5.2.1 - Reactions and Colours of Metal Aqua Ions
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Why are metal-aqua ion solutions acidic?
Hydrolysis occurs on the metal-aqua complex ions. For example, [Fe(H₂O)₆]²⁺ (aq) + H₂O (l) —-> [Fe(OH)(H₂O)₅]⁺ (aq) + H₃O⁺ (aq)
Why are 3⁺ metal-aqua solutions more acidic than 2⁺ metal-aqua solutions?
3⁺ metal-aqua solutions have a higher charge density due to their higher charge and smaller size. This makes them more polarising. As they attract the bonding pair of electrons, the O-H gets weaker which means the H⁺ ion is more likely to be released.
What colour are [Cu(H₂O)₆]²⁺ ions?
Blue.
What colour are [Fe(H₂O)₆]²⁺ ions?
Green.
What colour are [Al(H₂O)₆]³⁺ ions?
Colourless.
What colour are [Fe(H₂O)₆]³⁺ ions?
Although the solution is actually purple, it appears yellow because one of the H₂O ligands hydrolyses.
What is the overall reaction for [Cu(H₂O)₆]²⁺ reacting with sodium hydroxide dropwise? What colour does this turn?
[Cu(H₂O)₆]²⁺ + 2OH⁻ —-> Cu(H₂O)₄(OH)₂ + 2H₂O It forms a blue precipitate.
Does [Cu(H₂O)₆]²⁺ react differently in an excess of NaOH?
No, it remains the same as dropwise.
What is the overall reaction for [Cu(H₂O)₆]²⁺ reacting with ammonia dropwise? What colour does this turn?
[Cu(H₂O)₆]²⁺ + 2NH₃ —> Cu(H₂O)₄(OH)₂ + 2NH₄⁺ It forms a blue precipitate.
Does [Cu(H₂O)₆]²⁺ react differently in an excess of ammonia?
Yes, ligand substitution occurs. Cu(H₂O)₄(OH)₂ + 4NH₃ —-> [Cu(H₂O)₂(NH₃)₄]²⁺ + 2H₂O + 2OH⁻ This turns the solution deep blue.
What is the overall reaction for [Cu(H₂O)₆]²⁺ reacting with sodium carbonate? What colour does this turn?
[Cu(H₂O)₆]²⁺ + CO₃²⁻ —-> CuCO₃ + 6H₂O This forms a green-blue precipitate.
What is the overall reaction for [Fe(H₂O)₆]²⁺ reacting with sodium hydroxide dropwise? What colour does this turn?
[Fe(H₂O)₆]²⁺ + 2OH⁻ —-> Fe(H₂O)₄(OH)₂ + 2H₂O It forms a green precipitate. This will slowly turn brown in air.
Does [Fe(H₂O)₆]²⁺ react differently in an excess of NaOH?
No, it remains the same as dropwise.
What is the overall reaction for [Fe(H₂O)₆]²⁺ reacting with ammonia dropwise? What colour does this turn?
[Fe(H₂O)₆]²⁺ + 2NH₃ —> Fe(H₂O)₄(OH)₂ + 2NH₄⁺ It forms a green precipitate.
Does [Fe(H₂O)₆]²⁺ react differently in an excess of ammonia?
No, it is the same as dropwise.
What is the overall reaction for [Fe(H₂O)₆]²⁺ reacting with sodium carbonate? What colour does this turn?
[Fe(H₂O)₆]²⁺ + CO₃²⁻ —-> FeCO₃ + 6H₂O This forms a green precipitate.
What is the overall reaction for [Al(H₂O)₆]³⁺ reacting with sodium hydroxide dropwise? What colour does this turn?
[Al(H₂O)₆]³⁺ + 3OH⁻ —-> Al(H₂O)₃(OH)₃ + 3H₂O It forms a white precipitate.
Does [Al(H₂O)₆]³⁺ react differently in an excess of NaOH?
Yes it does. Further hydrolysis occurs. Al(H₂O)₃(OH)₃ + OH⁻ —-> [Al(H₂O)₂(OH)₄]⁻ + H₂O The white precipitate redissolves.
What is the overall reaction for [Al(H₂O)₆]³⁺ reacting with ammonia dropwise? What colour does this turn?
[Al(H₂O)₆]³⁺ + 3NH₃ —-> Al(H₂O)₃(OH)₃ + 3NH₄⁺ This forms a white precipitate.
Does [Al(H₂O)₆]³⁺ react differently in an excess of ammonia?
No, it remains the same as dropwise.
What is the overall reaction for [Al(H₂O)₆]³⁺ reacting with sodium carbonate? What colour does this turn?
2[Al(H₂O)₆]³⁺ + 3CO₃²⁻ —-> 2Al(H₂O)₃(OH)₃ + 3H₂O + 3CO₂ This forms a white precipitate and bubbles.
What is the overall reaction for [Fe(H₂O)₆]³⁺ reacting with sodium hydroxide dropwise? What colour does this turn?
[Fe(H₂O)₆]³⁺ + 3OH⁻ —-> Fe(H₂O)₃(OH)₃ + 3H₂O It forms a brown precipitate.
Does [Fe(H₂O)₆]³⁺ react differently in an excess of NaOH?
No, it remains the same as dropwise.
What is the overall reaction for [Fe(H₂O)₆]³⁺ reacting with ammonia dropwise? What colour does this turn?
[Fe(H₂O)₆]³⁺ + 3NH₃ —-> Fe(H₂O)₃(OH)₃ + 3NH₄⁺ This forms a brown precipitate.