What factors determine the acidity of metal aqua ions in aqueous solution?

The acidity of metal aqua ions in aqueous solution is determined by:

  1. The ionic radius of the metal ion.
  2. The charge on the metal ion.

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Compare and explain the acidity of M2+ and M3+ aqua ions.

Aqua ions of M3+ are generally more acidic than aqua ions of M2+.


This is because M3+ ions have a smaller radius and a greater charge - this means they are able to polarise water molecules and weaken the O-H bond in H2O to a greater extent. This releases a higher concentration of H+ ions into the solution.

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Explain, with the use of an equation, why [Fe(H2O)6]3+ is classed as a Brønsted-Lowry acid.

[Fe(H2O)6]3+ is a Brønsted-Lowry acid because it donates a proton to a water molecule.

[Fe(H2O)6]3+(aq) + H2O(l) ⇌ [Fe(H2O)5(OH)]2+(aq) + H3O+(aq)


An alternative equation is: [Fe(H2O)6]3+(aq) ⇌ [Fe(H2O)5(OH)]2+(aq) + H+(aq).

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What is the formula of the precipitate formed when aqueous [M(H2O)6]2+ ions react with hydroxide ions?

[M(H2O)4(OH)2


This could also be written as M(OH)2.

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What is the ionic equation for the reaction of carbonate ions (CO32-) with [Fe(H2O)6]2+?

[Fe(H2O)6]2+(aq) + 3CO32-(aq) ➔ FeCO3(s) + 6H2O(l)

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Why does iron(II) form a carbonate precipitate, while iron(III) does not?

  • [Fe(H2O)6]3+ is a stronger acid than [Fe(H2O)6]2+.
  • When carbonate ions come in contact with [Fe(H2O)6]3+, they remove protons to form iron(III) hydroxide and carbon dioxide.
  • However, carbonate ions cannot remove protons from the weaker acid [Fe(H2O)6]2+. Instead, iron(II) carbonate precipitates out when carbonates contact [Fe(H2O)6]2+.

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Describe how you can distinguish between dilute aqueous solutions of Fe2+ and Fe3+.

  • Add sodium hydroxide (NaOH).
  • Fe2+ forms a green precipitate of iron(II) hydroxide.
  • Fe3+ forms a brown precipitate of iron(III) hydroxide.

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What does the term amphoteric mean?

Amphoteric means a substance that can react with both acids and bases.


Aluminium hydroxide is an example of an amphoteric substance.

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What are the ionic equations for the reactions of aluminum hydroxide, Al(H2O)3(OH)3, with H+ and OH- ions?

Al(H2O)3(OH)3(s) + 3H+(aq) ➔ [Al(H2O)6]3+(aq)


Al(H2O)3(OH)3(s) + OH-(aq) ➔ [Al(H2O)2(OH)4]-(aq) + H2O(l)

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What is the formula of the precipitate formed when aqueous [M(H2O)6]3+ ions react with hydroxide ions?

[M(H2O)3(OH)3


This could also be written as M(OH)3.

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What is an aqua ion?

A positively-charged metal surrounded by water molecules acting as ligands.


An example of an aqua ion is [Cr(H2O)6]3+.

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