Reactions of Inorganic Compounds in Aqueous Solution

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Hydroysis Reaction

  • Hexaaquametal contains 2 type of bond: polar O-H bonds, and co-ordinate bonds between metal ion and water ligands
  • Hydrolysis breaks O-H bond 'heterolytically' (both bonding electrons remain with one of the two fragments between which the bond is broken)
  • M(II) hexaaqua cations, equilibrium lies to the left so solutions are weakly acidic
  • M(III) hexaaqua ions, equilibrium lies to the right so solutions are strongly acidic
  • Hydrolysis equilibria depends on the charge/size ratio
  • The greater the charge/size ration of the ion --> the more the O-H bonds are polarized --> therefore M(III) produce more acidic solutions because the ion is smaller and more highly charged than M(II)
  • (
  • LILAC       ---------------------->  INTENSE BROWN
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Reaction with alkalis

  • If a solution of NaOH or any other alkali is added to a solution of a transition metal (or hexaaquaaluminium), a precipitate of the metal hydroxide is produced.
  • Reactions:
  • [Fe(H2O)6]2+(aq) + 2OH-(aq) --> [Fe(H2O)4(OH)2](s) + 2H2O(l) iron (II) hydroxide produced
  • GREEN                                 ----> BROWN
  • [Cu(H2O)6]2+(aq) + 2OH-(aq) --> [Cu(H2O)4(OH)2](s) + 2H2O(l)                                             copper (II) hydroxide produced (pale blue precipitate)
  • [Co(H2O)6]2+(aq) + 2OH-(aq) --> [Co(H2O)4(OH)2](s) + 2H2O(l)                                         cobalt (II) hydroxide produced (blue precipitate turning pink on standing)
  • [Al(H2O)6]3+(aq) + 3OH-(aq) --> [Al(H2O)3(OH)3](s) + 3H2O(l)                                     aluminium hydroxide produced (white precipitate)
  • [Cr(H2O)6]3+(aq) + 3OH-(aq) --> [Cr(H2O)3(OH)3](s) + 3H2O(l)                                     chromium (III) hydroxide (dark green precipitate)
  • Both aluminium hydroxide and chromium (III) hydroxide react with both acid and alkali
  • Aluminium  hydroxide goes from white solid to colourless solution in both acid and alkali
  • Chromium (III) hydroxide goes from green solid to dark green solution in both acid and alkali


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Reaction with ammonia solution

  • The ammonia molecules behave in the same way as the hydroxide ions
  • The iron, copper and cobalt hexaaqua-ions react with 2NH3 to give their corresponding hydroxides and 2NH4+
  • The chromium, iron and aluminium hexaaqua-ions react with 3NH3 to give their corresponding hydroxides and 3NH4+
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Reaction with solution of carbonate ions

  • [M(H2O)6)]3+ solutions are acidic enough to provide the hydrated protons for the following reactions:
  • CO32-(aq) + H3O+(aq) ---> HCO3-(aq) + H2O(l) then...
  • HCO3-(aq) + H3O+(aq) ---> 2H2O(l) + CO2 (g)
  • The [M(H2O)6]2+ ions do not behave as acids. Instead they for a precipitate directly with the carbonate ion
  • [Fe(H2O)6)]2+(aq) + CO32-(aq) ---> FeCO3(s) (green precipitate) + 6H2O
  • [Co(H2O)6)]2+(aq) + CO32-(aq) ---> CoCO3(s) (light purple precipitate) + 6H2O
  • [Cu(H2O)6)]2+(aq) + CO32-(aq) ---> CuCO3(s) (blue precipitate) + 6H2O
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Substitution by ammonia ligands

  • Ammonia molecules are very similar in size to water molecules
  • On slow addition of ammonia to hexaaqua complexes, a precipitate is formed of the hydroxide, which redissolves in excess ammonia
  • For hexaaquacopper (pale blue solution) with ammonia, copper (II) hydroxide is produced and 2NH4+
  • Upon the addition of more ammonia solution the precipitate dissolves forming a dark blue solution of tetraaminebisaquacopper(II) ion [Cu(NH3)4(H2O)2]2+, water and OH- ions
  • In aqueous solution the substitution is incomplete and that only 4 of the water molecules are replaced
  • For hexaaquachromium (green solution) when reacted with ammonia, produces chromium (III) hydroxide (green precipitate) and 3NH4+
  • Chromium hydroxide then reacts with excess ammonia to produce [Cr(NH3)6]3+ (aq)            (lilac solution) and water and OH- ions
  • For hexaaquacobalt (pink solution) with ammonia, cobalt hydroxide is formed (blue precipitate) and 2NH4+
  • In excess ammonia [Co(NH3)6]2+ (straw coloured solution) is formed and water and OH- ions
  • Co(II) is much more stable than Co(III) so the ion is oxidised in the air and the solution turns dark brown
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Substitution by chloride ligands

  • Hexaaquacobalt (pink) reacts with conc. HCl to produce [CoCl4]2- (dark blue) and water
  • Conc. HCl is used because it has a high concentration of chloride ions
  • The high chloride ion concentration pushes the position of equilibrium to the right
  • Chloride ions are bigger than water molecules so the co-ordination number changes from 6 to 4
  • Hexaaquacopper (pale blue) reacts with conc. HCl to produce [CuCl4]2- (olive-green) and water
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The Chelate Effect

  • The chelate effect occurs when you replace water ligands around the central metal ions by multidentate ligands
  • Chelates are much more stable than complex ions formed from unidentate ligands
  • The chelate effect occurs because of entropy change
  • Any change which increases the amount of disorder increases the tendency of a reaction to happen
  • In the case of EDTA4- the number of species present is being increased from 2 on the left to 7 on the right, so disorder increases making it more likely to happen
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