🎓 Prepared by students from Boğaziçi University

What is Le Chatelier's Principle?

Q: What is Le Chatelier's Principle?

When equilibrium is stressed (concentration, pressure, temperature), the system shifts to counteract the change and re-establish equilibrium.

Q: What happens if you add more reactant?

Equilibrium shifts forward (right) to consume the added reactant, reducing the stress.

Q: What happens if you increase pressure?

Equilibrium shifts toward fewer moles of gas (e.g., 4 moles → 2 moles favors the side with 2).

Q: Does temperature shift change Kc?

Yes — temperature is the only factor that changes Kc. Exothermic shifts left when heated; endothermic shifts right.

Le Chatelier's Principle states that when a system at equilibrium is disturbed, it shifts to counteract the disturbance and restore equilibrium. Changes in concentration, pressure, or temperature trigger predictable shifts.

Short answer

When equilibrium is stressed (concentration/pressure/temperature change), the system shifts to consume or produce species, reducing the stress. This doesn't change Kc, but temporarily shifts the equilibrium position, eventually re-establishing constancy of concentrations.

Le Chatelier shifts in response to stress

1
System at equilibrium
Forward rate = Reverse rate; constant concentrations
↓
2
Stress applied
Add reactant, remove product, increase pressure, change temperature
↓
3
System shifts
One direction dominates (forward or reverse)
↓
4
New equilibrium reached
Rates equalize again; Kc unchanged, but new concentrations

Step-by-step worked examples

For N₂ + 3H₂ ⇌ 2NH₃ at equilibrium, you add more N₂. Which way does equilibrium shift?

Adding N₂ increases [N₂] (stress = too much reactant)
→ System shifts right (forward) to consume N₂
→ More NH₃ forms until equilibrium re-established
Result: [N₂] decreases, [NH₃] increases

For 2NO₂ ⇌ N₂O₄ (colorless), pressure increases. What happens?

Δn = 2 → 1 (moles decrease on right)
Increasing pressure favors fewer moles
→ Shift right (forward)
→ More N₂O₄ forms; color fades (colorless)
System absorbs the pressure increase

For N₂ + 3H₂ ⇌ 2NH₃ (exothermic), temperature increases. Shift?

Reaction is exothermic (releases heat)
Higher temperature = added heat (stress)
→ System shifts left (endothermic direction) to absorb heat
→ Less NH₃ at new equilibrium; efficiency decreases
Trade-off: high temp speeds reaction, but lowers yield

Flashcards

Quick quiz

Q1.For A ⇌ 2B at equilibrium, you remove some B. Which way shifts?

Correct answer: B. Removing product decreases [B] → forward shift (right) to replace the removed B.

Q2.For an exothermic reaction at equilibrium, temperature increases. Yield…

Correct answer: B. Exothermic reactions shift left (endothermic direction) when heated → yield decreases.

Q3.Increasing pressure favors which side of N₂ + 3H₂ ⇌ 2NH₃?

Correct answer: B. Right side has 2 moles total (1 N₂, 0 is liquid—actually 2 NH₃ only), left has 4 moles. Pressure favors fewer moles → right.

Q4.If you add a catalyst to an equilibrium system…

Correct answer: C. Catalyst speeds both directions equally → no net shift. Kc unchanged; reaches equilibrium faster.

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Common mistakes

✗

Confusing which direction equilibrium shifts (Le Chatelier). — Correct: Add product → shifts left. Add reactant → shifts right. Always opposite the stress.

✗

Thinking Le Chatelier changes Kc. — Correct: Kc depends only on T. Le Chatelier shifts positions of equilibrium (new concs), not the constant.

✗

Ignoring the stoichiometric coefficients in pressure shifts. — Correct: Pressure favors the side with fewer moles of gas (Δn < 0 is favored by higher P).

✗

Believing catalysts shift equilibrium. — Correct: Catalysts speed reactions equally both ways → no net shift, no change to Kc or equilibrium position.

FAQ

What is Le Chatelier's Principle?

When equilibrium is stressed (concentration, pressure, temperature), the system shifts to counteract the change and re-establish equilibrium.

What happens if you add more reactant?

Equilibrium shifts forward (right) to consume the added reactant, reducing the stress.

What happens if you increase pressure?

Equilibrium shifts toward fewer moles of gas (e.g., 4 moles → 2 moles favors the side with 2).

Does temperature shift change Kc?

Yes — temperature is the only factor that changes Kc. Exothermic shifts left when heated; endothermic shifts right.