A LevelChemistryAQARevision Notes6. Advanced Inorganic Chemistry (A Level only)6.3 Reactions of Ions in Aqueous Solution (A Level only)6.3.2 Reactions with Bases

Reactions with Bases (AQA A Level Chemistry)

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Reactions with Bases

  • The differences in the the chemistry of +2 and +3 aqua ions can be seen in their reactions with bases
  • The reactions of iron(II), iron(III), copper(II) and aluminium with bases is summarised below

The reactions of metal-aqua ions with bases table

Metal aqua ions with bases Table, downloadable AS & A Level Chemistry revision notes

Iron(II)

  • The dark green precipitate formed by reaction with hydroxide ions and with ammonia is hydrated iron(II) hydroxide
  • This is formed in a two-step process

[Fe(H2O)6] 2+ (aq)  + OH– (aq) →    [Fe(H2O)5(OH)] + (aq)  + H2O (l)

[Fe(H2O)5(OH)] + (aq)  + OH– (aq) →    Fe(H2O)4(OH)2 (s)  + H2O (l)

  • The ammonia behaves in the same way as sodium hydroxide as it is a base and removes protons from the water ligands; the overall reaction with ammonia is:

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

  • With carbonate ions, the iron(II)carbonate precipitates out:

[Fe(H2O)6] 2+ (aq)  + CO32- (aq) →   FeCO3 (s)  + 6H2O (l)

Copper(II)

  • The light blue precipitate formed by reaction with hydroxide ions is hydrated copper(II) hydroxide
  • This is formed in a two-step process

[Cu(H2O)6] 2+ (aq)  + OH– (aq) →    [Cu(H2O)5(OH)] + (aq)  + H2O (l)

[Cu(H2O)5(OH)] + (aq)  + OH– (aq) →    Cu(H2O)4(OH)2 (s)  + H2O (l)

  • The ammonia initially behaves in the same way as sodium hydroxide as it is a base and removes protons from the water ligands

[Cu(H2O)6] 2+ (aq)  + 2NH3 (aq) →    Cu(H2O)4(OH)2  (s)  + 2NH4+ (aq)

  • However, ammonia is a good ligand and in excess ammonia, the ammonia partially substitutes for water creating the deep blue complex ion, dihydroxytetraaminecopper(II)

Cu(H2O)4(OH)2 (s) + 4NH3 (aq) →  [Cu(NH3)4(H2O)2  ]2+ (aq) + + 2OH- (aq) + 2H2O (l)

  • With carbonate ions, the copper(II)carbonate precipitates out:

[Cu(H2O)6] 2+ (aq)  + CO32- (aq) →   CuCO3 (s)  + 6H2O (l)

Aluminium ions

  • The white precipitate formed by reaction with hydroxide ions and with ammonia is hydrated aluminium hydroxide
  • This is formed in a three-step process

[Al(H2O)6] 3+ (aq)  + OH– (aq) →    [Al(H2O)5(OH)] 2+ (aq)  + H2O (l)

[Al(H2O)5(OH)] 2+ (aq)  + OH– (aq) →    [Al(H2O)4(OH)2] + (aq)  + H2O (l)

[Al(H2O)4(OH)2] + (aq)  + OH– (aq) →    Al(H2O)3(OH)3 (s)  + H2O (l)

  • The ammonia behaves in the same way as sodium hydroxide as it is a base and removes protons from the water ligands; the overall reaction with ammonia is:

[Al(H2O)6] 3+ (aq)  + 3NH3 (aq) →    Al(H2O)3(OH)3 (s)  + 3NH4+ (aq)

  • With carbonate ions, the reaction is a little more complicated
  • In the previous section we saw that +3 ions are acidic in water, so the addition of carbonate ions liberates bubbles of carbon dioxide:

2H3O+ (aq)  + CO32- (aq) →   CO2 (g)  + 3H2O (l)

  • The reaction between aluminium and water exists in an equilibrium:

 [Al(H2O)6] 3+ (aq) + 3H2O (l)   ⇌   Al(H2O)3(OH)3  (s)  + 3H3O+ (aq)

  • Removal of the hydronium ions by carbonate ions pushes the equilibrium to the right and precipitates out the hydrated aluminium hydroxide
  • The overall equation can therefore shown as:

2[Al(H2O)6]3+ + 3CO32− (aq) → 2Al(H2O)3(OH)3 (s) + 3CO2 (g) + 3H2O (l)

Iron(III)

  • The red-brown precipitate formed by reaction with hydroxide ions and with ammonia is hydrated iron(III) hydroxide
  • This is formed in a three-step process

[Fe(H2O)6] 3+ (aq)  + OH– (aq) →    [Fe(H2O)5(OH)] 2+ (aq)  + H2O (l)

[Fe(H2O)5(OH)] 2+ (aq)  + OH– (aq) →    [Fe(H2O)4(OH)2] + (aq)  + H2O (l)

[Fe(H2O)4(OH)2] + (aq)  + OH– (aq) →    Fe(H2O)3(OH)3 (s)  + H2O (l)

  • The ammonia behaves in the same way as sodium hydroxide as it is a base and removes protons from the water ligands; the overall reaction with ammonia is:

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

  • With carbonate ions, the reaction is the same as with aluminium; the acidity of iron(III) ions removes the carbonate ion and produces bubbles of carbon dioxide, while the iron(III) precipitates out as the hydroxide

 [Fe(H2O)6] 3+ (aq) + 3H2O (l)   ⇌   Fe(H2O)3(OH)3  (s)  + 3H3O+ (aq)

  • The overall equation can therefore shown as:

2[Fe(H2O)6]3+ (aq)  + 3CO32− (aq) → 2Fe(H2O)3(OH)3 (s) + 3CO2 (g) + 3H2O (l)

Examiner Tip

Transition metals in the +3 state are acidic and do not form carbonate precipitates, unlike the +2 ions.

Amphoteric Hydroxides

  • Aluminium hydroxide is classified as an amphoteric hydroxide
  • The word amphoteric means it reacts with both acids and bases
  • Aluminium hydroxide is insoluble in water but readily dissolves in dilute hydrochloric acid producing the hexaaquaaluminium ion:

 Al(OH)3(H2O)3 (s)    + 3HCl (aq)    →    [Al(H2O)6] 3+ (aq)  + 3Cl- (aq)

  • Aluminium hydroxide dissolves in sodium hydroxide to form sodium tetrahydoxoaluminate

Al(OH)3(H2O)3 (s)    + NaOH (aq)    →   Na [Al(OH)4] (aq)  + 3H2O (l)

  • The same equation may be shown ionically as:

Al(OH)3(H2O)3 (s)    + OH- (aq)    →    [Al(OH)4] - (aq)  + 3H2O (l)

  • You need a strong base to carry out the reaction, so it is usually done with hot concentrated sodium hydroxide

Examiner Tip

You can also show the reactions with sodium hydroxide as:Al(OH)3 + NaOH → NaAl(OH)4orAl(OH)3(H2O)3 + OH– → [Al(OH)4(H2O)2]– + H2OorAl(OH)3 + OH– → [Al(OH)4]–

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    Stewart

    Author: Stewart

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