Reactions of Ions in Aqueous Solution (Oxford AQA International A Level (IAL) Chemistry) : Revision Note
Metal-Aqua Ions
Metal-aqua complex ions form when soluble transition metal salts dissolve in water
A metal-aqua complex ion is the complex of a central transition metal cation with co-ordinate bonds to ligands
Dissolving copper(II) sulfate in water
This is often shown as:
CuSO4 (s) + aq → Cu2+ (aq) + SO42- (aq)
But, the copper(II) ions actually form the hexaaqua copper(II) ion
CuSO4 (s) + aq → [Cu(H2O)6]2+ (aq) + SO42- (aq)
The water molecules form co-ordinate bonds to central transition metal cation
A lone pairs on the oxygen molecule provides the electrons for the bond
The hexaaqua copper(II) ion, [Cu(H2O)6]2+ (aq), causes the blue colour of the solution
Other metal-aqua complex ions
Iron(II) salts also form hexaaqua complex ions
Fe(NO3)2 (s) + aq → [Fe(H2O)6]2+ (aq) + 2NO3- (aq)
The hexaaqua iron(II) ion, [Fe(H2O)6] 2+ (aq), causes the green colour of the solution
Two common metal-aqua ions with a 3+ charge are iron(III) and aluminium:
Fe(NO3)3 (s) + aq → [Fe(H2O)6]3+ (aq) + 3NO3- (aq)
The hexaaqua iron(III) ion, [Fe(H2O)6]3+ (aq), causes the yellow-orange colour of the solution
Al2(SO4)3 (s) + aq → 2[Al(H2O)6] 3+ (aq) + 3SO42- (aq)
The hexaaqua aluminium(III) ion, [Al(H2O)6]3+ (aq), causes the lack of colour of the solution
Metal-aqua 2+ and 3+ complex ions
Acidity in Metal-Aqua Ions
Typically, when transition metal salts dissolve in water they form solutions that are not neutral
pH table for different transition metal solutions
0.1 mol dm-3 solution | pH |
|---|---|
Iron(III) chloride, FeCl3 | 2.0 |
Aluminium chloride, AlCl3 | 3.0 |
Copper(II) nitrate, Cu(NO3)2 | 4.0 |
Metal-aqua complex ions with a 3+ charge are more acidic than those with a 2+ charge
3+ ions, such as iron(III) and aluminium, are smaller than 2+ ions
This means that they have a higher charge density than +2 ions
The higher charge density attracts the lone pair from a water molecule more strongly
This weakens the O-H bonds
O-H bonds can dissociate into a hydroxide ligand, OH-, and a hydrogen ion, H+
The hydrogen ion causes the resulting solution to be more acidic
The metal ion polarises the water molecules
How metal(III) ions polarise water ligands
[Fe(H2O)6]3+ (aq) → [Fe(H2O)5(OH)]2+ (aq) + H+ (aq)
The loss of the hydrogen ion results in a pentaaqua 2+ complex ion
The remaining hydroxide ion from the water molecule has a negative charge
This cancels one of the charges on the original 3+ complex ion
The same reaction can be shown with a hydroxonium ion product
[Fe(H2O)6]3+ (aq) + H2O (l) → [Fe(H2O)5(OH)]2+ (aq) + H3O+ (aq)
These reactions can be called deprotonation reactions
Deprotonation reactions of [Fe(H2O)6]3+ (aq)
This usually occurs in several steps
The first two deprotonations of [Fe(H2O)6]3+ (aq) are:
[Fe(H2O)6]3+ (aq) → [Fe(H2O)5(OH)]2+ (aq) + H+ (aq)
[Fe(H2O)5(OH)]2+ (aq) → [Fe(H2O)4(OH)2]+ (aq) + H+ (aq)
The third deprotonation does not usually occur without the presence of a base
The base removes the third proton
This produces a red-brown precipitate of insoluble hydrated iron(III)hydroxide
[Fe(H2O)4(OH)2] + (aq) + OH- → Fe(H2O)3(OH)3 (s) + H2O (l)
Examiner Tips and Tricks
The splitting of water molecules can be called a hydrolysis reaction
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