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Reactions of Metal-Aqua Ions (Oxford AQA International A Level Chemistry)

Revision Note

Richard Boole

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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    Author
    Reviewer
    Richard Boole

    Author: Richard Boole

    Expertise: Chemistry

    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

    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.