Topic 17 · page 13 of 20
Eight ions with ammonia
A few drops of ammonia give the same precipitates as sodium hydroxide, because ammonia is a base. In excess, ammonia is a ligand and five of the precipitates dissolve.
Read first: Eight ions with sodium hydroxide
The observations 17.22
Ammonia solution does two jobs, one after the other.
- A few drops: NH₃ is a base. It removes H⁺ from water ligands, exactly as hydroxide does, and gives the same precipitates.
- Excess: NH₃ is a ligand. It replaces ligands in the precipitate, and five of the precipitates dissolve.
| Ion (colour of solution) | A few drops of NH₃(aq) | Excess NH₃(aq) |
|---|---|---|
| Cr³⁺ (green) | green precipitate | dissolves: purple solution |
| Mn²⁺ (very pale pink) | off-white precipitate | no change |
| Fe²⁺ (pale green) | green precipitate | no change |
| Fe³⁺ (yellow-brown) | orange-brown precipitate | no change |
| Co²⁺ (pink) | blue precipitate | dissolves: straw-coloured solution, darkens in air |
| Ni²⁺ (green) | green precipitate | dissolves: blue solution |
| Cu²⁺ (blue) | blue precipitate | dissolves: deep blue solution |
| Zn²⁺ (colourless) | white precipitate | dissolves: colourless solution |
Ammonia as a base 17.2217.23
Each M²⁺ aqua ion loses two H⁺ and each M³⁺ ion loses three. The NH₃ takes the H⁺ and becomes NH₄⁺:
M is Mn, Fe, Co, Ni, Cu or Zn for the first equation and Cr or Fe for the second. The precipitates are the same as on the previous page. Ammonia solution is alkaline because of a small amount of NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, so the OH⁻ route is also correct and the short ionic equation is accepted.
Ammonia as a ligand 17.2217.23
In excess, NH₃ replaces water and hydroxide ligands. The precipitate dissolves because a charged complex ion forms, and charged ions are soluble.
| Ion | Complex formed | Colour | Coordination number |
|---|---|---|---|
| Cr³⁺ | [Cr(NH₃)₆]³⁺ | purple | 6 |
| Co²⁺ | [Co(NH₃)₆]²⁺ | straw | 6 |
| Ni²⁺ | [Ni(NH₃)₆]²⁺ | blue | 6 |
| Cu²⁺ | [Cu(NH₃)₄(H₂O)₂]²⁺ | deep blue | 6 |
| Zn²⁺ | [Zn(NH₃)₄]²⁺ | colourless | 4 |
The equations, written from the precipitate:
Equations written straight from the aqua ion are also accepted, for example for nickel:
Copper keeps two water ligands: .
The straw-coloured cobalt complex turns darker brown in air, because cobalt(II) is oxidised to cobalt(III).
Sodium hydroxide and ammonia together
| Ion | Dissolves in excess NaOH? | Dissolves in excess NH₃? |
|---|---|---|
| Cr³⁺ | yes (amphoteric) | yes |
| Mn²⁺ | no | no |
| Fe²⁺ | no | no |
| Fe³⁺ | no | no |
| Co²⁺ | no | yes |
| Ni²⁺ | no | yes |
| Cu²⁺ | no | yes |
| Zn²⁺ | yes (amphoteric) | yes |
Chromium and zinc dissolve in both. Cobalt, nickel and copper dissolve only in ammonia. Manganese and the two irons dissolve in neither.
Check yourself
- Give the observations when ammonia solution is added slowly, then in excess, to Ni²⁺(aq).
- Write the equation for the reaction of [Ni(OH)₂(H₂O)₄] with excess ammonia.
- Give the observations for Fe³⁺(aq) with a few drops of ammonia, and then in excess. Write the ionic equation for the first step.
- State which of the eight ions dissolve in excess NaOH, and which dissolve in excess NH₃.
- A solution gives a blue precipitate with a few drops of ammonia, which dissolves in excess to a deep blue solution. Name the ion, give the formula of the complex in the final solution and write the equation for its formation from the precipitate.
- Explain why the zinc hydroxide precipitate dissolves in excess ammonia solution.
Answers cover these until you have tried the questions
- A green precipitate forms, which dissolves in excess ammonia to form a blue solution.
- An orange-brown precipitate forms, which does not dissolve in excess (no change).
- Excess NaOH: Cr³⁺ and Zn²⁺. Excess NH₃: Cr³⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺.
- Cu²⁺. The complex is [Cu(NH₃)₄(H₂O)₂]²⁺.
- In excess ammonia, NH₃ acts as a ligand and replaces the water and hydroxide ligands, forming the soluble complex ion [Zn(NH₃)₄]²⁺: (a colourless solution).