CAIE A-Level Chemistry 28.3.5 Colours of Cu(II) and Co(II) Complexes

Practise applying Cu(II) and Co(II) colour changes with water, ammonia, hydroxide and chloride ligands.

Exam points

recall pink [Co(H2O)6]2+ changing to blue [CoCl4]2− when concentrated chloride is added
distinguish pale-blue Cu(OH)2 precipitate from the deep-blue Cu(II) ammine solution in excess ammonia
state complex formulae alongside colours, including deep-blue [Cu(NH3)4(H2O)2]2+ and yellow [CuCl4]2−

[Maximum number: 1]

Copper is a transition element and has atomic number 29.

The following equilibrium exists between two complex ions of copper in the +2 oxidation state.

\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}+4 \mathrm{Cl}^{-} \rightleftharpoons\left[\mathrm{CuCl}_{4}\right]^{2-}+6 \mathrm{H}_{2} \mathrm{O}

State the colours of these two complex ions.
$\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ $\left[\mathrm{CuCl}_{4}\right]^{2-}$

$\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ blue and $\left[\mathrm{CuCl}_{4}\right]^{2-}$ yellow OR yellow / green OR green / yellow

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28.3.5—Colours of Cu(II) and Co(II) complexes question 1