AP Chemistry Le Chatelier's Principle
Predicting shifts with stress rules and with Q vs. K, three full worked examples.
Le Chatelier's principle is usually taught as a list of "shifts left" and "shifts right" rules, but the AP exam rewards the reasoning behind those rules: compare the reaction quotient Q to the equilibrium constant K, and the direction of the shift follows. Here are both approaches, worked on real reactions.
Le Chatelier's Principle: The Stress-and-Shift Rules
A system at equilibrium that is disturbed shifts in the direction that partially counteracts the disturbance. Adding a species shifts the reaction away from it; removing a species shifts the reaction toward it. Squeezing a gas mixture into a smaller volume shifts it toward the side with fewer moles of gas. Heating shifts an equilibrium in the endothermic direction, treating heat as a reactant or product. Of all these stresses, only temperature changes the value of K; the others change Q and the system shifts until Q returns to the same K. The equilibrium constant itself never moves with concentration or pressure.
Worked Example 1: Predicting Shifts for the Haber Process
N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = −92 kJ (exothermic). For each stress, predict the shift and whether K changes.
| Stress | Shift | Why | K |
|---|---|---|---|
| Add N2 | Right | Reactant added; reaction consumes it | Unchanged |
| Remove NH3 | Right | Product removed; reaction replaces it | Unchanged |
| Decrease volume (raise pressure) | Right | 2 mol of gas on the right vs. 4 mol on the left | Unchanged |
| Raise temperature | Left | Heat is a product; adding heat is like adding product | Decreases |
| Add a catalyst | None | Speeds both directions equally | Unchanged |
| Add argon at constant volume | None | No concentration changes | Unchanged |
The temperature row is the only one where K changes: for an exothermic reaction, heating lowers K, meaning less ammonia at the new equilibrium.
Worked Example 2: Comparing Q to K for H₂ + I₂ ⇌ 2HI
H2(g) + I2(g) ⇌ 2HI(g) has Kc = 50.3 at a given temperature. A mixture has [H2] = 0.50 M, [I2] = 0.50 M, and [HI] = 2.0 M. In which direction does the reaction proceed?
Q = [HI]2 / ([H2][I2]) = (2.0)2 / [(0.50)(0.50)] = 4.0 / 0.25 = 16.0
Since Q (16.0) < K (50.3), there is too little product relative to equilibrium, so the reaction proceeds forward, forming more HI until Q rises to 50.3.
Worked Example 3: Adding HI to an Equilibrium Mixture
The same system sits at equilibrium with [H2] = 0.22 M, [I2] = 0.22 M, [HI] = 1.56 M (the equilibrium concentrations found in the Kc and Kp guide, where Kc = 50.3). Suddenly 1.00 M of HI is added. Which way does the system shift?
New [HI] = 1.56 + 1.00 = 2.56 M, the others unchanged:
Q = (2.56)2 / [(0.22)(0.22)] = 6.554 / 0.0484 = 135
Q (135) > K (50.3), so the system shifts in reverse, consuming HI to form H2 and I2 until Q falls back to 50.3. This matches the stress rule (adding a product shifts the reaction away from it), but the Q-vs-K comparison shows why, and works even when the rule of thumb is ambiguous. The same logic explains the common-ion effect on solubility in Ksp problems.
Common Le Chatelier Mistakes
- Saying a concentration or pressure change changes K. Only temperature changes K; the others change Q, and the system shifts until Q equals the same K again.
- Shifting for a pressure change when both sides have equal moles of gas. If Δngas = 0, a volume change shifts nothing.
- Treating an inert gas or a catalyst as a stress that shifts equilibrium. Neither changes the equilibrium position.
- Forgetting heat is part of the reaction for temperature stresses. Treat it as a product (exothermic) or reactant (endothermic) before deciding the shift.
This same stress-and-shift reasoning applies across the rest of AP Chemistry too -- for every other free tool and guide on this site, start from the AP Chem Score Calculator.
Related Resources
- AP Chem Score Calculator
- Unit 7 Review: Equilibrium
- AP Chemistry Equilibrium Constant Kc and Kp
- How to Solve ICE Table Equilibrium Problems
- AP Chemistry Ksp and Molar Solubility
- AP Chemistry Study Guide
Frequently Asked Questions
What is Le Chatelier's principle?
When a system at equilibrium is disturbed by a change in concentration, pressure/volume, or temperature, it shifts in the direction that partially counteracts the disturbance until a new equilibrium is established.
Which stresses change the value of K?
Only temperature. Changes in concentration, volume, or pressure change the reaction quotient Q and cause a shift, but the system shifts until Q returns to the same, unchanged K. A temperature change actually changes K itself.
How do you predict the effect of a temperature change?
Treat heat as if it were a product of an exothermic reaction, or a reactant of an endothermic one, then apply the same shift logic as for any concentration change. Heating an exothermic reaction shifts it toward the reactants and lowers K; heating an endothermic reaction shifts it toward the products and raises K.
Does adding a catalyst or an inert gas shift an equilibrium?
No to both. A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but at the same position. An inert gas added at constant volume raises total pressure without changing any reactant or product concentration, so Q still equals K and nothing shifts.
How does Q compare to K tell you the direction of the shift?
If Q < K, products are under-represented, so the reaction proceeds forward (toward products). If Q > K, products are over-represented, so the reaction proceeds in reverse (toward reactants). If Q = K, the system is already at equilibrium.
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