Reactions of ions in aqueous solution

• Reactions of Inorganic Compounds in Aqueous Solution
• Lewis acid


  Electron pair acceptor
• Lewis base


  Electron pair donator
• In the formation of complex ions the ligand is the Lewis base because it is donating a pair of electrons in the dative covalent bond and the metal ion is the Lewis acid
• Metal-aqua ions


  Metal aqua ions are formed in aqueous solution
• Metal-aqua ions


  • [M(H2O)6]2+, limited to M = Fe (green) and Cu (blue)
  • [M(H2O)6]3+, limited to M = Al (colourless), and Fe (violet)
• In solution Fe(III) appears yellow/brown due to hydrolysis reactions. The violet colour is only really seen in solid hydrated salts that contain these complexes
• Acidity or hydrolysis reactions


  [M(H2O)6]3+ + H2O [M(H2O)5(OH)]2+ + H3O+
• The equilibria lead to generation of acidic solutions with M3+ ions, and very weakly acidic solutions with M2+ ions. The 3+ ions are noticeably more acidic
• Acidity of [M(H2O)6]3+


  Greater than that of [M(H2O)6]2+ because the 3+ metal ions have higher charge density (charge/size ratio) and have greater polarising power. The greater the polarising power, the more strongly it attracts the water molecule. This weakens the O-H bond so it breaks more easily releasing H+ ions
• Reaction with limited OH- and limited NH3

  1. M(OH)2(H2O)4 (s) : Cu blue ppt, Fe (II) green ppt

  2. M(OH)3(H2O)3 (s) : Fe(III) brown ppt, Al white ppt
• This process can happen step wise removing one proton at a time
• Reaction with excess OH-


  Al becomes [Al(OH)4]- (aq) colourless solution
• Aluminium hydroxide is classed as amphoteric because it reacts and dissolves in both acids and bases
• Reaction with excess NH3


  Cu becomes [Cu(NH3)4(H2O)2]2+ deep blue solution
• In this reactions NH3 is acting as a Lewis base donating an electron pair
• MCO3 is formed with 2+ ions but M2(CO3)3 is not formed with 3+ ions. The difference is explained by the greater polarising power of the 3+ ion due to its higher charge density