Solution A contains the compound [Cu(H2O)6]Cl2 - AQA - A-Level Chemistry - Question 7 - 2017 - Paper 1

The chloride ions (Cl-) do not donate a lone pair to the copper ion. Instead, they are present as counterions and do not form coordinate (dative) bonds with the metal ion.

Equation:

$[Cu(H2O)6]^{2+} + 4 NH_3 \rightarrow [Cu(NH_3)_{4}]^{2+} + 6 H_2O$

Colour:

Royal blue or dark blue.

The blue-green solid C is likely to be copper carbonate (CuCO3).

Identify of reagent D:

Reagent D is hydrochloric acid (HCl).

Equation:

$Cu^{2+} + 4 Cl^- \rightarrow [CuCl_4]^{2-}$

Colorimetry cannot be used due to the fact that [Cu^2+] has an incomplete d-orbital configuration, which prevents it from absorbing visible light effectively. Furthermore, the presence of chloride ions interferes with the detection through colorimetric methods.