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A-Level Chemistry October/November 2024 Q4(b)(iii): Co2+ ions form a different complex ion, M. Each M ion contains two Co2+ ions, both of w…
A-Level Chemistry · Paper 9701/41 · October/November 2024 · Question 4(b)(iii) · [2 marks]
Co2+ ions form a different complex ion, M. Each M ion contains two Co2+ ions, both of which are octahedrally coordinated, but the ligands are different from the ligands in G. Explain why G and M have different colours.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
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The colour of a transition metal complex depends on the ligands, the central metal ion, and its oxidation state. In this case, the central Co²⁺ ions in complexes G and M have different ligands.
- Different ligands cause a different degree of splitting of the d-orbitals. This means the energy gap, ΔE, between the split d-orbitals is different for complex G and complex M.
- As a result, the energy required to promote an electron from a lower d-orbital to a higher d-orbital is different. Each complex will therefore absorb light of a different frequency (and wavelength) from the visible spectrum, as .
The colour we observe is the complementary colour to the light that is absorbed. Since G and M absorb different frequencies of light, they will transmit/reflect different colours, and thus appear as different colours.
How the marks are awarded
- B1 — Stating that the different ligands cause the d-orbital energy gap (ΔE) to be different in the two complexes.
- B1 — Linking the different energy gap to the absorption of a different frequency or wavelength of light from the visible spectrum.
Common mistakes
- Simply stating 'they have different ligands' without explaining the effect on d-orbital splitting and energy absorption.
- Confusing the absorbed colour with the observed colour, for example, saying 'they emit different colours' instead of explaining that the observed colour is the complement of the absorbed light.
- Forgetting to mention the d-orbital splitting or the energy gap (ΔE), which is the fundamental reason for the colour.
- Incorrectly stating that the oxidation state or coordination number of the cobalt ion has changed, when the question specifies both are Co²⁺ and octahedral.
Examiner tip: To explain differences in colour, always connect the four key points in order: different ligand/metal ion → different d-orbital splitting (ΔE) → different frequency of light absorbed → different complementary colour observed.
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