Le Chatelier's Principle and Equilibrium Shifts
Predict how changes in conditions affect equilibrium position.
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Le Chatelier's Principle and Equilibrium Shifts
Le Chatelier's Principle
When a system at equilibrium is disturbed, it will shift to partially counteract the disturbance and establish a new equilibrium.
Concentration Changes
| Disturbance | Shift | Effect on K |
|---|---|---|
| Add reactant | Forward → | No change |
| Remove reactant | ← Reverse | No change |
| Add product | ← Reverse | No change |
| Remove product | Forward → | No change |
K does not change when concentrations change (at constant T).
Pressure/Volume Changes (gases only)
| Disturbance | Shift Direction |
|---|---|
| Decrease volume (increase P) | Toward fewer moles of gas |
| Increase volume (decrease P) | Toward more moles of gas |
| Add inert gas (constant V) | No shift |
If , pressure changes have no effect.
Temperature Changes
Temperature is the only factor that changes K.
| Reaction Type | Increase T | Decrease T |
|---|---|---|
| Exothermic () | Shift left, K decreases | Shift right, K increases |
| Endothermic () | Shift right, K increases | Shift left, K decreases |
Think of heat as a product (exothermic) or reactant (endothermic).
Catalyst Effects
A catalyst:
- Does NOT shift equilibrium
- Does NOT change K
- Reaches equilibrium faster
- Lowers activation energy equally for forward and reverse reactions
Common Ion Effect
Adding an ion already present in solution shifts the equilibrium away from that ion.
Adding NaCl (source of Cl⁻) shifts left → less AgCl dissolves.
AP Chemistry Tip: Always state both what happens (shift direction) AND why (Le Chatelier's principle counteracts the stress).
📋 AP Chemistry — Exam Format Guide
| Section | Format | Questions | Time | Weight | Calculator |
|---|---|---|---|---|---|
| Multiple Choice | MCQ | 60 | 90 min | 50% | ✅ |
| Free Response (Long) | FRQ | 3 | 69 min | 30% | ✅ |
| Free Response (Short) | FRQ | 4 | 36 min | 20% | ✅ |
📊 Scoring: 1-5
💡 Key Test-Day Tips
- ✓Memorize common polyatomic ions
- ✓Practice dimensional analysis
- ✓Know your gas laws
⚠️ Common Mistakes: Le Chatelier's Principle and Equilibrium Shifts
Avoid these 3 frequent errors
🌍 Real-World Applications: Le Chatelier's Principle and Equilibrium Shifts
See how this math is used in the real world
📝 Worked Example: Stoichiometry — Limiting Reagent
mol of reacts with mol of . How many grams of water are produced? Which is the limiting reagent? ()
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