Lesson 9 of 12 · 7 min
Common ion effect and hydrolysis of salts
NCERT § "Common Ion Effect in the Ionization of Acids and Bases"; § "Hydrolysis of Salts and the pH of their Solutions"
Stir sodium acetate into the 0.1 M acetic acid. It is not a base you would recognise, yet the pH jumps from 2.87 to 4.74. How?
The lesson in notes
In short
Adding an ion already present in a weak electrolyte's equilibrium pushes the equilibrium back and suppresses ionisation; adding CH₃COONa to acetic acid lowers [H₃O⁺] and raises pH.
The common ion effect is a direct application of Le Chatelier's principle.
Salts of a strong acid and a strong base (NaCl) do not hydrolyse and give neutral solutions.
Salts of a weak acid and a strong base (CH₃COONa) hydrolyse to give basic solutions.
Salts of a strong acid and a weak base (NH₄Cl) hydrolyse to give acidic solutions.
For a salt of a weak acid and a weak base (CH₃COONH₄), pH = 7 + ½(pKa − pKb); the solution is neutral only if pKa = pKb.
Hydrolysis is the reaction of a salt's cation or anion (or both) with water, which changes the pH away from 7.