Reactions of metal ions with sodium hydroxide and ammoniaEdexcel International A Level Chemistry: Revision notes
Section 1
Why precipitates form
Aqueous metal ions exist as hexaaqua complexes such as [Cu(H₂O)₆]²⁺ and [Fe(H₂O)₆]³⁺. Hydroxide ions, or ammonia acting as a base, remove H⁺ from the water ligands (deprotonation). This forms a neutral hydroxide complex that is insoluble, a precipitate.
M²⁺: [M(H₂O)₆]²⁺ + 2OH⁻ → [M(H₂O)₄(OH)₂] + 2H₂O
M³⁺: [M(H₂O)₆]³⁺ + 3OH⁻ → [M(H₂O)₃(OH)₃] + 3H₂O
With ammonia: [M(H₂O)₆]²⁺ + 2NH₃ → [M(H₂O)₄(OH)₂] + 2NH₄⁺
Simple ionic equations are also accepted, e.g. Cu²⁺ + 2OH⁻ → Cu(OH)₂.
Section 2
Observations with sodium hydroxide
Add NaOH dropwise, then in excess.
- Cr³⁺ (green): green precipitate, dissolves in excess to give a dark green solution
- Mn²⁺ (pale pink): off-white precipitate, turns brown in air; insoluble in excess
- Fe²⁺ (pale green): green precipitate, brown at the surface in air; insoluble
- Fe³⁺ (yellow): orange-brown precipitate; insoluble
- Co²⁺ (pink): blue precipitate; insoluble
- Ni²⁺ (green): green precipitate; insoluble
- Cu²⁺ (blue): pale blue precipitate; insoluble
- Zn²⁺ (colourless): white precipitate, dissolves in excess to give a colourless solution
Only Cr³⁺ and Zn²⁺ dissolve in excess sodium hydroxide, so they are the amphoteric ones.
Section 3
Amphoteric hydroxides
An amphoteric substance reacts with both acids and bases.
Chromium(III) and zinc hydroxides dissolve in acid (acting as bases) and in excess alkali (acting as acids):
[Cr(H₂O)₃(OH)₃] + 3H⁺ → [Cr(H₂O)₆]³⁺
[Cr(H₂O)₃(OH)₃] + 3OH⁻ → [Cr(OH)₆]³⁻ + 3H₂O
[Zn(H₂O)₄(OH)₂] + 2OH⁻ → [Zn(OH)₄]²⁻ + 4H₂O
The hydroxide ligands replace water ligands, so this is also a ligand exchange.
Section 4
Observations with ammonia and ligand exchange
A few drops of NH₃ give the same precipitates, by deprotonation. In excess ammonia some precipitates dissolve by ligand exchange:
- Cu²⁺: pale blue precipitate dissolves to give a deep blue solution, [Cu(NH₃)₄(H₂O)₂]²⁺
- Ni²⁺: green precipitate dissolves to give a blue solution, [Ni(NH₃)₆]²⁺
- Co²⁺: blue precipitate dissolves to give a brown or straw coloured solution, [Co(NH₃)₆]²⁺
- Zn²⁺: white precipitate dissolves to give a colourless solution, [Zn(NH₃)₄]²⁺
- Cr³⁺: green precipitate; with excess it dissolves only slightly, giving a purple solution, [Cr(NH₃)₆]³⁺
- Mn²⁺, Fe²⁺, Fe³⁺: precipitates do not dissolve
Example: [Cu(H₂O)₄(OH)₂] + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 2H₂O + 2OH⁻
Do not forget that the precipitate with a few drops of ammonia forms by deprotonation, not ligand exchange. Exchange happens only in excess ammonia.
Section 5
Identifying an unknown ion
Work through the observations in order.
- Note the colour of the solution (colourless means Zn²⁺).
- Note the precipitate colour with NaOH.
- Does it dissolve in excess NaOH? Yes means Cr³⁺ or Zn²⁺.
- Does it dissolve in excess NH₃? Yes means Cu²⁺, Ni²⁺, Co²⁺ or Zn²⁺.
- Does the precipitate darken in air? Fe²⁺ and Mn²⁺ turn brown.
Then write the equation for each step, naming the precipitate and complex ion formed.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Reactions of metal ions with sodium hydroxide and ammonia
- A technician has four solutions, each containing one of the ions Cu²⁺(aq), Fe²⁺(aq), Fe³⁺(aq) or Zn²⁺(aq). She adds aqueous sodium hydroxide to a sample of each solution, first dropwise and then in excess.State what is seen when sodium hydroxide is added to the Cu²⁺(aq) solution and write an ionic equation for the reaction, using the simple formulae of the species.2 marks
- Aqueous ammonia is added first dropwise and then in excess to separate solutions of Ni²⁺(aq), Co²⁺(aq) and Cu²⁺(aq). In each case a precipitate forms at first.Explain, with an equation, why a precipitate forms when a few drops of aqueous ammonia are added to Cu²⁺(aq).2 marks
- Aqueous chromium(III) ions, Cr³⁺(aq), are green and aqueous zinc ions, Zn²⁺(aq), are colourless. A student adds sodium hydroxide solution to each, first dropwise and then in excess, and later adds dilute sulfuric acid to the precipitates.Describe what is seen when sodium hydroxide solution is added, first dropwise and then in excess, to the Cr³⁺(aq) 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).