1. Acidity or Hydrolysis Reactions

2.  Definition: A Lewis acid is an electron pair acceptor  Definition: A Lewis base is an electron pair donor  In a complex, ligands donate a lone pair of electrons to the central metal ion  The ligands act as Lewis bases  The central metal ion act as a Lewis acid

3.  In aqueous solutions, metal aqua ions are formed  The metal ions form hexaaqua complexes such as [M(H 2 O) 6 ] 2+ and [M(H 2 O) 6 ] 3+  These complexes have a octahedral shape with a co-ordination number of 6

4.  Copper, cobalt and iron(II) form hexaaqua complexes in aqueous solution  Anhydrous white copper(II) sulfate dissolves in water to produce blue hexaaquacopper(II) CuSO 4 + 6H 2 O → [Cu(H 2 O) 6 ] 2+ + SO 4 2- White solid Blue

5.  Blue anhydrous cobalt(II) chloride dissolves in water to produce pink hexaaquacobalt(II) CoCl 2 + 6H 2 O → [Co(H 2 O) 6 ] 2+ + 2Cl - Blue solid Pink

6.  Pale green iron(II) sulfate dissolves in water to produce pale green hexaaquairon(II) FeSO 4 + 6H 2 O → [Fe(H 2 O) 6 ] 2+ + SO 4 2- Pale green solid Pale green

7.  Aluminium is not a transition metal  Aluminium forms a Al 3+ ion  Aluminium chloride dissolves in water to produce hexaaquaaluminium(III)  Hexaaquaaluminium(III) is colourless as it is not a transition metal and therefore it has no d to d transitions AlCl 3 + 6H 2 O → [Al(H 2 O) 6 ] 3+ + 3Cl - Colourless

8.  Chromium(III) compounds such as chromium chloride dissolve in water to produce ruby hexaaquachromium(III) CrCl 3 + 6H 2 O → [Cr(H 2 O) 6 ] 3+ + 3Cl - Ruby

9.  Iron(III) compounds such as iron(III) chloride dissolve in water to produce pale violet hexaaquairon(III) FeCl 3 + 6H 2 O → [Fe(H 2 O) 6 ] 3+ + 3Cl - Pale violet

10.  Summary of hexaaqua colours: Oxidation StateComplexColour +2[Cu(H 2 O) 6 ] 2+ Blue +2[Co(H 2 O) 6 ] 2+ Pink +2[Fe(H 2 O) 6 ] 2+ Pale green +3[Al(H 2 O) 6 ] 3+ Colourless +3[Cr(H 2 O) 6 ] 3+ Ruby +3[Fe(H 2 O) 6 ] 3+ Pale violet