Reactions with Bases (AQA A Level Chemistry)

• 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 (aq) →    Fe(H₂O)₄(OH)₂  (s)  + 2NH₄⁺ (aq)

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

[Fe(H₂O)₆] ²⁺ (aq)  + CO₃^2- (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)  + CO₃^2- (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)  + CO₃^2- (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)

• 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