2.3.4—Le Châtelier's principle
- Syllabus
- First assessment 2025
- Objective
- 2.3.4
- Level
- SL
Le Châtelier's Principle
An equilibrium shifts to partially counteract an imposed change. Pressure favours the side with fewer gaseous molecules; temperature favours the endothermic direction; concentration changes alter composition.
At fixed temperature, concentration and pressure changes do not change K. Temperature changes K. A catalyst changes rates in both directions and does not change equilibrium position.
For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), compression favours the two-mole gas side, but K is unchanged if temperature is fixed. Heating favours the endothermic direction and changes K; a catalyst reaches the same equilibrium faster by accelerating both directions.
| Disturbance | Immediate evidence | K at fixed/new T | Direction check |
|---|---|---|---|
| concentration or pressure change | Q changes before composition readjusts | unchanged if T is fixed | compare the new Q with K |
| raise temperature | heat favours the endothermic direction | K increases if the forward reaction is endothermic; decreases if it is exothermic | use the stated forward ΔH |
| catalyst | both forward and reverse rates increase | unchanged | equilibrium composition is unchanged; it is reached sooner |
For gas pressure, count gaseous coefficients only. Use Q/K or opposing-rate evidence to justify the shift rather than the phrase “counteracts the change” alone.
Predicting Equilibrium Shifts
Assessment in practice
Representative question
Question 1
[Maximum number: 2]
Explain why an increase in pressure shifts the position of equilibrium towards the products and how this affects the value of the equilibrium constant, Kc.
shifts to the side with fewer moles «of gas» OR shifts to right as there is a reduction in volume
«value of » Kc unchanged
Marking guidance:
Accept " Kc only affected by changes in temperature".
Extent of Chemical Change Summary
Retrieve the route: define dynamic equilibrium, write K, interpret its magnitude, predict Le Châtelier shifts, compare Q with K, solve a RICE table, and connect K with ΔG.
Check closed-system and equal-rate language, exponents and direction, whether a change affects K, current versus equilibrium concentrations, stoichiometric x changes, and kelvin/unit consistency in ΔG calculations.