What is Salt Hydrolysis?
When a salt dissolves in water, its ions can react with water molecules—a process called hydrolysis. This reaction may produce H⁺ or OH⁻ ions, shifting the solution's pH away from neutral.
Salt hydrolysis is the reaction of salt ions with water molecules, producing H⁺ or OH⁻. Salts of weak acids or bases hydrolyze, making solutions acidic or basic.
- 1↓1. Salt dissolvesNaA → Na⁺ + A⁻ in water
- 2↓2. Ion meets waterA⁻ (weak conjugate base) attracts H⁺
- 3↓3. Hydrolysis reactionA⁻ + H₂O ⇌ HA + OH⁻
- 44. Solution becomes basicExcess OH⁻ raises pH > 7
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Step-by-step worked examples
A 0.1 M solution of NaA (weak acid A). Ka = 1×10⁻⁵. Will it be acidic or basic?
Kh = Kw/Ka = (1×10⁻¹⁴)/(1×10⁻⁵) = 1×10⁻⁹ A⁻ is a weak base, hydrolyzes: A⁻ + H₂O ⇌ HA + OH⁻ The anion forms OH⁻ → solution is basic (pH > 7).
In a solution of NH₄Cl, which ion hydrolyzes? Is the solution acidic or basic?
NH₄⁺ is conjugate acid of weak base NH₃. NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺ Produces H⁺ → solution is acidic (pH < 7).
NaCl dissolved in water. Explain pH.
Na⁺ does not hydrolyze (conjugate acid of strong base). Cl⁻ does not hydrolyze (conjugate base of strong acid). No hydrolysis occurs → pH ≈ 7 (neutral).
Flashcards
Quick quiz
Q1.Which salt solution is acidic?
Q2.Hydrolysis constant Kh = Kw/Ka. Why divide?
Q3.Does NaCl hydrolyze?
Q4.Rank by pH (highest to lowest): NaA, NaCl, NH₄Cl.
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Common mistakes
All salts make solutions acidic or basic. — Correct: Only salts of weak acids/bases hydrolyze; strong salt—neutral.
Hydrolysis means breaking apart completely. — Correct: It's an equilibrium—only partial reaction.
Higher Ka always means more hydrolysis. — Correct: Higher Ka means lower Kh—weaker hydrolysis.
Na⁺ ions cause acidity in NaNO₃. — Correct: Na⁺ doesn't hydrolyze; NO₃⁻ is too weak to hydrolyze.
FAQ
What causes salt hydrolysis?
Salt ions react with water's H⁺ and OH⁻, shifting the hydrolysis equilibrium.
Why do weak acid salts form basic solutions?
The anion (weak conjugate base) pulls H⁺ from water, leaving OH⁻.
Is all hydrolysis acidic?
No—anion hydrolysis is basic, cation hydrolysis is acidic.
pH of 0.1 M NaA if Ka(HA) = 10⁻⁵?
Kh = 10⁻⁹; [OH⁻] ≈ √(Kh·C) → basic pH.