# Equilibrium and Le Chatelier's Principle
AP Chemistry Unit 7 covers dynamic equilibrium, equilibrium expressions, calculations with ICE tables, and predicting shifts using Le Chatelier's principle and Q vs K analysis.
1. Equilibrium Expressions
- Gases and aqueous species included
- Solids and pure liquids excluded
- depends only on temperature
for gases: use partial pressures. where = moles gas products − moles gas reactants.
2. Q vs K
| Comparison | Direction |
|---|---|
| Forward → (more products needed) | |
| At equilibrium | |
| Backward ← (more reactants needed) |
3. ICE Tables
| A | B | C | |
|---|---|---|---|
| Initial | known | known | 0 |
| Change | |||
| Equilibrium | I−ax | I−bx | cx |
Substitute into K expression and solve for x.
Simplification: if is very small, assume initial concentration.
4. Le Chatelier's Principle
| Stress | Response |
|---|---|
| Add reactant | Shifts forward |
| Remove product | Shifts forward |
| Increase T (exo forward) | Shifts backward; K decreases |
| Increase T (endo forward) | Shifts forward; K increases |
| Increase pressure | Shifts toward fewer gas moles |
| Add catalyst | No shift; K unchanged; equilibrium reached faster |
| Add inert gas (constant V) | No effect |
5. Practice Questions
- Write for:
- At equilibrium: [N₂] = 0.50, [H₂] = 1.50, [NH₃] = 0.10. Calculate for .
- . Initially [A] = 1.0, [B] = 0. Use ICE to find equilibrium concentrations for .
- , . Which direction does the reaction shift?
- For an exothermic reaction, what happens to K when temperature increases?
Want to check your answers and get step-by-step solutions?
Summary
- = products/reactants at equilibrium; only changes with temperature
- → forward; → backward
- ICE tables for equilibrium calculations
- Le Chatelier: system opposes changes
- Catalysts don't change K