A-Level Chemistry May/June 2025 Q6(a)(ii): Explain why a solution containing complex ion A and a solution containing complex ion B…

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  The colour of transition metal complexes is due to the splitting of the d-orbitals into two different energy levels by the ligands.

  For complex ion A and complex ion B, the degree of d-orbital splitting is different. This results in a different energy gap, $ΔE$, between the lower and higher energy d-orbitals for each complex.

  Consequently, the two complexes absorb different frequencies (or wavelengths) of visible light, as the energy of the absorbed photon must be equal to $ΔE$. The colour observed is the complementary colour of the light absorbed, so because different frequencies are absorbed, the solutions appear as different colours.

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  How the marks are awarded

  • M1 — Stating that the energy gap between the split d-orbitals, $ΔE$, is different for the two complex ions.
  • M2 — Linking the different energy gap to the absorption of different frequencies or wavelengths of visible light.

  Common mistakes

  • Simply stating 'they absorb different colours' which is too vague; the answer must refer to frequency or wavelength of light.
  • Forgetting to mention the difference in the d-orbital splitting energy ($ΔE$) as the fundamental reason for the colour difference.
  • Confusing the absorbed colour with the observed (transmitted) colour.
  • Incorrectly describing the process as being due to the emission of light from the complex.

  Examiner tip: Always connect the observed colour of a transition metal complex to the specific frequency of light absorbed, which in turn is determined by the magnitude of the d-orbital splitting energy, $ΔE$.

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