Le Chatelier's principle helps predict how a chemical equilibrium responds when conditions change. It matters because many important reactions, from industrial ammonia production to blood oxygen transport, are reversible and can shift in either direction. At equilibrium, the forward and reverse reaction rates are equal, but the amounts of reactants and products do not have to be equal.
A stress such as adding reactant, changing volume, or heating the system causes the equilibrium position to shift to reduce that stress.
Key Facts
- At equilibrium, rate forward = rate reverse.
- Reaction quotient: Q = [products]^coefficients / [reactants]^coefficients.
- If Q < K, the reaction shifts right toward products.
- If Q > K, the reaction shifts left toward reactants.
- For gases, decreasing volume increases pressure and shifts equilibrium toward the side with fewer moles of gas.
- For an exothermic reaction, heat is a product, so increasing temperature shifts left.
Vocabulary
- Dynamic equilibrium
- A state in which forward and reverse reactions continue at equal rates so concentrations remain constant.
- Le Chatelier's principle
- The rule that an equilibrium system shifts to oppose a change in concentration, pressure, volume, or temperature.
- Reaction quotient
- A value calculated like the equilibrium constant using current concentrations to predict the direction of shift.
- Equilibrium constant
- A constant value at a fixed temperature that compares product concentrations to reactant concentrations at equilibrium.
- Endothermic reaction
- A reaction that absorbs heat, so heat can be treated as a reactant in equilibrium predictions.
Common Mistakes to Avoid
- Saying the system shifts toward the side with more particles after compression is wrong because increased pressure favors the side with fewer moles of gas.
- Including pure solids or pure liquids in K or Q expressions is wrong because their activities are treated as constant and do not appear in the expression.
- Assuming a catalyst changes the equilibrium position is wrong because a catalyst speeds up both forward and reverse reactions equally and only helps the system reach equilibrium faster.
- Forgetting that temperature changes alter K is wrong because concentration and pressure changes shift position at the same K, but changing temperature changes the equilibrium constant itself.
Practice Questions
- 1 For N2(g) + 3H2(g) ⇌ 2NH3(g), a container at equilibrium has [N2] = 0.20 M, [H2] = 0.30 M, and [NH3] = 0.60 M. Calculate Q and compare it with K = 100 to predict the shift direction.
- 2 For 2SO2(g) + O2(g) ⇌ 2SO3(g), predict the shift when the volume of the container is cut in half. Then state which side has fewer gas moles.
- 3 For Co(H2O)6^2+(aq) + 4Cl−(aq) ⇌ CoCl4^2−(aq) + 6H2O(l), adding chloride ion makes the solution more blue because CoCl4^2− is blue. Explain the shift using Le Chatelier's principle.