Equilibrium and Le Chatelier's principle (HT)
When a reversible reaction reaches equilibrium in a closed system, the position of equilibrium can be shifted by changing conditions. Le Chatelier's principle predicts which way it shifts.
Le Chatelier's principle
If a system at equilibrium is disturbed, the position shifts to oppose the change.
Three changes you must analyse: concentration, temperature, pressure.
Concentration
- Add more reactant → equilibrium shifts to make more product (uses up the added reactant).
- Remove product → equilibrium shifts to make more product (replaces removed amount).
- Add more product → shifts back to reactants.
Temperature
- For an exothermic forward reaction: heat is a "product".
- Increase T → shifts towards reactants (consumes heat). Less product.
- Decrease T → shifts towards products. More product.
- For an endothermic forward reaction: opposite.
Pressure (gases only)
Look at the moles of gas on each side.
- Increase P → equilibrium shifts to the side with fewer moles of gas (relieves the pressure).
- Decrease P → shifts to the side with more moles.
If both sides have the same number of moles of gas, pressure has no effect.
Catalysts
A catalyst does not change the position of equilibrium. It speeds up forward and reverse reactions equally so equilibrium is reached faster.
The Haber process — worked example
N₂(g) + 3H₂(g) ⇌ 2NH₃(g), exothermic forward.
- Reactants: 1 + 3 = 4 mol gas. Products: 2 mol gas.
- High pressure favours products (fewer moles right). Industrial: ~200 atm.
- Low temperature favours products (exothermic forward). But low T also slows the reaction. Compromise: ~450 °C (high enough for reasonable rate, not too high to lose yield).
- Iron catalyst speeds up reaction without changing yield.
⚠Common mistakes
- "Catalyst increases yield" — it does NOT. Equilibrium position is unchanged; only rate to equilibrium changes.
- Forgetting "moles of gas, not just particles" for pressure rules.
- Mixing up forward/reverse for temperature on endo vs exo reactions. Always identify direction of forward.
- Saying "increase T always favours forward" — depends on whether forward is exo or endo.
Standard exam phrasing
"Predict the effect on the equilibrium position of [change]." Answer:
- State the direction (left/right, towards reactants/products).
- Justify using Le Chatelier (system opposes the change).
Links
Extends C6.4. Builds on C5.1 (exo/endo). Used in C10.10 (Haber HT) and links to A-level Kc/Kp.
AI-generated · claude-opus-4-7 · v3-deep-chemistry