AP Chemistry: Le Chatelier’s Principle (Concentration, Temperature, Pressure)

AP Chemistry – Le Chatelier’s Principle (Concentration, Temperature, Pressure)

AP Chemistry Study Guide: Le Chatelier’s Principle (Concentration, Temperature, Pressure)

What This Is

Le Chatelier’s Principle explains how a chemical system at equilibrium responds to disturbances (like changes in concentration, temperature, or pressure) by shifting to counteract the change and re-establish equilibrium. This is critical for the AP exam because it appears in multiple-choice questions, FRQs, and lab-based scenarios—often tied to real-world applications like industrial processes (e.g., the Haber process for ammonia synthesis) or biological systems (e.g., hemoglobin binding oxygen in your blood). For example, if you’ve ever seen a cold pack (endothermic reaction) or a hot pack (exothermic reaction), you’ve witnessed Le Chatelier’s Principle in action: the system shifts to absorb or release heat when disturbed.

Key Terms & Concepts

• Le Chatelier’s Principle: When a system at equilibrium is disturbed, it shifts in the direction that reduces the disturbance to re-establish equilibrium.
• Equilibrium: A state where the forward and reverse reaction rates are equal, and concentrations of reactants/products remain constant (not necessarily equal!).
• Stress: A change imposed on a system at equilibrium (e.g., adding/removing reactants/products, changing temperature or pressure).
• Shift Right (→): The forward reaction speeds up, producing more products.
• Shift Left (←): The reverse reaction speeds up, producing more reactants.
• Concentration Changes:
• Add reactant → Shift right (to use up the added reactant).
• Remove product → Shift right (to replace the lost product).
• Add product → Shift left (to use up the added product).
• Pressure Changes (for gases only!):
• Increase pressure (decrease volume) → Shift to the side with fewer moles of gas (to reduce pressure).
• Decrease pressure (increase volume) → Shift to the side with more moles of gas (to increase pressure).
• ⚠️ No shift if moles of gas are equal on both sides!
• Temperature Changes:
• Exothermic reaction (ΔH < 0): Heat is a product.
  • Increase temp → Shift left (to absorb heat).
  • Decrease temp → Shift right (to release heat).
• Endothermic reaction (ΔH > 0): Heat is a reactant.
  • Increase temp → Shift right (to absorb heat).
  • Decrease temp → Shift left (to release heat).
• Catalysts: Do not affect equilibrium position—they only speed up the rate at which equilibrium is reached.
• Inert Gases (e.g., He, Ar): Adding an inert gas at constant volume has no effect on equilibrium (partial pressures don’t change). At constant pressure, it increases volume, shifting equilibrium to the side with more moles of gas.

Step-by-Step / Process Flow

How to predict equilibrium shifts like a pro:

1. Identify the stress (change in concentration, pressure, or temperature).
2. Determine the direction of the shift based on Le Chatelier’s Principle:
3. Concentration: Shift to use up added or replace removed species.
4. Pressure (gases only): Shift to the side with fewer moles of gas if pressure increases (or more if pressure decreases).
5. Temperature: Treat heat as a reactant (endothermic) or product (exothermic) and shift accordingly.
6. Check for exceptions:
7. No shift if moles of gas are equal (pressure change).
8. No shift if an inert gas is added at constant volume.
9. Predict the effect on concentrations:
10. If the reaction shifts right, products increase and reactants decrease.
11. If the reaction shifts left, reactants increase and products decrease.
12. Verify with an example:
13. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = –92 kJ/mol (exothermic):
    • Add N₂ → Shift right (more NH₃).
    • Increase pressure → Shift right (2 moles gas vs. 4 moles gas).
    • Increase temperature → Shift left (heat is a product).

Common Mistakes

• Mistake: Thinking adding a catalyst shifts equilibrium.
  Correction: Catalysts only speed up the rate of reaching equilibrium—they don’t change the equilibrium position.

• Mistake: Assuming pressure changes always shift equilibrium.
  Correction: Pressure changes only matter for gases and only if moles of gas differ on each side. If moles are equal, no shift occurs.

• Mistake: Forgetting that temperature changes affect K (equilibrium constant).
  Correction: Unlike concentration/pressure changes, temperature changes alter K because they change the energy of the system.

• Mistake: Treating heat as a reactant for exothermic reactions.
  Correction: For exothermic reactions (ΔH < 0), heat is a product. For endothermic reactions (ΔH > 0), heat is a reactant.

• Mistake: Ignoring inert gases in pressure changes.
  Correction: Adding an inert gas at constant volume has no effect. At constant pressure, it increases volume, shifting equilibrium to the side with more moles of gas.

AP Exam Insights

• FRQ Hotspots:
• Lab-based questions (e.g., "Explain how adding HCl affects the equilibrium of a weak acid dissociation").
• Industrial applications (e.g., "Why is the Haber process run at high pressure but moderate temperature?").

• Graph interpretation (e.g., "A reaction shifts left when temperature increases—is it endothermic or exothermic?").

• Multiple-Choice Traps:

• Pressure changes with equal moles of gas (no shift, but students often pick a direction).
• Inert gases (students forget they only matter at constant pressure).

• Temperature vs. concentration (students mix up how each affects K).

• Tricky Distinction:

• K vs. Q: Le Chatelier’s Principle explains how a system responds to stress, but K (equilibrium constant) only changes with temperature. Q (reaction quotient) helps predict the direction of shift when concentrations/pressures change.

Quick Check Questions

1. Multiple Choice

For the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = –198 kJ/mol, which change will increase the amount of SO₃ at equilibrium? (A) Adding a catalyst (B) Increasing temperature (C) Decreasing pressure (D) Adding O₂

Answer: (D) Adding O₂ Explanation: Adding O₂ shifts the reaction right to use up the added reactant, producing more SO₃.

2. Short FRQ

Consider the equilibrium: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) ΔH > 0
(a) Predict the shift if Cl₂ is removed.
(b) Predict the shift if temperature is decreased.
(c) Explain why adding He gas at constant volume has no effect.

Answers:
(a) Shift right (to replace the removed Cl₂).
(b) Shift left (endothermic reaction, so decreasing temp favors reactants).
(c) At constant volume, partial pressures don’t change, so equilibrium isn’t disturbed.

Last-Minute Cram Sheet

1. Le Chatelier’s Principle: System shifts to counteract stress.
2. Add reactant → Shift right (use it up).
3. Remove product → Shift right (replace it).
4. Increase pressure (gases only) → Shift to fewer moles of gas.
5. Decrease pressure (gases only) → Shift to more moles of gas.
6. Exothermic (ΔH < 0): Heat is a product → increase temp = shift left.
7. Endothermic (ΔH > 0): Heat is a reactant → increase temp = shift right.
8. Catalysts → No effect on equilibrium position.
9. Inert gases at constant volume → No effect.
10. ⚠️ Temperature changes K; concentration/pressure changes do not!