🎓 Prepared by students from Boğaziçi University

What are Hydrolysis Reactions?

Hydrolysis is a reaction in which water (H₂O) breaks chemical bonds and becomes incorporated into products. In acid–base hydrolysis, ions from a salt react with water, often affecting the solution's pH.

Short answer

Hydrolysis occurs when water molecules split and donate H⁺ or OH⁻ to another species, breaking bonds. Common examples: salts of weak acids/bases hydrolyze, making solutions acidic or basic; esters + water → carboxylic acid + alcohol.

Salt Hydrolysis Example: Sodium Acetate

1
1. Salt dissolved
NaCH₃COO dissolves in water → Na⁺ + CH₃COO⁻
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2. Water reacts
Acetate ion accepts H⁺ from water: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻
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3. pH effect
OH⁻ is released → solution becomes basic (pH > 7)
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4. Equilibrium
Mostly salt + water remain; small [OH⁻]

Step-by-step worked examples

Sodium acetate (NaCH₃COO) is dissolved in water. Will the solution be acidic, basic, or neutral?

Acetate ion is the conjugate base of weak acetic acid.
It will accept H⁺ from water: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻
OH⁻ is produced, making the solution basic (pH > 7).
This is acid hydrolysis of a salt of a weak acid.

An ester (like ethyl acetate, CH₃COOC₂H₅) reacts with water. What are the products?

Ester hydrolysis: CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH
(With catalyst or base, reaction is faster)
Products: acetic acid + ethanol

Why is a solution of ammonium chloride (NH₄Cl) acidic?

NH₄⁺ is the conjugate acid of weak ammonia (NH₃).
It donates H⁺ to water: NH₄⁺ ⇌ NH₃ + H⁺
H⁺ is released, lowering pH (pH < 7), making solution acidic.
This is base hydrolysis of a salt of a weak base.

Flashcards

Quick quiz

Q1.Sodium acetate solution will be…

Correct answer: B. Acetate (CH₃COO⁻) is a weak base, accepting H⁺ from water and releasing OH⁻.

Q2.Which salt hydrolyzes to make an acidic solution?

Correct answer: B. NH₄⁺ is a weak acid; it donates H⁺ to water, lowering pH.

Q3.In ester hydrolysis (R-COO-R' + H₂O), the products are…

Correct answer: B. Ester hydrolysis produces carboxylic acid and alcohol.

Q4.Hydrolysis of a strong acid salt (e.g., NaCl) will produce…

Correct answer: C. Neither Na⁺ nor Cl⁻ hydrolyze (both from strong base and strong acid), so solution remains neutral.

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Common mistakes

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All salts hydrolyze. — Correct: Only salts of weak acids or weak bases hydrolyze significantly.

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Hydrolysis always makes a solution basic. — Correct: Hydrolysis can make it basic (if anion of weak acid) or acidic (if cation of weak base).

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Water doesn't participate in hydrolysis; it's just a solvent. — Correct: Water is a key reactant in hydrolysis, donating or accepting H⁺ and OH⁻.

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Hydrolysis and neutralization are the same. — Correct: Neutralization is acid + base → salt + water. Hydrolysis is salt + water → weak acid/base + water.

FAQ

What is the difference between hydrolysis and neutralization?

Neutralization: strong acid + strong base → salt + water (no pH effect). Hydrolysis: salt + water → weak acid/base + water (pH changes).

Which salts undergo hydrolysis?

Salts where the anion is from a weak acid (e.g., CH₃COO⁻) or the cation is from a weak base (e.g., NH₄⁺).

Why doesn't NaCl hydrolyze?

Na⁺ is a very weak acid (won't donate H⁺); Cl⁻ is a very weak base (won't accept H⁺). Neither drives hydrolysis.

Hydrolysis constant Kh—how is it related to Ka?

For an anion: Kh = Kw / Ka. For a cation: Kh = Kw / Kb. This links salt hydrolysis to parent acid/base strength.