Reactions of Metal-Aqua Ions (Oxford AQA International A Level Chemistry)
Revision Note
Written by: Richard Boole
Reviewed by: Stewart Hird
Reactions of Metal-Aqua Ions with Bases
Reactions with Bases
The differences in the chemistry of +2 and +3 aqua ions can be seen in their reactions with:
Hydroxide ions, OH-
Ammonia, NH3
Carbonate ions, CO22-
Hexaaqua iron(II) reactions
With hydroxide ions
The hexaaqua iron(II) undergoes deprotonation reactions with dilute and excess hydroxide ions
This is 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 green [Fe(H2O)6]2+ solution reacts to form a dark green precipitate of hydrated iron(II) hydroxide, Fe(H2O)4(OH)2 (s)
The green hydrated iron(II) hydroxide will slowly turn orange-brown
This is due to an oxidation reaction forming hydrated iron(III) hydroxide
There is no further reaction with excess hydroxide ions
With ammonia solution
Ammonia behaves in the same way as sodium hydroxide because it is a base
It 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)
Again, the green [Fe(H2O)6]2+ solution reacts to form a dark green precipitate of hydrated iron(II) hydroxide, Fe(H2O)4(OH)2 (s)
The green hydrated iron(II) hydroxide will oxidise to orange-brown hydrated iron(III) hydroxide
There is no further reaction with excess ammonia
With carbonate ions
With carbonate ions, iron(II) carbonate precipitates out:
[Fe(H2O)6]2+ (aq) + CO32- (aq) → FeCO3 (s) + 6H2O (l)
The green [Fe(H2O)6]2+ solution reacts to form a green precipitate of iron(II) carbonate, FeCO3 (s)
Hexaaqua copper(II) reactions
With hydroxide ions
The hexaaqua copper(II) undergoes deprotonation reactions with dilute and excess hydroxide ions
This is 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 blue [Cu(H2O)6]2+ solution reacts to form a blue precipitate of copper(II) hydroxide, Cu(H2O)4(OH)2 (s)
There is no further reaction with excess hydroxide ions
With ammonia solution
Initially, ammonia behaves in the same way as sodium hydroxide because it is a base
It removes protons from the water ligands
This reaction with ammonia is
[Cu(H2O)6]2+ (aq) + 2NH3 (aq) → Cu(H2O)4(OH)2 (s) + 2NH4+ (aq)
A further reaction occurs with excess ammonia
Ammonia is a stronger ligand than water
So, ammonia partially substitutes for water
Cu(H2O)4(OH)2 (s) + 4NH3 (aq) → [Cu(NH3)4(H2O)2]2+ (aq) + 2OH- (aq) + 2H2O (l)
The blue Cu(H2O)4(OH)2 precipitate reacts to form a deep blue solution of dihydroxytetraaminecopper(II), [Cu(NH3)4(H2O)2]2+ (aq)
With carbonate ions
With carbonate ions, copper(II) carbonate precipitates out:
[Cu(H2O)6] 2+ (aq) + CO32- (aq) → CuCO3 (s) + 6H2O (l)
The blue [Cu(H2O)6]2+ solution reacts to form a blue / blue-green precipitate of copper(II) carbonate, CuCO3 (s)
Hexaaqua iron(III) reactions
With hydroxide ions
The hexaaqua iron(III) undergoes deprotonation reactions with dilute and excess hydroxide ions
This is 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 yellow-orange [Fe(H2O)6]3+ solution reacts to form a red-brown precipitate of hydrated iron(III) hydroxide, Fe(H2O)3(OH)3 (s)
There is no further reaction with excess hydroxide ions
With ammonia solution
Ammonia behaves in the same way as sodium hydroxide because it is a base
It 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)
Again, the yellow-orange [Fe(H2O)6]3+ solution reacts to form a red-brown precipitate of hydrated iron(III) hydroxide, Fe(H2O)3(OH)3 (s)
There is no further reaction with excess ammonia
With carbonate ions
Hexaaqua 3+ ions are acidic in water
