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Acidity (A2 Only)

Chemistry: AQA A Level, topic 4.3.1

Acidity

Some metal-aqua ions have an acidic pH.

Metal-aqua ions

Metal-aqua ions

  • When transition metal compounds are dissolved in water, they form complex aqua ions.
  • These are ions with water molecules as ligands.
  • Most of these ions (and all the ones you should worry about) have six water ligands.
  • Aluminium (III) will also form a hexaaqua ion.
Acidity

Acidity

  • When you dissolve transition metal ions in water, you get hexaaqua ions.
  • But these ions are actually acidic.
  • M2+ ions will release some H+ into the solution.
  • M3+ ions are much more acidic.
Why are they acidic?

Why are they acidic?

  • The ions are positively charged.
  • This means they pull electron density off the water ligands and towards the metal ions.
  • This weakens the O-H bonds in the water molecules.
  • This makes H+ ions more likely to dissociate from the water molecules.
M2+ vs M3+

M2+ vs M3+

  • M3+ ions are smaller, and have a higher charge.
  • They have a higher charge density.
  • Because they have a higher charge density, they pull harder on the electron density of the water ligands.
  • This weakens the O-H bonds more.
  • So M3+ ions are much more acidic than M2+ ions.

Reactions With Base

Metal aqua-ions will react with a base.

Insoluble metal precipitates

Insoluble metal precipitates

  • Adding sodium hydroxide to a solution of transition metal ions will remove multiple protons from the ligands.
  • You remove protons until the overall charge is neutralised.
  • These ions then precipitate out of solution because they are no longer charged.
M3+ ions

M3+ ions

  • With [M(H2O)6]3+ ions, you remove three protons to get [M(H2O)3(OH)3]. The charge is neutralised and the hydroxide precipitates out of solution.
M2+ ions

M2+ ions

  • With [M(H2O)6]2+ ions, you remove two protons to get [M(H2O)4(OH)2].
  • This also precipitates out of solution.

Amphotericity

Some compounds will react with both acids and bases.

Amphotericity

Amphotericity

  • An amphoteric compound is one which reacts with both acid and base.
  • This is pretty counter-intuitive because they can act as both acid and base!
  • All of the metal hydroxides we talked about in the previous concept module will react with acid.
  • They can all accept protons and go back to being hexaaqua ions.
  • Some of them can react with bases as well.
Aluminium hydroxide

Aluminium hydroxide

  • Aluminium hydroxide is actually [Al(OH)3(H2O)3].
  • If you add base, you can remove protons from the water ligands according to this equation:
  • [Al(OH)3(H2O)3](s) + OH-(aq) ⇋ [Al(OH)4]-(aq) + 3H2O(l)
  • But if you add acid, you can add protons according to this equation:
  • [Al(OH)3(H2O)3](s) + H+(aq) ⇋ [Al(H2O)6]3+(aq)
  • So aluminium hydroxide will react with acids and bases. It is amphoteric.

Test yourself on Acidity (A2 Only)

  • Which of the following is an example of a non-transition metal that also forms a hexaaqua ion?

  • Acidity of Aqueous Transition Metal Ions

  • What is the reason for higher charged metal ions being more acidic?

  • What will happen if we add sodium hydroxide to a solution of transition metal ions?

  • Adding sodium hydroxide to aqueous transition metal solution usually results in what type of complex?

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