 Metal aqua ions can undergo two types of reactions, depending on which bonds in the complex are broken  If the co-ordinate bond between an aqua ligand and the central metal ion breaks, the aqua ligand can be replaced by another ligand  This is called a ligand substitution reaction  If the O-H bond in an aqua ligand breaks, a hydrogen ion, H + is produced  This is called a hydrolysis reaction

 Aluminium chloride dissolves in water to produce colourless hexaaquaaluminium(III) AlCl 3 + 6H 2 O → [Al(H 2 O) 6 ] Cl -  The solution formed is acidic, with a pH of 3  This is because the [Al(H 2 O) 6 ] 3+ complex reacts with a water molecule (H 2 O) to produce H 3 O +  A hydrolysis reaction occurs where a O-H bond in an aqua ligand in the [Al(H 2 O) 6 ] 3+ complex is broken

[Al(H 2 O) 6 ] 3+ + H 2 O ⇌ [Al(H 2 O) 5 (OH)] 2+ + H 3 O + [Al(H 2 O) 6 ] 3+ ⇌ [Al(H 2 O) 5 (OH)] 2+ + H +  The reaction is reversible  The position of equilibrium lies to the left, which is why the solution is weakly acidic  This type of reaction is also called an acidity reaction  Metal hexaaqua ions where the central metal ion has a +2 or +3 charge undergo hydrolysis reactions

 Chromium(III) compounds dissolve in water to produce ruby hexaaquachromium(III) CrCl 3 + 6H 2 O → [Cr(H 2 O) 6 ] Cl -  Hexaaquachromium(III) undegoes hydrolysis to produce a weakly acidic solution [Cr(H 2 O) 6 ] 3+ + H 2 O ⇌ [Cr(H 2 O) 5 (OH)] 2+ + H 3 O + [Cr(H 2 O) 6 ] 3+ ⇌ [Cr(H 2 O) 5 (OH)] 2+ + H +

 Iron(III) compounds dissolve in water to produce pale violet hexaaquairon(III) FeCl 3 + 6H 2 O → [Fe(H 2 O) 6 ] Cl -  Hexaaquairon(III) undegoes hydrolysis to produce a weakly acidic solution [Fe(H 2 O) 6 ] 3+ + H 2 O ⇌ [Fe(H 2 O) 5 (OH)] 2+ + H 3 O + [Fe(H 2 O) 6 ] 3+ ⇌ [Fe(H 2 O) 5 (OH)] 2+ + H +

 Copper(II) compounds dissolve in water to produce blue hexaaquacopper(II)  Hexaaquacopper(II) undegoes hydrolysis to produce a very weakly acidic solution of pH 6 [Cu(H 2 O) 6 ] 2+ + H 2 O ⇌ [Cu(H 2 O) 5 (OH)] + + H 3 O + [Cu(H 2 O) 6 ] 2+ ⇌ [Cu(H 2 O) 5 (OH)] + + H +  The position of equilibrium lies very far to the left, which is why the solution is very weakly acidic

 The same hydrolysis reaction occurs with pink hexaaquacobalt(II) and pale green hexaaquairon(II) [Co(H 2 O) 6 ] 2+ + H 2 O ⇌ [Co(H 2 O) 5 (OH)] + + H 3 O + [Co(H 2 O) 6 ] 2+ ⇌ [Co(H 2 O) 5 (OH)] + + H + [Fe(H 2 O) 6 ] 2+ + H 2 O ⇌ [Fe(H 2 O) 5 (OH)] + + H 3 O + [Fe(H 2 O) 6 ] 2+ ⇌ [Fe(H 2 O) 5 (OH)] + + H +  The position of equilibrium lies very far to the left, which is why the solution is very weakly acidic

 M 3+ hexaaqua ions form weakly acidic solutions [M(H 2 O) 6 ] 3+ + H 2 O ⇌ [M(H 2 O) 5 (OH)] 2+ + H 3 O + [M(H 2 O) 6 ] 3+ ⇌ [M(H 2 O) 5 (OH)] 2+ + H +  M 2+ hexaaqua ions form very weakly acidic solutions [M(H 2 O) 6 ] 2+ + H 2 O ⇌ [M(H 2 O) 5 (OH)] + + H 3 O + [M(H 2 O) 6 ] 2+ ⇌ [M(H 2 O) 5 (OH)] + + H +

 M 3+ ions such as Al 3+, Cr 3+ and Fe 3+ have a high charge to size ratio compared to M 2+ ions such as Cu 2+, Co 2+ and Fe 2+  A high charge to size ratio means the metal ion is highly charged and small in size

 Therefore M 3+ ions have greater polarising power  This means they attract the electron density more strongly from the oxygen in aqua ligands  This makes the O-H bond weaker, so less energy is needed to break the bond and produce a hydrogen ion  Therefore the acidity of [M(H 2 O) 6 ] 3+ complexes is greater than [M(H 2 O) 6 ] 2+ complexes