Hydrolysis

  • Most salts are strong electrolytes and exists as ions in aqueous solution. These ions do not react with water, and have no effect on the pH of the solution
  • Some ions, however, react to form H3O+ or OH ions. These reactions are called hydrolysis reactions.
1. Salt of a strong acid and a strong base yields a neutral solution
  • NaCl
  • Neither Na+ nor Cl react with H2O to form hydroxide or hydronium ions
2. Salt of a weak acid and a strong base yield a cation that hydrolyses, forming OH ions and a basic solution.
  • NaCO2
  • CO32- (aq) + H2O (l) ⇌ HCO3 (aq) + OH(aq)
3. Salt of a strong acid and a weak base yield an anion that hydrolyses, forming H3O+ ions and an acidic solution.
  • NH4Cl
  • NH4+ (aq) + H2O (l) ⇌ NH3 (aq) + H3O+ (aq)
4. Salt of a weak acid and a weak base yields both ions that hydrolyse.

It could be anything: it depends on the relative extent to which each ion hydrolyses.

  • NH4CH3COO
  • NH4+ (aq) + H2O (l) ⇌ NH3 (aq) + H3O+ (aq)
  • CH3COO(aq) + H2O (l) ⇌ CH3COOH (aq) + OH (aq)

Since the two are relatively the same strength, the solution is neutral, pH ~7

5. Cations with large positive charge (Fe3+ and Al3+) form complex ions.

These complex ions can hydrolyse, forming H+ ions, hence acidic solutions.

  • Fe3+ –> Fe(H2O)6 3+
  • Fe(H2O)6 3+ ⇌ Fe(H2O)5(OH) 2+ (aq) + H+ (aq)

 

EXCEPTION:

The SO4 ion has no basic properties at all!!!

So Na2SO4 solution would still be pH 7.

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