18.1.5 Salt Hydrolysis

• An ionic salt is formed from the neutralisation reaction of an acid and base

Neutralisation 

• The ionic salt, MA, formed will dissociate in water
  • Hydrolysis is where water is used to break a bond within a compound, which results in the aqueous ions for an ionic salt

• The reaction of the salt will vary depending on the strength of the acids and bases used in the neutralisation reaction
• The use of the differing strengths of the acids and bases will directly influence the type of salt hydrolysis and the pH of the final solution

Strong Acids and Strong Bases

• A common example of this is the reaction between hydrochloric acid, HCl (aq), and sodium hydroxide (aq):

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

• The Na⁺ and Cl^- ions do not act as Brønsted-Lowry acids or bases as they can not release or accept H⁺ ions
• Therefore, they do not affect the pH

Strong Acid and Weak Base

• The salt formed by a strong acid such as hydrochloric acid, HCl (aq), and a weak base such as ammonia, NH₃ (aq), will form an acidic solution:

HCl (aq) + NH₃ (aq) → NH₄Cl (aq)

• In this reaction, the conjugate acid of ammonia is formed, NH₄⁺, and can react with water to produce H₃O⁺

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

• Therefore, the solution becomes more acidic
• The hydrolysis of this salt demonstrates why the equivalence point of a strong acid - weak base pH curve is below 7

Strong Base and Weak Acid

• The salt formed by a strong base such as sodium hydroxide, NaOH (aq), and a weak acid such as ethanoic acid, CH₃COOH (aq), will form an acidic solution:

NaOH (aq) + CH₃COOH (aq) → CH₃COONa (aq) + H₂O (l)

• In this reaction, the conjugate base of ethanoic acid is produced, CH₃COO^– (aq), and this will react with water to form hydroxide ions, OH^- (aq)

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

• Therefore, the solution becomes more basic
• The hydrolysis of this salt demonstrates why the equivalence point of a strong base - weak acid pH curve is above 7

Weak Acid and Weak Base

• In order 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) 

How to calculate the vales for the values of K_a (cation) and 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 metal ions that have a high charge will exhibit a high charge density
  • An example is Al³⁺
• This makes the highly charged metal ions ideal for forming complexes as they can coordinately bond with ligands
• The complex formed can then act as a weak acid by releasing hydrogen ions when hydrolysed, H⁺
• The high charge density of the metal ion increases the polarity of the water molecule pulling the electrons towards itself, until the O-H bond finally breaks

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

• The metal ion must have a high enough charge and small radius for this to occur, consequently, 1+ and 2+ ions will not release H⁺ ions and therefore decrease the pH of a solution

The [Al(H₂O)₆]³⁺ (aq) releases an H+ ion decreasing the pH of the solution