Reactions of Ions in Aqueous Solution
Al(H2O)3(OH)3 (s) + OH- (aq) → [Al(OH)4]^- (aq) + 3H2O (l)
- In aqueous solution, the following metal-aqua ions are formed:
- [M(H2O)6]^2+ when M = Fe or Cu
- [M(H2O)6]^3+ when M = Al or Fe
- The acidity of [M(H2O)6]^3+ is greater than that of [M(H2O)6]2+. This is because M^3+ ions are smaller and have a higher charge, so the electrons from the oxygen atoms of the water ligands are more strongly attracted to the M^3+ ions. This weakens the O-H bonds in the water ligands, meaning H^+ ions are more easily lost
- Aluminium hydroxide shows amphoteric character by dissolving in both acids and bases
Al(H2O)3(OH)3 (s) + OH- (aq) → [Al(OH)4]^- (aq) + 3H2O (l)
Precipitate Reactions of Transition Metals
- A precipitate reaction is when two solutions containing soluble ions are mixed, forming an insoluble compound
- Reaction of metal ions with carbonate:
- With aqueous M^3+ ions, the carbonate will behave as a base by removing protons from the water, forming a hydroxide precipitate:
- 2[M(H2O)6]^3+ (aq) + 3CO3^2- (aq) → 2M(OH)3(H2O)3 (s) + 3CO2 (g) + 3H2O (l)
- Aqueous M^2+ ions aren’t acidic enough, so form a solid metal (II) carbonate instead of H3O^+.
- [M(H2O)6]^2+ (aq) + CO3^2- (aq) → MCO3 (s) + 6H2O (l)
- With aqueous M^3+ ions, the carbonate will behave as a base by removing protons from the water, forming a hydroxide precipitate:
- Reaction of metal ions with hydroxides:
Metal-aqua ion | Reaction with OH^- (aq) | Reaction with NH3 (aq) |
M2^+ (aq) | [M(H2O)6]^2+ (aq) + 2OH^- (aq) → [M(OH)2(H2O)4] (s) + 2H2O (l) | [M(H2O)6]^2+ (aq) + 2NH3 (aq) → [M(OH)2(H2O)4] + 2NH4^+ (aq) |
M3^+ (aq) | [M(H2O)6]^3+ (aq) + 3OH^- (aq) → M3+(OH)3(H2O)3 (s) + 3H2O (l) | [M(H2O)6]^3+ (aq) + 3NH3 (aq) → [M(OH)3(H2O)3 (s) + 3NH4^+ (aq) |
Metal-aqua ion | Dropwise OH-/NH3 | Excess OH^- | Excess NH3 | CO3^2- |
[Fe(H2O)6]^2+ (aq) Pale green solution | [Fe(H2O)4(OH)2] Green ppt* | [Fe(H2O)4(OH)2] Green ppt | [Fe(H2O)4(OH)2] Green ppt* | FeCO3 Green ppt |
[Cu(H2O)6]^2+ (aq) Pale blue solution | [Cu(H2O)4(OH)2] Pale blue ppt | [Cu(H2O)4(OH)2] Pale blue ppt | [Cu(H2O)2(NH3)4]^2+ Deep blue solution | CuCO3 Blue-green ppt |
[Fe(H2O)6]^3+ (aq) Purple/yellow/brown solution | [Fe(H2O)3(OH)3] Brown ppt | [Fe(H2O)3(OH)3] Brown ppt | [Fe(H2O)3(OH)3] Brown ppt | [Fe(H2O)3(OH)3] Brown ppt and bubbles of CO2 |
[Al(H2O)6]^3+ (aq) Colourless solution | [Al(H2O)3(OH)3] White ppt | [Al(OH)4]^- Colourless solution | [Al(H2O)3(OH)3] White ppt | [Al(H2O)3(OH)3] White ppt and bubbles of CO2 |
- Fe2+ eventually oxidised by air to Fe3+ making brown [Fe(H2O)3(OH)3] (s)