3.2.6 Reactions of ions 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.
• [M(H2O)6]2+, limited to M = Fe (green) and Cu (blue)

  [M(H2O)6]3+, limited to M = Al (colourless), and Fe (purple)
• Acidity or hydrolysis reactions by adding zinc and hcl

  [M(H2O)6]2+ + H2O ⇌  [M(H2O)5(OH)]+ + H3O+ 

  [M(H2O)6]3+ + H2O  ⇌ [M(H2O)5(OH)]2+ + H3O+
• The acidity of [M(H2O)6]3+ is 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 limitedNH3

  [Cu(H2O)6]2+ (aq) + 2OH- (aq) -> Cu(H2O)4(OH)2 (s) + 2H2O (l) 

  [Al(H2O)6]3+(aq) + 3OH- (aq) -> Al(H2O)3(OH)3 (s) + 3H2O (l)

  [Fe(H2O)6]2+ (aq) + 2NH3 (aq) -> Fe(H2O)4(OH)2 (s) + 2NH4+ (aq)

  [Fe(H2O)6]3+ (aq) + 3NH3 (aq) -> Fe(H2O)3(OH)3 (s) + 3NH4+ (aq)

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

  M(OH)3(H2O)3 (s) : Fe(III) brown ppt, Al white ppt 
• Reaction with excess OH-

  With excess NaOH the aluminium hydroxide dissolves. Al becomes [Al(OH)4]- (aq) colourless solution.

  Al(H2O)3(OH)3(s) + OH-(aq)-> [Al(OH)4]-(aq)+ 3H2O(l)
• Al(H2O)3(OH)3 is classed as amphoteric because it reacts and dissolves in both acids and bases.

  Al(H2O)3(OH)3(s) + 3H+(aq) -> [Al(H2O)6]3+ (aq)
• Reaction with excess NH3

  With excess NH3 a ligand substitution reaction occurs with Cu and its precipitate dissolves to form [Cu(NH3)4(H2O)2]2+ a deep blue solution.

  Cu(OH)2(H2O)4(s) + 4NH3 (aq) -> [Cu(NH3)4(H2O)2]2+ (aq) + 2H2O (l) + 2OH- (aq)
• Reactions with carbonate solution

  The 2+ ions react differently to the 3+ ions with carbonate solutions.The 2+ ions with carbonate solution results in MCO3 ppt being formed (Cu = blue/green , Fe(II) green). These are precipitation reactions

  (CO3)2- + [Cu(H2O)6]2+ -> CuCO3 + 6H2O

  (CO3)2- + [Fe(H2O)6]2+ -> FeCO3 + 6H2O
• Reactions with carbonate solution

  The 3+ ions with carbonate solution form a M(OH)3(H2O)3 ppt and CO2 gas is produced. These are classed as acidity reactions.

  2[Fe(H2O)6]3+ (aq) + 3(CO3)2- (aq) -> 2[Fe(OH)3(H2O)3] (s) + 3CO2 + 3H2O(l) 

  2[Al (H2O)6]3+(aq) + 3(CO3)2- (aq) -> 2[Al(OH)3(H2O)3] (s) + 3CO2 + 3H2O(l)
• 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.
• 2+ ion summary

  A) CuCO3 blue green ppt

  B) FeCO3 green ppt

  C) Fe(OH)3(H2O)3 Brown ppt

  D) Air

  E) CO3 2-

  F) excess NH3

  G) [Cu(NH3)4(H2O)2] 2+

  H) deep blue solution

  I) Limited NH3 or OH-

  J) Acid

  K) conc HCl

  L) H2O

  M) [CuCl4]2-

  N) yellow solution

  O) blue ppt

  P) green ppt

  Q) blue solution

  R) green solution

  
• 3+ ion summary

  A) white ppt

  B) brown ppt

  C) colourless solution

  D) purple solution

  E) white ppt and CO2

  F) Brown ppt and CO2

  G) CO3 2-

  H) Acid

  I) limited NH3 or OH-

  J) excess OH-

  K) colourless solution

  L) [Al(OH)4]-

  M) [Al(H2O)6]3+

  N) [Fe(H2O)6]3+

  O) Al(OH)3(H2O)3

  P) Fe(OH)3(H2O)3