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Reactions with Bases (AQA A Level Chemistry)

Revision Note

Stewart Hird

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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 Tips and Tricks

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 Tips and Tricks

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 Hird

    Author: Stewart Hird

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.