3.1.12 (HL)—Salt hydrolysis

Trace each salt ion to its parent acid or base. A conjugate base from a weak acid can hydrolyse water to produce OH− and an alkaline solution; a conjugate acid from a weak base can produce H3O+.

$A− + H2O ⇌ HA + OH−$

Spectator ions from strong parents do not control pH. NH₄Cl is acidic because NH₄⁺ donates a proton to water, while a carbonate salt is basic because CO₃²⁻ accepts one; write the hydrolysing ion, not the intact salt, in the equilibrium.

Representative question

[Maximum number: 1]

Explain, with reference to acid-base equilibria, why the sodium benzoate solution formed has a pH>7.

«benzoate ion» hydrolysis/reaction with water AND forms $\mathrm{OH}^{-}$ OR
$\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COO}^{-}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq}) \quad$

Retrieve the route: track proton transfer and conjugates, calculate pH and Kw, distinguish strength, balance neutralization, read titration curves, use Ka/Kb and hydrolysis, select indicators, and explain and calculate buffer behaviour.

Check donor versus acceptor, one-proton differences, logarithm direction, ion comparison, strength versus concentration, equivalence versus endpoint, pKa landmarks, conjugate equations and dilution ratios.

3.1.12 (HL)—Salt hydrolysis

Salt Hydrolysis

Predicting Salt-Solution pH

Assessment in practice

Proton Transfer Reactions Summary

Question 1