Magnetic Properties and Colour of Compounds - UNSOLVED PRACTICE SET

Q7. Explain why transition metal ions are coloured. What is the role of d-d transitions in producing colour?

Q8. Calculate the spin-only magnetic moment for Fe²⁺ (d⁶) in a high-spin octahedral complex.

Q9. Why is [Cu(H₂O)₆]²⁺ blue while [Cu(NH₃)₄(H₂O)₂]²⁺ is deep blue-violet? Explain the difference.

Q10. Zn²⁺ salts are colourless and diamagnetic. Explain why.

Q11. What is the difference between paramagnetism and diamagnetism? Give one example of each from transition metal compounds.

Q12. Why does KMnO₄ appear purple in colour despite Mn being in the +7 oxidation state (d⁰ configuration)?

Q13. (a) Explain why most transition metal compounds are coloured.

(b) What are d-d transitions? How do they give rise to colour?

(c) Why are compounds of Zn²⁺, Cd²⁺, and Cu⁺ generally colourless?

(d) Explain why the colour of a transition metal compound changes when the ligand is changed, even though the metal ion remains the same.

Q14. (a) Explain the origin of magnetic properties in transition metal compounds.

(b) Calculate the spin-only magnetic moment for the following ions:

(i) Ti³⁺

(ii) V³⁺

(iii) Cr³⁺

(iv) Mn²⁺ (high spin)

(v) Fe²⁺ (high spin)

(vi) Co²⁺ (high spin)

(c) A compound of iron has a magnetic moment of 5.92 BM. What is the oxidation state and electronic configuration of iron in this compound?

Q15. (a) What is meant by the spectrochemical series? How does it relate to the colour of coordination compounds?

(b) Explain why [Co(NH₃)₆]³⁺ is yellow while [CoF₆]³⁻ is green.

(c) The absorption maximum of [Ti(H₂O)₆]³⁺ is at 500 nm. Calculate the crystal field splitting energy in kJ/mol. (Given: h = 6.626 × 10⁻³⁴ J s, c = 3 × 10⁸ m/s, Nₐ = 6.022 × 10²³ mol⁻¹)

Q16. The following data give the magnetic moments of some transition metal ions:

(a) Verify the observed magnetic moments using the spin-only formula μ = √[n(n+2)] BM, where n = number of unpaired electrons.

(b) Plot a graph of the number of unpaired electrons vs. the observed magnetic moment.

(c) Why does Fe²⁺ show two different magnetic moments? What does this tell you about the ligand field?

(d) Calculate the number of unpaired electrons in a complex with observed μ = 4.90 BM. Identify the possible metal ion and its oxidation state.

(e) A compound has μ = 0 BM but contains a transition metal. Is this possible? Explain with

Q17. The following complexes show different colours:

(a) Calculate the crystal field splitting energy (Δ₀) for each complex in kJ/mol.

(b) Arrange these complexes in order of increasing crystal field splitting energy.

(c) Which complex absorbs light of the highest energy? Which absorbs light of the lowest energy?

(d) Predict the colour of [Cr(H₂O)₆]³⁺ if the ligand is changed from H₂O to NH₃. Will the absorption shift to a longer or shorter wavelength? Explain.

(e) A student claims that the colour of a complex is the same as the colour of light it absorbs. Is this correct? Explain using the concept of complementary colours.

Q18. In a school science exhibition, students demonstrate magnetic properties of transition metal compounds.

(a) A student brings a magnet near a sample of FeSO₄·7H₂O crystals. The crystals are attracted to the magnet. What property does this demonstrate? Calculate the spin-only magnetic moment of Fe²⁺ in this compound.

(b) Another student shows that CuSO₄·5H₂O crystals are also attracted to a magnet, but much more weakly. Explain why the attraction is weaker than for FeSO₄·7H₂O.

(c) A third student shows that ZnSO₄·7H₂O crystals are not attracted to the magnet at all. Explain this observation.

(d) The students prepare solutions of these compounds and test them with a magnet. They find that the solutions show no magnetic attraction. Why?

(e) The teacher asks: "If you had to design a magnetic storage device, would you use a compound with paired or unpaired electrons? Why?" Give your answer with reasoning.