Colour and Magnetic Properties from CFT - UNSOLVED PRACTICE SET
Chapter: Coordination Compounds | Topic: Colour and Magnetic Properties from CFT
COLOUR AND MAGNETIC PROPERTIES FROM CFT - UNSOLVED PRACTICE SET
Topic: Colour and Magnetic Properties from CFT
Multiple Choice Questions
Q1. The colour of a coordination compound is due to:
- Reflection of light
- d-d transitions
- Nuclear transitions
- Radioactive decay
Q2. A complex that absorbs light in the red region of the spectrum will appear:
- Red
- Green
- Blue
- Yellow
Q3. The magnetic moment of a complex depends on:
- The atomic number of the metal
- The number of unpaired electrons
- The charge on the complex
- The geometry only
Q4. A d⁰ configuration in an octahedral complex will be:
- Coloured and paramagnetic
- Colourless and diamagnetic
- Coloured and diamagnetic
- Colourless and paramagnetic
Q5. The spin-only magnetic moment formula is:
- μ = √[n(n+1)]
- μ = √[n(n+2)]
- μ = n(n+2)
- μ = n(n+1)
Q6. [Cu(H₂O)₆]²⁺ is blue in colour because:
- It absorbs blue light
- It absorbs orange-red light and transmits blue
- It reflects all light except blue
- It emits blue light
Short Answer Questions
Q7. Explain why [Ti(H₂O)₆]³⁺ is purple in colour while [Sc(H₂O)₆]³⁺ is colourless.
Q8. Calculate the spin-only magnetic moment for a complex with 3 unpaired electrons.
Q9. Why is [Co(CN)₆]³⁻ colourless while [CoF₆]³⁻ is coloured? Explain using CFT.
Q10. A complex has a magnetic moment of 1.73 BM. How many unpaired electrons does it contain?
Q11. Explain why KMnO₄ is intensely coloured even though Mn⁷⁺ has a d⁰ configuration.
Q12. [Ni(H₂O)₆]²⁺ is green and paramagnetic, while [Ni(CN)₄]²⁻ is colourless and diamagnetic. Explain the difference.
Long Answer Questions
Q13. Explain how Crystal Field Theory accounts for the colour of coordination compounds. Discuss:
(a) The origin of colour in d-d transitions
(b) Why some complexes are colourless
(c) The relationship between Δ₀ and the wavelength of absorbed light
(d) Why KMnO₄ and K₂Cr₂O₇ are coloured despite having d⁰ and d⁰ configurations respectively
Q14. (a) Explain the relationship between the number of unpaired electrons and the magnetic properties of coordination compounds.
(b) Calculate the spin-only magnetic moment for the following:
(i) [Fe(CN)₆]⁴⁻ (low spin)
(ii) [FeF₆]³⁻ (high spin)
(iii) [Co(NH₃)₆]³⁺ (low spin)
(iv) [NiCl₄]²⁻ (tetrahedral)
Q15. (a) What is meant by crystal field stabilization energy (CFSE)? How is it calculated for octahedral complexes?
(b) Calculate CFSE for the following octahedral complexes:
(i) [Ti(H₂O)₆]³⁺
(ii) [Fe(CN)₆]⁴⁻ (low spin)
(iii) [CoF₆]³⁻ (high spin)
(iv) [Ni(NH₃)₆]²⁺
Numerical / Application-Based Problems
Q16. A coordination compound of Cr³⁺ shows an absorption maximum at 500 nm.
(a) Calculate the crystal field splitting energy (Δ₀) in Joules per photon.
(b) Convert this energy to cm⁻¹ (wavenumbers).
(c) Predict the colour of the complex. (Use the colour wheel: absorbed colour → observed complementary colour.)
(d) If the same metal is complexed with a stronger field ligand, how would the absorption maximum shift? Explain.
(e) Is this complex paramagnetic or diamagnetic? Calculate its spin-only magnetic moment.
Q17. The following table gives data for some coordination compounds:
| Complex | Unpaired Electrons | Colour | Geometry |
|---|---|---|---|
| [Co(NH₃)₆]³⁺ | 0 | Yellow | Octahedral |
| [CoF₆]³⁻ | 4 | Green | Octahedral |
| [Ni(CN)₄]²⁻ | 0 | Colourless | Square planar |
| [Cu(H₂O)₆]²⁺ | 1 | Blue | Octahedral |
(a) Calculate the spin-only magnetic moment for each complex.
(b) Explain why [Co(NH₃)₆]³⁺ is yellow while [CoF₆]³⁻ is green.
(c) Why is [Ni(CN)₄]²⁻ colourless despite having d⁸ configuration?
(d) [Cu(H₂O)₆]²⁺ has one unpaired electron but appears blue. Explain the origin of its colour.
Q18. In your school science fair, you set up an exhibit on "The Chemistry of Gemstones." You display models of:
Ruby: Contains Cr³⁺ in Al₂O₃ matrix (red)
Emerald: Contains Cr³⁺ in Be₃Al₂Si₆O₁₈ matrix (green)
Sapphire: Contains Fe²⁺/Ti⁴⁺ in Al₂O₃ (blue)
(a) Both ruby and emerald contain Cr³⁺, yet they have different colours. Explain this using CFT. (Hint: Think about the crystal field created by the surrounding matrix.)
(b) Calculate the spin-only magnetic moment of Cr³⁺ in these gemstones.
(c) Why is pure Al₂O₃ (corundum) colourless, but ruby (Cr³⁺-doped) is red?
(d) The absorption spectrum of ruby shows peaks at 400 nm and 550 nm. Which colours are absorbed, and why does ruby appear red?
(e) How does this exhibit demonstrate that the colour of a coordination compound depends not just on the metal ion, but also on its environment?