This means that they undergo neutralisation reactions with carbonate ions
The reaction between hexaaqua iron(III) ions and water exists in an equilibrium
[Fe(H2O)6]3+ (aq) + 3H2O (l) ⇌ Fe(H2O)3(OH)3 (s) + 3H3O+ (aq)
The hydronium ions, H3O+, react with carbonate ions to produce carbon dioxide
2H3O+ (aq) + CO32- (aq) → CO2 (g) + 3H2O (l)
This reaction removes hydronium ions from the hexaaqua iron(III) equilibrium
This pushes the equilibrium to the right
Therefore, the hydrated iron(III) hydroxide precipitates out
The overall reaction equation is:
2[Fe(H2O)6]3+ (aq) + 3CO32− (aq) → 2Fe(H2O)3(OH)3 (s) + 3CO2 (g) + 3H2O (l)
Hexaaqua aluminium reactions
With hydroxide ions
The hexaaqua aluminium undergoes deprotonation reactions with dilute and excess hydroxide ions
This is 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 colourless [Al(H2O)6]3+ solution reacts to form a white precipitate of Al(H2O)3(OH)3 (s)
Further deprotonation reactions occur with excess hydroxide ions
Al(H2O)3(OH)3 (s) + OH- (aq) ⇋ [Al(H2O)2(OH)4]- (aq) + H2O (l)
[Al(H2O)2(OH)4]- (aq) + OH- (aq) ⇋ [Al(H2O)(OH)5]2- (aq) + H2O (l)
[Al(H2O)(OH)5]2- (aq) + OH- (aq) ⇋ Al(OH)63- (aq) + H2O (l)
The white Al(H2O)3(OH)3 (s) precipitate reacts to form a colourless solution of aluminium hydroxide, Al(OH)6 (aq)
With ammonia solution
Ammonia behaves in the same way as sodium hydroxide because it is a base
It 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)
Again, the colourless [Al(H2O)6]3+ solution reacts to form a white precipitate of aluminium hydroxide, Al(H2O)3(OH)3 (s)
There is no further reaction with excess ammonia
With carbonate ions
Hexaaqua 3+ ions are acidic in water
This means that they undergo neutralisation reactions with carbonate ions
The reaction between hexaaqua aluminium ions and water exists in an equilibrium
[Al(H2O)6]3+ (aq) + 3H2O (l) ⇌ Al(H2O)3(OH)3 (s) + 3H3O+ (aq)
The hydronium ions, H3O+, react with carbonate ions to produce carbon dioxide
2H3O+ (aq) + CO32- (aq) → CO2 (g) + 3H2O (l)
This reaction removes hydronium ions from the hexaaqua iron(III) equilibrium
This pushes the equilibrium to the right
Therefore, the aluminium hydroxide precipitates out
The overall reaction equation is:
2[Al(H2O)6]3+ (aq) + 3CO32− (aq) → 2Al(H2O)3(OH)3 (s) + 3CO2 (g) + 3H2O (l)
Summary of reactions
Ion | Colour of hexaaqua solution | Reaction with OH- | Reaction with NH3 | Reaction with CO32- |
---|---|---|---|---|
Fe2+ | Pale green | Forms a dark green precipitate Insoluble in excess | Forms a dark green precipitate Insoluble in excess | Forms a green precipitate |
Cu2+ | Blue | Forms a blue precipitate Insoluble in excess | Forms a blue precipitate Redissolves in excess to form a deep blue solution | Forms a blue precipitate |
Al3+ | Colourless | Forms a white precipitate Redissolved in excess to form a colourless solution | Forms a white precipitate Insoluble in excess | Forms a white precipitate and bubbles of gas |
Fe3+ | Yellow-orange | Forms a red-brown precipitate Insoluble in excess | Forms a red-brown precipitate Insoluble in excess | Forms a red-brown precipitate and bubbles of gas |
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
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)
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(H2O)3 (s) + OH- (aq) → [Al(OH)4]- (aq) + 3H2O (l)
Al(OH)3(H2O)3 + OH– → [Al(OH)4(H2O)2]– + H2O
Al(OH)3 + NaOH → NaAl(OH)4
Al(OH)3 + OH– → [Al(OH)4]-
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