CAIE A-Level Chemistry 28.3.3 Why Transition Compounds Are Coloured

Practise building the full explanation from split d orbitals and electron promotion to absorption and complementary colour.

Exam points

state that ligands split the metal ion’s d orbitals into non-degenerate energy levels
explain that an electron absorbs visible-light energy equal to ΔE and is promoted between the levels
state that the complementary colour to the absorbed frequency or wavelength is the colour observed

[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]

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28.3.3—Why transition elements form coloured compounds question 1