Chemical Equilibrium and Acid-Base Chemistry
What this domain tests
The equilibrium and acid-base domain combines ICE tables, Le Chatelier's principle, weak-acid and buffer calculations, and the salt-hydrolysis reasoning used to predict whether a solution is acidic, basic or neutral.
Core rules
- For aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d/([A]^a[B]^b). Pure solids and liquids are omitted. Kp = Kc(RT)^Δn, where Δn counts only gaseous moles.
- Q compared with K gives the direction: Q < K shifts right, Q > K shifts left.
- Le Chatelier: changing concentration or pressure shifts the position but not K; only temperature changes K. A catalyst changes nothing about the position or K, only the rate of approach.
- Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C, so pH + pOH = 14. Ka·Kb = Kw for a conjugate pair.
- Buffers: pH = pKa + log([A⁻]/[HA]), valid when the ratio is between 0.1 and 10.
Worked example
Find the pH of 0.100 mol dm⁻³ acetic acid, Ka = 1.8 × 10⁻⁵. Let x = [H⁺]. x²/(0.100 − x) = 1.8 × 10⁻⁵. Assuming x ≪ 0.100: x² = 1.8 × 10⁻⁶, x = 1.34 × 10⁻³ mol dm⁻³ (1.3% of 0.100, so the assumption is valid). pH = −log(1.34 × 10⁻³) = 2.87.
Traps that cost marks
- Skipping the 5% check on the small-x assumption.
- Adding an inert gas at constant volume - no shift at all, because no partial pressure changes.
- Forgetting that a salt of a weak acid and strong base is basic (hydrolysis of the anion).
- Using pH + pOH = 14 at temperatures other than 25 °C, where Kw differs.