Colour in transition metal complexesEdexcel A-Level Chemistry: Subtopic test

1. 1
   A solution containing the complex ion [Ti(H₂O)₆]³⁺ is purple. It absorbs visible light most strongly at a wavelength of 500 nm. Planck constant, h = 6.63 × 10⁻³⁴ J s; speed of light, c = 3.00 × 10⁸ m s⁻¹; Avogadro constant, L = 6.02 × 10²³ mol⁻¹.

   (a)

   What is the electron configuration of the Ti³⁺ ion in this complex?

   [1 mark]

   • A[Ar] 4s¹
   • B[Ar] 3d¹
   • C[Ar] 3d²
   • D[Ar] 3d³

   (b)

   What is the energy gap, ΔE, between the d orbitals that corresponds to absorption at 500 nm, in J per ion?

   [1 mark]

   • A3.98 × 10⁻²⁸ J
   • B1.99 × 10⁻²⁵ J
   • C2.40 × 10⁵ J
   • D3.98 × 10⁻¹⁹ J

   (c)

   Explain why the solution appears purple.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student compares the aqueous ions of several d-block elements. The ions are Sc³⁺, Ti³⁺, Fe²⁺, Cu²⁺ and Zn²⁺, each in the form of a hexaaqua complex.

   (a)

   Which of these aqueous ions is colourless?

   [1 mark]

   • ASc³⁺
   • BTi³⁺
   • CFe²⁺
   • DCu²⁺

   (b)

   Which statement correctly explains why the aqueous Zn²⁺ ion is colourless?

   [1 mark]

   • AIt has no d electrons
   • BThe ligands do not split the d orbitals of Zn²⁺
   • CIts 3d subshell is full, so no electron can be promoted to a higher d orbital
   • DIt absorbs all wavelengths of visible light

   (c)

   Explain why the aqueous Sc³⁺ ion is colourless.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student adds reagents to a pale blue solution of copper(II) sulfate, which contains [Cu(H₂O)₆]²⁺. Adding an excess of aqueous ammonia gives a deep blue solution containing [Cu(NH₃)₄(H₂O)₂]²⁺. Adding concentrated hydrochloric acid to a fresh sample gives a yellow-green solution containing [CuCl₄]²⁻.

   (a)

   Explain why [Cu(NH₃)₄(H₂O)₂]²⁺ is a different colour from [Cu(H₂O)₆]²⁺, even though copper is in the +2 oxidation state in both.

   [3 marks]

   (b)

   State the coordination number and shape of [Cu(H₂O)₆]²⁺ and of [CuCl₄]²⁻, and explain why the colour changes when chloride ligands replace water.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A solution of [Co(H₂O)₆]²⁺ is pink and absorbs light most strongly at a wavelength of 510 nm. Adding concentrated hydrochloric acid turns the solution blue, because [CoCl₄]²⁻ forms. A solution of zinc(II) sulfate under the same conditions is colourless. Planck constant, h = 6.63 × 10⁻³⁴ J s; speed of light, c = 3.00 × 10⁸ m s⁻¹.

   (a)

   Explain, in terms of d orbitals, why the solution of [Co(H₂O)₆]²⁺ is coloured and appears pink, and calculate the energy gap ΔE, in J per ion, between the d orbitals involved.

   [6 marks]

   (b)

   Explain the colour of [CoCl₄]²⁻ compared with [Co(H₂O)₆]²⁺, and explain why the zinc(II) solution is colourless.

   [6 marks]

   Total for question 4: 12 marks

End of questions