Le Chatelier's Principle
High-Yield Summary
- Le Chatelier's Principle: a system at equilibrium shifts to counteract an applied stress and re-establish equilibrium.
- Concentration: adding reactant/removing product drops Q below Keq → shifts forward; removing reactant/adding product raises Q above Keq → shifts reverse.
- Pressure/volume (gas-phase only): increased pressure/decreased volume shifts toward fewer moles of gas; decreased pressure/increased volume shifts toward more moles of gas.
- Temperature is the one stress that changes Keq itself: heat acts as a reactant in endothermic reactions, a product in exothermic reactions.
- Catalysts speed up reaching equilibrium but never shift its position or change Keq.
Concentration Stress
| Change | Effect |
|---|---|
| Add reactant / remove product | Q < Keq → shifts forward (more product) |
| Remove reactant / add product | Q > Keq → shifts reverse (more reactant) |
Pressure/Volume Stress (Gas-Phase Only)
| Change | Effect |
|---|---|
| Increased pressure (decreased volume) | Shifts toward the side with FEWER moles of gas |
| Decreased pressure (increased volume) | Shifts toward the side with MORE moles of gas |
| Solids/liquids | Unaffected — essentially incompressible, only gas moles matter |
Temperature Stress (Changes Keq Itself)
| Reaction Type | Effect of Increasing Temperature |
|---|---|
| Endothermic (ΔH > 0) — heat acts as a reactant | Shifts toward products (Keq increases) |
| Exothermic (ΔH < 0) — heat acts as a product | Shifts toward reactants (Keq decreases) |
Must-Know Points
- Concentration and pressure/volume shifts move the equilibrium POSITION without changing Keq's value.
- Temperature is unique — it's the only stress that changes the value of Keq itself, because it's tied to ΔH.
- Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) has 4 mol gas (reactants) vs. 2 mol gas (products) — increasing pressure shifts right (toward NH₃, fewer moles).
Common MCAT Trap
- A catalyst does NOT shift equilibrium position or change Keq — it speeds up forward and reverse rates equally, only shortening the time to reach equilibrium.
- Pressure/volume stress only matters for gas-phase species — don't apply it to solids or liquids, which are incompressible.
- Don't mix up which stress changes Keq: concentration and pressure/volume shift the POSITION only; temperature is the only stress that changes Keq itself.
Quick Recall
For an exothermic reaction, does increasing temperature favor reactants or products?
Does adding a catalyst to a system at equilibrium shift the equilibrium position?
For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), which direction does increasing pressure shift the equilibrium?