CAIE A-Level Chemistry A2 28.3 Colour of Complexes Questions

Practise explaining complex colours through d-orbital splitting, visible-light absorption and ligand-dependent energy gaps.

Exam points

use non-degenerate d orbitals, electron promotion and visible-light absorption to explain observed colour
explain colourless d0 or d10 species by the absence of a possible d–d electron transition
link a ligand change to a different ΔE, absorbed frequency or wavelength, and complementary colour

Covered syllabus points

28.3.1—Terms: degenerate and non-degenerate d orbitals 28.3.2—d-orbital splitting 28.3.3—Why transition elements form coloured compounds 28.3.4—Ligand effects on orbital splitting 28.3.5—Colours of Cu(II) and Co(II) complexes

[Maximum number: 5]

(a)

The 3d orbitals in an isolated gaseous $\mathrm{Cu}^{2+}$ ion are degenerate.

[ 1 ]

(i)

Define the term degenerate.

[ 1 ]

(b)

State the colours of the aqueous solutions for the two copper(II) complex ions shown.
- $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}(\mathrm{aq})$
- $\left[\mathrm{CuCl}_{4}\right]^{2-}(\mathrm{aq})$

[ 1 ]

$\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}(\mathrm{aq})$ deep / dark / royal blue

AND

$\left[\mathrm{CuCl}_{4}\right]^{2-}(\mathrm{aq})$ yellow

(c)

Explain why aqueous complex ions of transition elements are usually coloured.

[ 3 ]

[Maximum number: 2]

The 3d orbitals in an isolated $\mathrm{Cu}^{2+}$ ion are degenerate.

Complete the diagram to show the relative energies of the 3 d orbitals in an isolated $\mathrm{Cu}^{2+}$ ion and in $\mathrm{Cu}^{2+}$ in a tetrahedral complex.

Figure for Question 2 — CAIE A-Level Chemistry A2

[2]

five 3d orbitals (lines, boxes) in the isolated $\mathrm{Cu}^{2+}$ ion of the same energy

- splitting: three higher and two lower d orbitals

- energy of all five d orbitals in complex higher than all d orbitals in isolated ion any two [1], all three [2]

[Maximum number: 2]

Titanium is a transition element in Period 4. It is commonly found as $\mathrm{TiO}_{2}$ in minerals.

The $\mathrm{TiO}^{2+}$ ion forms when $\mathrm{TiO}_{2}$ reacts with an excess of sulfuric acid.
$\mathrm{TiO}^{2+}$ can be reduced by zinc metal in acidic conditions to form a purple solution containing $\mathrm{Ti}^{3+}(\mathrm{aq})$ .

$\mathrm{TiO}^{2+}(\mathrm{aq})$ is a colourless ion.

Suggest why.

Ti is in +4 oxidation state so no d electrons $/ \mathrm{d}^{0} \mathbf{OR ~ T i}$ in $\mathrm{TiO}^{2+}$ has no d electrons $/ \mathrm{d}^{0}$ [1]
cannot absorb photons / light in visible spectrum OR no wavelength / frequency absorbed in visible spectrum [1]

[Maximum number: 1]

The 3d orbitals in an isolated gaseous $\mathrm{Cu}^{2+}$ ion are degenerate.

Define the term degenerate.

[Maximum number: 1]

Copper is a transition element. It forms a wide variety of compounds.

An aqueous solution of copper(II) sulfate, $\mathrm{CuSO}_{4}$ , contains $\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ complex ions. If an excess of concentrated hydrochloric acid is added to this solution a ligand exchange reaction occurs and $\left[\mathrm{CuCl}_{4}\right]^{2-}$ complex ions are formed.

Explain why the solutions of the two complex ions in Table 5.1 are different colours.

[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

1

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Covered syllabus points

Question 1

Question (a)

Question (i)

Question (b)

Question (c)

Question 2

Question 3

Question 4

Question 5

Question 6