Knowledge Organiser: Dynamic Equilibrium & Le Chatelier's Principle
Key Terms
- Dynamic equilibrium — forward rate = backward rate
- Closed system — required for equilibrium
- Le Chatelier's Principle — system opposes change
- Equilibrium position — left (reactants) or right (products)
Le Chatelier Quick Rules
- Temp ↑ → endothermic direction
- Temp ↓ → exothermic direction
- Conc of reactant ↑ → shift right
- Pressure ↑ → fewer gas molecules side
- Catalyst → no shift (faster equilibrium)
Haber Process
- N₂ + 3H₂ ⇌ 2NH₃ (exothermic forward)
- Temperature: 450°C (compromise)
- Pressure: 200 atm (compromise)
- Catalyst: iron
- High P → more NH₃ (4→2 gas moles)
Common Exam Errors
- Saying equal concs at equilibrium
- Catalyst shifts equilibrium (wrong)
- Not counting gas molecules for pressure
- Missing "closed system" in definition
Key Equations
- N₂ + 3H₂ ⇌ 2NH₃ (Haber process — 4 gas moles → 2 gas moles)
- At equilibrium: rate of forward reaction = rate of reverse reaction
- Le Chatelier: system shifts to oppose any change imposed on it
Memory Aid
- Add heat? → System absorbs heat (endothermic direction)
- Remove heat? → System releases heat (exothermic direction)
- Add reactant? → System uses it up (shifts to products)
- Increase pressure? → System reduces molecules (smaller side)
Common Mistakes
Misconception 1: "Equilibrium means equal amounts of products and reactants" At equilibrium, the rates of the forward and backward reactions are equal — not the concentrations. The equilibrium mixture may contain much more product than reactant, or vice versa. The ratio of products to reactants depends on the specific reaction and the conditions. Misconception 2: "A catalyst shifts the equilibrium position to the right" A catalyst speeds up both the forward and backward reactions equally. Therefore it has no effect on the equilibrium position — the ratio of products to reactants at equilibrium is unchanged. It only makes equilibrium reached faster. Misconception 3: "Increasing temperature always increases the yield of products" Increasing temperature shifts equilibrium in the endothermic direction. If the forward reaction is exothermic (as in the Haber process), increasing temperature shifts equilibrium left — towards reactants — actually reducing the yield of products. Temperature and yield trade off against each other in industrial processes.