Salt Hydrolysis (HL) (DP IB Chemistry): Revision Note

Salt hydrolysis

• When an acid reacts with a base, a neutralisation reaction occurs:

  • Salt and water are formed.

  HA (acid) + MOH (base) → MA (salt) + H₂O

• The salt dissolves in water and splits into ions (dissociates)

  These ions can react with water in a process called hydrolysis

  The pH of the final solution depends on:

  • Whether the original acid and base were strong or weak

  • Whether the ions formed react with water to make the solution acidic or basic

Strong acids and strong bases

• A typical example is the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH):

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O

• The resulting ions, Na⁺ and Cl⁻, do not react with water:

  • They do not act as Brønsted–Lowry acids or bases

  • They do not donate or accept H⁺ ions

• Therefore, the solution remains neutral and the pH stays at 7

Strong acid and weak base

• When a strong acid (like HCl) reacts with a weak base (like NH₃), the salt formed (e.g. NH₄Cl) creates an acidic solution

• This happens because the NH₄⁺ ion (conjugate acid of ammonia) reacts with water to produce H₃O⁺, increasing the acidity:

NH₄⁺ (aq) + H₂O (l) → H₃O⁺ (aq) + NH₃ (aq) 

• The pH drops, which explains why the equivalence point of a strong acid–weak base titration is below 7

Strong base and weak acid

• When a strong base (like NaOH) reacts with a weak acid (like CH₃COOH), the resulting salt (e.g. CH₃COONa) makes the solution basic.

• The CH₃COO^– ion (conjugate base of ethanoic acid) reacts with water to produce OH⁻:

CH₃COO^– (aq) + H₂O (l) → CH₃COOH (aq) + OH^- (aq)

• The solution becomes more alkaline, and the equivalence point is above 7.

Weak acid and weak base

• To determine the pH of the resulting solution of a reaction between a weak acid and weak base we must take into account the K_a and K_b values

• Using the reaction between ammonia, NH₃ (aq), and ethanoic acid, CH₃COOH (aq), as an example:

NH₃ (aq) + CH₃COOH (aq)→ CH₃COONH₄ (aq)

• Both the cation (positive ion) and anion ion (negative) produced will have acid-base properties

CH₃COO^– (aq) + H₂O (l) → CH₃COOH (aq) + OH^- (aq)

NH₄⁺ (aq) + H₂O (l) → H₃O⁺ (aq) + NH₃ (aq) 

K_a(cation) = 

K_b(anion) = 

• If the K_a is larger, the solution will be acidic

• If the K_b is larger the solution will be basic

• If K_a = K_b, then the pH will be 7

Metals

• Small, highly charged metal ions (e.g. Al³⁺) have a high charge density

• These ions attract water molecules and form complex ions such as:

[Al(H₂O)₆]³⁺

• This complex can act as a weak acid by releasing H⁺ ions through hydrolysis:

[Al(H₂O)₆]³⁺ (aq)  [Al(H₂O)₅(OH)]²⁺ (aq) + H⁺ (aq)

• The high charge density pulls electrons from the O–H bonds in water, making it easier for H⁺ to be released

• This release of H⁺ lowers the pH of the solution

• Only 3+ ions (like Al³⁺ or Fe³⁺) are strong enough to do this

  • 1+ and 2+ metal ions do not release H⁺ and therefore do not significantly affect pH

Diagram to show how the aluminium complex forms an acidic solution