Metal-Aqua Ions (AQA A-Level Chemistry): Revision Notes

6.3.1 Metal-Aqua Ions

Metal-Aqua Ions

In aqueous solutions, some metal ions form metal-aqua ions by coordinating with water molecules.

These complexes consist of a central metal ion surrounded by six water molecules, forming $[ \text{M}(\text{H}_2\text{O})_6 ]^{2+}$ or $[ \text{M}(\text{H}_2\text{O})_6 ]^{3+}$ ions, depending on the metal's oxidation state.

Types of Metal-Aqua Ions

• $[ \text{M}(\text{H}_2\text{O})_6 ]^{2+}$ ions: Formed with transition metals such as iron ($Fe$) and copper ($Cu$) in the +2 oxidation state.
• $[ \text{M}(\text{H}_2\text{O})_6 ]^{3+}$ ions: Formed with metals like aluminium ($Al$) and iron ($Fe$) in the +3 oxidation state.

The Acidity of Metal-Aqua Ions

The acidity of these metal-aqua ions varies with the oxidation state of the metal ion. Generally:

$[ \text{M}(\text{H}_2\text{O})_6 ]^{3+}$ ions are more acidic than $[ \text{M}(\text{H}_2\text{O})_6 ]^{2+}$ ions.

Explanation of Acidity: Charge/Size Ratio

The difference in acidity between $[ \text{M}(\text{H}_2\text{O})_6 ]^{3+}$ and $[ \text{M}(\text{H}_2\text{O})_6 ]^{2+}$ can be explained by the charge/size ratio of the metal ion:

Higher Charge Density:

A 3+ metal ion has a higher charge density than a 2+ ion because of its greater positive charge relative to its size.

Polarization of Water Molecules:

• The higher charge density of $[ \text{M}(\text{H}_2\text{O})_6 ]^{3+}$ strongly attracts the electron density from the oxygen atoms of the water molecules.
• This weakens the $O-H$ bonds in the coordinated water, making it easier for one of the hydrogen atoms to dissociate as a $\text{H}^+$ ion. Increased Acidity:

Due to this polarisation, $[ \text{M}(\text{H}_2\text{O})_6 ]^{3+}$ ions are more likely to release $\text{H}^+$ ions into solution, increasing their acidity compared to $[ \text{M}(\text{H}_2\text{O})_6 ]^{2+}$ ions.