase temperature | towards the endothermic direction |
Practice
Increasing the pressure shifts the equilibrium towards:
Moving to fewer gas molecules reduces the pressure, opposing the increase.
Practice
Increasing the temperature shifts the equilibrium towards:
The endothermic direction absorbs the added heat, opposing the temperature rise.
Catalysts
- A catalyst speeds up both the forward and reverse reactions equally.
- So equilibrium is reached faster, but its position does not change.
Practice
Adding a catalyst to a system at equilibrium:
A catalyst speeds up both directions equally, so the position of equilibrium is unchanged.
You've got it
Key idea
- Le Chatelier: the equilibrium shifts to oppose any change you make
- more reactant → towards products; more pressure → fewer-gas-molecules side; more heat → endothermic direction
- a catalyst speeds both ways equally → reaches equilibrium faster, no shift