Salt Hydrolysis

IB Chemistry: Salt Hydrolysis Salt Hydrolysis 1. Deduce whether salts form acidic, alkaline or neutral aqueous solutions. Salts formed from a strong acid and base e.g. NaCl, will be neutral in aqueous solution. However, salts made from a strong base and a weak acid will be alkaline, such as sodium ethanoate. Ethanoate ions and sodium ions are created when the salt in the solution dissociates: NaCH 3 COO (aq) → Na + (aq) + CH 3 COO - (aq) Don’t forget that water partly dissociates as well: H 2 O (l) ↔ H + (aq) + OH - (aq) The ethanoate ions react with the hydrogen ions in the solution and form undissociated ethanoic acid. Only the Na + and OH - ions are left in the solutions, and because of the high concentration of OH- ions, making the pH alkaline. Conversely, a salt formed from a weak base and a strong acid will become acidic when aqueous e.g. NH 4 Cl. What determines the acidity of a solution when a salt dissolves is the charge of the positive ions that dissociate. Salts that contain positive ions with a high charge density and small ionic radius e.g. AlCl 3 and MgCl 2 will form acidic solutions when they dissolve in water. For example, when AlCl 3 dissolves in water, complex ions are formed: AlCl 3 → [Al(H 2 O) 6 ] 3+ + 3Cl - The lone pair of electrons in H 2 O are strongly attracted to the high charge density of the aluminum ion and will begin to surround it, forming a complex ion. The charge density on the aluminum is so strong that the bound water molecules will undergodeprotonation, breaking the bond between H + and OH - in the ligand. The OH - remains attached to the aluminum central ion whilst the H + ion is ejected into the solution in the process, turning the solution slightly acidic. This process continues until aluminum hydroxideis formed. [Al(H 2 O) 6 ] 3+ ↔ [Al(H 2 O) 5 OH] 2+ ↔ [Al(H 2 O) 4 (OH) 2 ] + ↔ [Al(H 2 O) 3 (OH) 3 ] Keep in mind, this is an equilibrium reaction. By adding more OH- ions to the solution, you shift the equilibrium to the left by encouraging deprotonation to occur. Conversely, by addingmore H+ ions, you shift the equilibrium to the right.