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6.1.13Additional lessons

Salt Hydrolysis (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 127 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 5Why Salts Can Change pH

6.1.13 Salt Hydrolysis (HL)

When a salt dissolves, its ions can react with water and shift the pH away from 7.

Hydrolysis
  • Hydrolysis is the reaction of an ion with water in which the pH of the solution is changed.
Spectator ions
  • The ions of strong acids and strong bases, such as Na⁺, K⁺, Cl⁻ and NO₃⁻, do not react with water.
Conjugate base
  • The anion of a weak acid is a base and accepts a proton from water, producing OH⁻.
Conjugate acid
  • The cation of a weak base is an acid and donates a proton to water, producing H₃O⁺.

Part 2 of 5Predicting the Solution

Predicting the Solution
Strong acid + strong base
  • Salts such as NaCl give a neutral solution with a pH of 7.
Weak acid + strong base
  • Salts such as CH₃COONa are basic: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻.
Strong acid + weak base
  • Salts such as NH₄Cl are acidic: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺.
Metal ions
  • Small, highly charged cations such as Al³⁺ polarise water in [Al(H₂O)₆]³⁺ and give acidic solutions.
Salt fromExampleSolution
Strong acid, strong baseNaClNeutral
Weak acid, strong baseCH₃COONaBasic
Strong acid, weak baseNH₄ClAcidic

Part 3 of 5Working Through

1
Split the salt into its cation and anion and decide which acid and base it could have come from.
2
Ignore any ion that is the conjugate partner of a strong acid or strong base.
3
For the remaining ion, write the equation with water and identify whether OH⁻ or H₃O⁺ is formed.
4
State the pH as acidic or basic, according to the ion that is produced in excess.
5
For a quantitative question, use Kb = Kw/Ka for the anion and solve like a weak base.

Part 4 of 5Worked Example

Question Calculate the pH of 0.100 mol dm⁻³ sodium ethanoate, given Ka(CH₃COOH) = 1.74 × 10⁻⁵.

  1. Kb of ethanoate = 1.00 × 10⁻¹⁴ / 1.74 × 10⁻⁵ = 5.75 × 10⁻¹⁰.
  2. [OH⁻] = √(Kb × c) = √(5.75 × 10⁻¹¹) = 7.58 × 10⁻⁶ mol dm⁻³.
  3. pOH = 5.12, so pH = 14.00 − 5.12 = 8.88.

Answer pH = 8.88 (basic)

Part 5 of 5Exam Checklist & Tips

6.1.13 – Key Points for the Exam

  • Hydrolysis The reaction of an ion with water that changes the pH of the solution.
  • Salt An ionic compound made of a cation and an anion.
  • Spectator ion An ion that does not take part in the reaction.
  • Conjugate base The species left when an acid has lost a proton.

Common Mistakes

  • Saying that all salts give neutral solutions.
  • Writing hydrolysis equations for Na⁺ or Cl⁻.
  • Choosing a basic solution for NH₄Cl.
  • Forgetting that the conjugate base of a weak acid is itself a base.

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