Examkin

Question

Difficulty: MediumCopper: Extraction, Physical and Chemical Properties, and Compounds

When aqueous ammonia is added dropwise until in excess to an aqueous solution containing copper(II) ions, the pale blue precipitate that initially forms dissolves to produce a characteristic deep blue solution. Which complex ion is responsible for this deep blue color?

  1. [Cu(NH3)4]2+[\text{Cu}(\text{NH}_3)_4]^{2+}Answer
  2. B
    [Cu(NH3)2]+[\text{Cu}(\text{NH}_3)_2]^+
  3. C
    [Cu(OH)4]2−[\text{Cu}(\text{OH})_4]^{2-}
  4. D
    [Cu(H2O)4]2+[\text{Cu}(\text{H}_2\text{O})_4]^{2+}

Answer

The complex ion responsible for the deep blue solution is the tetraamminecopper(II) ion, [Cu(NH3)4]2+[\text{Cu}(\text{NH}_3)_4]^{2+}.
When excess aqueous ammonia is added to a solution containing Cu2+\text{Cu}^{2+} ions, ammonia molecules act as ligands to coordinate with the Cu2+\text{Cu}^{2+} ion, forming the deep blue tetraamminecopper(II) complex ion, [Cu(NH3)4]2+[\text{Cu}(\text{NH}_3)_4]^{2+}.

Step-by-Step Solution

1
Identify the initial reaction of copper(II) ions with aqueous ammonia.
A pale blue precipitate of copper(II) hydroxide, Cu(OH)2\text{Cu(OH)}_2, is formed when small amounts of aqueous ammonia are added: Cu(aq)2++2NH3(aq)+2H2O(l)→Cu(OH)2(s)+2NH4(aq)+\text{Cu}^{2+}_{(\text{aq})} + 2\text{NH}_{3(\text{aq})} + 2\text{H}_2\text{O}_{(\text{l})} \rightarrow \text{Cu(OH)}_{2(\text{s})} + 2\text{NH}_{4(\text{aq})}^+.
Aqueous ammonia acts as a weak base, generating hydroxide ions.
2
Determine the effect of adding excess aqueous ammonia.
The copper(II) hydroxide precipitate dissolves due to ligand substitution, forming the soluble complex ion [Cu(NH3)4](aq)2+[\text{Cu}(\text{NH}_3)_4]^{2+}_{(\text{aq})}.
Ammonia molecules act as unidentate neutral ligands that bind strongly to copper(II) ions, displacing hydroxide ions and forming the deep blue tetraamminecopper(II) ion.

Key Concept

Complex ion formation and qualitative test for copper(II) ions using aqueous ammonia.
Estimated Time:1m 0s
Rate this question

Topics

Copper: Extraction, Physical and Chemical Properties, and Compounds

More questions from this topic