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Identification (A2 Only)

Chemistry: AQA A Level, topic 4.3.2

Precipitates

Precipitates can form with ammonia solution and with carbonates. They are different precipitates to those you get with sodium hydroxide ions.

Ammonia precipitates

Ammonia precipitates

  • Ammonia is a base.
  • When you add it to water, you get a reaction according to the following equation:
  • NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
  • The OH-(aq) ions will deprotonate the water ligands and generate an insoluble metal hydroxide precipitate.
Ammonia ligands

Ammonia ligands

  • If you add a small amount of ammonia, you get the same as with sodium hydroxide.
  • If you add an excess of ammonia, you can get a ligand exchange reaction.
  • The ammonia can replace some of the water ligands and some of the hydroxide ligands.
  • This happens for the reaction with [Cu(H2O)6]2+.
  • The reaction with excess ammonia is:
  • [Cu(H2O)6]2+(aq) + 4NH3(aq) → [Cu(H2O)2(NH3)4]2+(aq)
Carbonate reactions

Carbonate reactions

  • M2+ ions react with sodium carbonate according to the following equation:
  • [M(H2O)6]2+(aq) + CO32-(aq) ⇌ MCO3(s) + 6H2O(l)
M3+ ions and carbonates

M3+ ions and carbonates

  • M3+ ions are stronger acids. This means they react with the carbonates in a different way.
  • They release H+ ions which react with the carbonate ions:
  • CO32-(aq) + 2H+(aq) → H2O(l) + CO2(g)
  • In practice, this means that the carbonates act as bases. You get the same product as for hydroxide with M3+ ions.

Test Tube Reactions

To identify a metal ion, you can test its behaviour with hydroxide, ammonia, and carbonates in a test tube.

Procedure

Procedure

  • To test your ions, you want to put a small sample in three test tubes:
  • To the first, add sodium hydroxide and record any changes. Then add sodium hydroxide to excess to see if anything changes.
  • To the second, repeat the process with ammonia.
  • To the third, add some sodium carbonate.
  • In the following slides, we’ll see how to tell the difference between copper(II), iron(II), iron(III) and aluminium(III).
Sodium hydroxide

Sodium hydroxide

  • The four ions will all form precipitates with sodium hydroxide.
  • When you add excess sodium hydroxide to aluminium(III), it will re-dissolve because [Al(OH)4]−(aq) forms.
  • The rest will do nothing with an excess.
Ammonia

Ammonia

  • The four ions will all form precipitates with ammonia.
  • When you add excess ammonia to copper(II), it re-dissolves.
  • This is because you get the following ligand exchange:
  • [Cu(H2O)6]2+(aq) + NH3(aq) → [Cu(H2O)2(NH3)4]2+(aq)
  • The rest will do nothing on excess.
Sodium carbonate

Sodium carbonate

  • Sodium carbonate will always generate a precipitate.
  • If the solution bubbles, you have an M(III) ion. If not, it’s an M(II) ion.
  • This can be used to distinguish between iron(II) and iron(III).

Colour Changes

In addition to the behaviours in each test tube reaction, you need to know the colours of everything produced.

Solutions

Solutions

  • These are the colours of the aqueous solutions of the above ions.
Hydroxides

Hydroxides

  • These are the colours of the precipitates produced when hydroxide ions are added to the solutions.
Ammonia

Ammonia

  • The only change on adding ammonia is that the copper ion goes a deeper blue.
Carbonates

Carbonates

  • Upon adding sodium carbonate, the copper precipitates as a green-blue carbonate.

Test yourself on Identification (A2 Only)

  • What happens to every metal ion in sodium hydroxide, sodium carbonate and ammonia?

  • Adding hydroxide ions to an aqueous solution causes a precipitate to form. What is the only ion that forms a precipitate of a different colour to the original aqueous solution?

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