Unit 5 Section 4 Metal-Aqua Ions

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Hydration of Metal ions

Coordinate Bonds: A covalent bond in which both electrons in the shaired pair come from the same atom.

Metal-aqua Complex Ion: A species formed when transition metal ions dissolve in water. The water molecules form coordinate bonds with the metal ions.

Metal ions act as lewis acids in aqueous solution because they accept electron pairs from water molecules around them.

The water molecules, like any ligands, are electron pair donors so they must be lewis bases.

Six water molecules form coordinate bonds with each metal ion by donating a non-bonding pair of electrons from their oxygen.

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The Acidity of Metal-aqua ion Solutions

In a solution containing metal-aqua 2+ ions, there's a reaction between the metal-aqua ion and the water - this a hydrolysis or acidity reaction.

The metal-aqua 2+ ions release H+ ions, so an acidic solution is formed.

There is only slight dissociation, so the solution is only weakly acidic.

  • E.g.  [Fe(H2O)6],2+(aq) + H2O(l) <<<>>> [Fe(H2O)5(OH)],+(aq) + H3O,+(aq)

Metal-aqua 3+ ions react in the same way. They form more acidic solutions though.

  • E.g. [Al(H2O)6],3+(aq) + H2O(l) <<<>>> [Al(H2O)5(OH)],2+(aq) + H3O,+(aq)
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The Relative Acidity of 2+ and 3+ Metal-aqua ion S

Metal 3+ ions have a high charge density (and charge/size ratio).

Metal 2+ ions have a much lower charge density.

This makes the 3+ ions more polarising than the 2+ ions. More polarising power means that they attract electrons from the oxygen in the coordinated water molecules more strongly, weakening the O-H bond.

So it's more likely a H+ will be released. More H+ ions means a more acidic solution - so metal 3+ ions are more acidic than metal 2+ ions.

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Amphoteric Character

Amphoteric: Having both acidic and basic properties.

Amphoteric Metal Hydroxides will dissolve in an excess of base as well as acids.


As an acid: [Al(H2O)3(OH)3](s) + OH,-(aq) <<<>>> [Al(H2O)2(OH)4],-(aq) + H2O(l)

As a base: [Al(H2O)3(OH)3(s) + 3H+(aq) <<<>>> [Al(H2O)6],3+(aq)

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Hydrolysis of Metal-Aqua Ions Using Na2CO3

Metal 2+ ions:
Metal 2+ ions react with Na2CO3 to form insoluble metal carbonates, like this:
[M(H2O)6]^2+(aq) + CO3^2-(aq) <<<>>> MCO3(s) + 6H2O(l)

Metal 3+ ions:
Metal 3+ ions don't form M2(CO3)3 species when you react them with NaCO3.

They are stronger acids so they always form hydroxide precipitates instead.

The CO3^2- ions react with the H3O,+ ions, removing them from the solution and forming bubbles of CO2 gas.

[2M(H2O)6]^3+(aq) + 3CO3^2-(aq) <<<>>> [2M(H2O)3(OH)3](s) + 3CO2(g) + 3H2O(l)

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