AQA A Level Chemistry

Revision Notes

6.3.2 Reactions with Bases

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

 

 

 

 

 

 

 

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 →    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(OH)2  ]2+ (aq) + 4H2O (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] 2+ (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:

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

  • The reaction between aluminium and water exists in an equilibrium

 [Al(H2O)63+ (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

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] 2+ (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)63+ (aq) + 3H2O (l)   ⇌   Fe(H2O)3(OH)3  (s)  + 3H3O+ (aq)

Exam 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)  + H2O (l)

  • The same equation may be shown ionically as:

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

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