Acidity or Hydrolysis Reactions

 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

 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

 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

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

 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

 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 ] Cl - Colourless

 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 ] Cl - Ruby

 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 ] Cl - Pale violet

 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