Reactions of Ions in Aqueous Solution
Quick Notes
- Metal-aqua ions form when a metal ion is in aqueous solution:
- [M(H2O)6]2+ (M = Fe2+, Cu2+)
- [M(H2O)6]3+ (M = Fe3+, Al3+)
- [Fe(H2O)6]3+ is more acidic than [Fe(H2O)6]2+ due to the metal ion having a higher charge/size ratio.
- Some metal hydroxides show amphoteric behaviour, dissolving in both acids and bases (e.g., Al(OH)3).
- Summary Table: Reactions of Metal-Aqua Ions
| Ion | OH− (NaOH) | NH3 | CO32− |
|---|---|---|---|
| Fe2+ (aq) | Green ppt Fe(OH)2 | Green ppt Fe(OH)2 | Green ppt FeCO3 |
| Cu2+ (aq) | Blue ppt Cu(OH)2 | Blue ppt Cu(OH)2 (if dropwise NH3) Deep blue solution [Cu(NH3)4(H2O)2]2+ (in excess NH3) | Blue-green ppt CuCO3 |
| Fe3+ (aq) | Brown ppt Fe(OH)3 | Brown ppt Fe(OH)3 | Brown ppt Fe(OH)3 + CO2 gas |
| Al3+ (aq) | White ppt Al(OH)3 (redissolves in excess OH−) | White ppt Al(OH)3 | White ppt Al(OH)3 + CO2 gas |
Full Notes
A pdf download sheet for all metal-aqua ion reactions for AQA (Fe2+, Fe3+, Cu2+ and Al3+) can be found here.
Formation of Metal-Aqua Ions
For AQA you need to know the colours and formulas of these aqua ions
General equation: Mn+ + 6H2O → [M(H2O)6]n+
Acidity of Metal-Aqua Ions
[M(H2O)6]3+ is more acidic than [M(H2O)6]2+ because the charge/size ratio of M3+ is higher than M2+. This means the metal polarises the O–H bonds in water ligands more, making them easier to break and releasing H+ ions more easily.
For Example:
Iron (III) [Fe(H2O)6]3+ has a pH of 2 to 3 whereas Iron (II) [Fe(H2O)6]2+ has a pH of 3 to 5
Fe3+ and Al3+ solutions are more acidic than Fe2+ and Cu2+ solutions.
Reactions of Metal-Aqua Ions in Test-Tube Reactions
The following precipitation reactions and colour changes are expected for AQA A-level students.
Iron (II), Fe2+ reactions
Iron (III), Fe3+ reactions
Copper (II), Cu2+ reactions
Aluminium (III), Al3+ reactions
Key ‘interesting’ things to remember from the metal aqua-ion reactions – Al(OH)3 is amphoteric and dissolves in excess OH− to form [Al(OH)4]−. Only Cu2+ dissolves in excess NH3, forming a deep blue complex. Fe2+ and Cu2+ form metal carbonates with CO32− ions. Fe3+ and Al3+ form hydroxide precipitates with CO32− ions. The 3+ ions react with carbonates forming carbon dioxide gas because they are more acidic than the 2+ ions
Summary Table: Reactions of Metal-Aqua Ions
| Metal Ion | OH− (NaOH) | NH3 | CO32− |
|---|---|---|---|
| Fe2+ (aq) | Green ppt Fe(OH)2 | Green ppt Fe(OH)2 | Green ppt FeCO3 |
| Cu2+ (aq) | Blue ppt Cu(OH)2 | Blue ppt Cu(OH)2 (if dropwise NH3) Deep blue solution [Cu(NH3)4(H2O)2]2+ (in excess NH3) | Blue-green ppt CuCO3 |
| Fe3+ (aq) | Brown ppt Fe(OH)3 | Brown ppt Fe(OH)3 | Brown ppt Fe(OH)3 + CO2 gas |
| Al3+ (aq) | White ppt Al(OH)3 (redissolves in excess OH−) | White ppt Al(OH)3 | White ppt Al(OH)3 + CO2 gas |
Summary
- Hexaaqua ions form in water and their acidity increases with metal ion charge/size ratio.
- Characteristic test-tube reactions with OH−, NH3, and CO32− allow identification of Fe2+, Fe3+, Cu2+ and Al3+.
- Al(OH)3 shows amphoteric behaviour; Cu2+ forms a deep blue ammine complex in excess NH3.