Le Chatelier's Principle
You now know how to calculate K and Q, and how to predict which direction a reaction shifts. Le Chatelier’s principle is the conceptual framework that ties it all together - and it is tested on virtually every MCAT.
The Principle
Le Chatelier’s principle states: if a stress is applied to a system at equilibrium, the system will shift in the direction that partially relieves that stress.
The key word is “partially.” The system never completely undoes the change - it just moves toward a new equilibrium that partially offsets whatever you did.
Three Types of Stress
Le Chatelier’s principle applies to three types of changes:
- Changes in concentration (adding or removing a reactant or product)
- Changes in pressure or volume (for gas-phase reactions)
- Changes in temperature (the only one that changes K)
Each of the next three sections covers one of these in depth. Here is the summary table:
| Stress applied | Equilibrium shifts… | Does K change? |
|---|---|---|
| Add reactant | Toward products (right) | No |
| Remove reactant | Toward reactants (left) | No |
| Add product | Toward reactants (left) | No |
| Remove product | Toward products (right) | No |
| Increase pressure (decrease volume) | Toward side with fewer moles of gas | No |
| Decrease pressure (increase volume) | Toward side with more moles of gas | No |
| Increase temperature | Toward endothermic direction | Yes |
| Decrease temperature | Toward exothermic direction | Yes |
| Add catalyst | No shift | No |
| Add inert gas at constant volume | No shift | No |
How to Think About It Using Q
Le Chatelier’s principle is really just Q vs. K logic in disguise:
- Add reactant - the denominator of Q gets bigger, so Q decreases below K. Since Q < K, the reaction shifts forward.
- Remove product - the numerator of Q gets smaller, so Q decreases below K. Since Q < K, the reaction shifts forward.
- Add product - the numerator gets bigger, Q > K, so the reaction shifts backward.
Every Le Chatelier prediction can be confirmed by checking what happens to Q relative to K.