AL Chem一小題

When dissolved in water, aluminium sulphate(VI) dissociates to give aqueous aluminium and sulphate(VI) ions. Actually, an aqueous aluminium ion exists as a hydrated aluminium ion, [Al(H2­O)6]³⁺, which hydrolyzes in water according to the following equation:
[Al(H2­O)6]³⁺(aq) + H2O(l) ⇌ [Al(H2O)5(OH)]²⁺(aq) + H3O⁺(aq) …… (*)

In both [Al(H2­O)6]³⁺ and [Al(H2O)5(OH)]²⁺ ions, the central ion is Al³⁺. Al³⁺ ion has a small size and carries a charge as high as +3. Therefore, the charge density (charge : volume ratio) of Al³⁺ is high, and this leads to high polarizing power of Al³⁺ ion. The acidity of aluminium sulphate(VI) can thus be explained in either of the following two ways:

(1) Older explanation (may be out-dated):
In [Al(H2­O)6]³⁺ ion, H2­O molecules are highly polarized by the Al³⁺ ion. Therefore, the electron clouds of the H2O molecules are distorted to a great extent. This makes the H2O molecules unstable and they would thus dissociate in reaction (*). The solution becomes acidic as the formation of H3O⁺ ion in the forward reaction of reaction (*).

(2) Newer explanation:
In [Al(H2O)5(OH)]²⁺ ion, the strong attraction between the highly polarizing Al³⁺ ion and the OH¯ ion increase the stability of the [Al(H2O)5(OH)]²⁺ ion. This favours the forward reaction of reaction (*), and the solution becomes acidic as the formation of H3O⁺ ion in the forward reaction of reaction (*).