Home · A-Level Chemistry 9701 · Transition elements · Colour of complexes · Practice
A2 · Practice questions · Colour of complexes

Mind the gap.

Six original Cambridge-style questions on the colour of transition metal complexes.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Split, absorb, see the complement.

01
Explain
[4 marks]

Explain why aqueous [Cu(H2O)6]2+ is coloured.

  • Cu2+ is 3d9, a partly filled d subshell. ✓
  • The water ligands split the 3d orbitals into two non-degenerate sets (three lower, two higher), separated by ΔE. ✓
  • An electron is promoted from a lower to a higher d orbital by absorbing visible light of energy ΔE (red-orange light). ✓
  • The remaining (transmitted) light, the complementary colour, is seen: blue. ✓
02
Calculation
[3 marks]

[Ti(H2O)6]3+ absorbs light of wavelength 500 nm. Calculate ΔE in kJ mol−1. (h = 6.63 × 10−34 J s, c = 3.00 × 108 m s−1, L = 6.02 × 1023 mol−1)

  • ΔE = hc/λ = 6.63 × 10−34 × 3.00 × 108 ÷ 5.00 × 10−7 = 3.98 × 10−19 J. ✓
  • Per mole: 3.98 × 10−19 × 6.02 × 1023 = 2.39 × 105 J mol−1. ✓
  • ΔE = 239 kJ mol−1. ✓
03
Recall
[2 marks]

Draw energy level diagrams to show how the 3d orbitals split in (a) an octahedral and (b) a tetrahedral complex.

  • (a) Octahedral: three orbitals at lower energy, two at higher energy, gap ΔE. ✓
  • (b) Tetrahedral: two lower, three higher, with a smaller ΔE. ✓
04
Explain
[2 marks]

Explain why copper(I) compounds such as CuCl are usually white, but copper(II) compounds are coloured.

  • Cu+ is 3d10: the upper d orbitals are full, so no d-d electron transition can occur and no visible light is absorbed. ✓
  • Cu2+ is 3d9, so an electron can be promoted across ΔE, absorbing visible light. ✓
05
Analysis
[3 marks]

Adding concentrated HCl to pink [Co(H2O)6]2+ gives blue [CoCl4]2−. Give two reasons why ΔE changes, and say which colour of light each complex mainly absorbs.

  • The ligand changes from H2O to Cl−, which splits the d orbitals differently. ✓
  • The shape changes from octahedral to tetrahedral, which gives a smaller, reversed splitting. ✓
  • Pink [Co(H2O)6]2+ absorbs mainly green light; blue [CoCl4]2− absorbs mainly orange-red light. ✓
06
Explain
[2 marks]

Explain why [Fe(H2O)6]2+ and [Fe(H2O)6]3+ have different colours although they have the same ligands and shape.

  • Fe3+ has a higher charge and different d electron configuration (3d5) from Fe2+ (3d6). ✓
  • The ligands are held closer and the d orbitals split by a different ΔE, so a different frequency of light is absorbed: pale green versus yellow-brown. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.