Equilibrium
What to know
Equilibrium questions cover the equilibrium constant, Le Chatelier's principle, and acid–base/solubility equilibria. For aA + bB ⇌ cC + dD, K = [C]^c[D]^d / [A]^a[B]^b; pure solids and liquids are omitted. K > 1 favors products; K < 1 favors reactants. The reaction quotient Q compared to K predicts direction: Q < K shifts right, Q > K shifts left, Q = K at equilibrium.
Le Chatelier's principle: adding reactant or removing product shifts right; changing pressure affects only gaseous species with unequal mole counts; temperature changes shift K itself (exothermic favors reactants when heated). Catalysts speed attainment but do not shift position.
For acids and bases, know pH = −log[H⁺], Kw = 1×10⁻¹⁴ at 25 °C, and the conjugate pairs of strong/weak acids. Solubility product Ksp governs precipitation.
Traps
- Including solids or pure liquids in the K expression.
- Thinking a catalyst changes equilibrium position.
- Assuming a temperature change behaves like a concentration change (it alters K).
- Forgetting that adding inert gas at constant volume does not shift equilibrium.
Worked example
For N2 + 3H2 ⇌ 2NH3, increasing pressure shifts toward fewer gas moles-the product side (2 vs 4). Removing NH3 also shifts right. Rule of thumb: count gas moles on each side to predict pressure effects.