Reactions of Ions in Aqueous Solution (Oxford AQA International A Level Chemistry)

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) + SO₄^2- (aq)

• But, the copper(II) ions actually form the hexaaqua copper(II) ion

CuSO₄ (s) + aq → [Cu(H₂O)₆]²⁺ (aq) + SO₄^2- (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)   + 3SO₄^2- (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

• 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)