Detailed and comprehensive notes on unit 3.2.6 (metal-aqua ions) of AQA A level chemistry. Covers: reactions with hydroxide ions, ammonia and carbonate ions (with explanations and equations), and a summary table of observations.
Metal-Aqua Ions
Lewis Acids and Bases
• Lewis Acid: Electron Pair Acceptor (metal ion accepts the pair of electrons)
• Lewis Base: Electron Pair Donor (ligand donates lone pair in the dative covalent bond)
Metal-Aqua Ions
• When transition metal compounds dissolve in water, the water molecules form coordinate bonds
with the metal ions.
• This forms metal-aqua complex ions.
• The water molecules donate a lone pair of electrons, from their oxygen, to the metal ion.
• Generally, 6 water molecules form coordinate bonds with each ion.
• Fe2+ ions and Cu2+ ions form [M(H2O)6]2+ complexes.
• Fe3+ ions and Al3+ ions form [M(H2O)6]3+ complexes.
Acidity / Hydrolysis Reactions
M3+ vs. M2+ Ions
• M3+ ions have a greater charge than M2+ ions, so have a higher charge density.
• M3+ ions are therefore more polarising than M2+ ions.
• M3+ ions more strongly attract electrons from the oxygen atoms in coordinated water bonds,
weakening the O–H bond.
• This makes it more likely that a hydrogen ion will be released, increasing the acidity of the solution.
M2+ Ions and Water
• [M(H2O)6]2+ + H2O ⇌ [M(H2O)5(OH)]+ + H3O+
• There is only slight dissociation, so the solution is weakly acidic.
• The [M(H2O)6]2+ ion is the predominant species present.
M3+ and Water
• The following equilibria occurs in aqueous solutions of metal 3+ ions:
o [M(H2O)6]3+ + H2O ⇌ [M(H2O)5(OH)]2+ + H3O+
• There is more dissociation, so the solution is more acidic.
Adding Hydroxide Ions
M3+:
1. [M(H2O)6]3+ (aq) + H2O (l) ⇌ [M(H2O)5(OH)]2+ (aq) + H3O+ (aq)
• The equilibrium lies well to the left.
• Adding OH– ions will remove H3O+ ions, shifting the position of equilibrium to the right.
2. [M(H2O)5(OH)]2+ (aq) + H2O (l) ⇌ [M(H2O)4(OH)2]+ (aq) + H3O+ (aq)
3. [M(H2O)4(OH)2]+ (aq) + H2O (l) ⇌ [M(H2O)3(OH)3] (s) + H3O+ (aq)
• This produces the insoluble [M(H2O)3(OH)3] metal hydroxide.
M2+:
• The same process occurs with M2+ ions, producing an insoluble [M(H2O)4(OH)2] metal hydroxide.
1. [M(H2O)6]2+ + H2O ⇌ [M(H2O)5(OH)]+ + H3O+
2. [M(H2O)5(OH)]+ + H2O ⇌ [M(H2O)4(OH)2] + H3O+
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