Reactions of metal ions with hydroxide, ammonia and carbonateAQA A-Level Chemistry: Revision notes
Section 1
Hydroxide and ammonia as bases
The aqua ions [Cu(H₂O)₆]²⁺, [Fe(H₂O)₆]²⁺, [Al(H₂O)₆]³⁺ and [Fe(H₂O)₆]³⁺ react with bases. A base such as OH⁻ removes H⁺ from the water ligands. A few drops give an uncharged hydroxide precipitate, for example:
[Fe(H₂O)₆]³⁺ + 3OH⁻ → Fe(OH)₃(H₂O)₃ + 3H₂O
Aqueous ammonia also acts as a base (NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, or it accepts H⁺ directly), so a small amount gives the same precipitates.
Section 2
Colours with sodium hydroxide
With NaOH(aq), added dropwise and then in excess:
- Cu²⁺: pale blue precipitate, Cu(OH)₂(H₂O)₄, insoluble in excess
- Fe²⁺: green precipitate, Fe(OH)₂(H₂O)₄, insoluble in excess; darkens or turns brown at the surface in air as it is oxidised to Fe(III)
- Fe³⁺: brown precipitate, Fe(OH)₃(H₂O)₃, insoluble in excess
- Al³⁺: white precipitate, Al(OH)₃(H₂O)₃, dissolves in excess to give a colourless solution
Say brown for Fe³⁺ and green for Fe²⁺. Do not just write orange or yellow.
Section 3
Amphoteric aluminium hydroxide
Aluminium hydroxide dissolves in both acids and bases, so it is amphoteric.
With excess OH⁻ (acting as an acid): Al(OH)₃(H₂O)₃ + OH⁻ → [Al(OH)₄(H₂O)₂]⁻ + H₂O
With acid (acting as a base): Al(OH)₃(H₂O)₃ + 3H⁺ → [Al(H₂O)₆]³⁺
It does not dissolve in excess ammonia, because ammonia is a weak base and does not provide enough OH⁻.
Do not say aluminium hydroxide dissolves in excess ammonia. Only copper(II) hydroxide does, by ligand substitution.
Section 4
Excess ammonia with copper(II)
Excess ammonia dissolves the pale blue precipitate to give a deep blue solution because ammonia ligands replace water ligands in a ligand substitution:
[Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O
The charge is unchanged, because NH₃ and H₂O are both neutral. The other hydroxides in the specification do not dissolve in excess ammonia.
Section 5
Reactions with carbonate
Carbonate ions behave differently with the 2+ and 3+ ions because the 3+ aqua ions are more acidic.
M²⁺ (Cu, Fe): the metal carbonate precipitates, with no gas: [Cu(H₂O)₆]²⁺ + CO₃²⁻ → CuCO₃ + 6H₂O (blue-green) [Fe(H₂O)₆]²⁺ + CO₃²⁻ → FeCO₃ + 6H₂O (green)
M³⁺ (Al, Fe): the aqua ion is acidic enough to give up H⁺ to carbonate, so CO₂ and a hydroxide precipitate form: 2[Fe(H₂O)₆]³⁺ + 3CO₃²⁻ → 2Fe(OH)₃(H₂O)₃ + 3CO₂ + 3H₂O
White Al(OH)₃ forms in the same way with Al³⁺.
Effervescence with carbonate points to a 3+ ion.
Section 6
Required practical 11
In the test-tube practical, add each reagent dropwise to about 1 cm depth of solution, shake gently, and note the colour and state at once, then after adding excess and after standing. Always note whether a precipitate dissolves. Use clean droppers and fresh samples, as Fe²⁺ darkens in air.
Record the colour of the precipitate and the colour of the solution separately.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Reactions of metal ions with hydroxide, ammonia and carbonate
- A technician has four unlabelled bottles of aqueous solution. They contain copper(II) sulfate, iron(II) sulfate, iron(III) chloride and aluminium nitrate. She adds sodium hydroxide solution dropwise to a sample of each, and then in excess, and records the observations.Write equations to show that aluminium hydroxide is amphoteric.2 marks
- A student adds aqueous ammonia dropwise to copper(II) sulfate solution. A pale blue precipitate forms. When more ammonia is added the precipitate dissolves, giving a deep blue solution.Write an equation for the formation of the pale blue precipitate and state the role of ammonia in this reaction.2 marks
- A student adds sodium carbonate solution to separate samples of iron(II) sulfate solution and iron(III) sulfate solution of the same concentration.Describe the observations made with each solution.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).