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

1. 1
   A teacher compares two aqueous solutions of the same concentration. The solution containing Cu²⁺(aq) ions is blue, whereas the solution containing Sc³⁺(aq) ions is colourless.

   (a)

   Which statement describes the effect of ligands on the 3d orbitals of a transition metal ion?

   [1 mark]

   • AAll five 3d orbitals are lowered equally in energy
   • BThe 3d orbitals are lowered to the same energy as the 4s orbital
   • CThe ligands split the five 3d orbitals into two groups at different energies
   • DThe ligands remove all electrons from the 3d orbitals

   (b)

   Which statement explains why Sc³⁺(aq) is colourless?

   [1 mark]

   • ASc³⁺ has a full 3d sub-shell
   • BSc³⁺ has no electrons in the 3d sub-shell, so none can be promoted
   • CWater ligands cannot bond to Sc³⁺
   • DSc³⁺ absorbs all wavelengths of visible light

   (c)

   Explain why the Cu²⁺(aq) solution appears blue.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Aqueous copper(II) ions form the pale blue complex [Cu(H₂O)₆]²⁺. Adding excess aqueous ammonia gives the deeper blue complex [Cu(NH₃)₄(H₂O)₂]²⁺, and adding concentrated hydrochloric acid gives the yellow-green complex [CuCl₄]²⁻.

   (a)

   Which changes cause the colour change when [Cu(H₂O)₆]²⁺ becomes [CuCl₄]²⁻?

   [1 mark]

   • AA change of ligand and a change of coordination number
   • BA change of oxidation number only
   • CA change in the number of 3d electrons
   • DA change from a dative bond to an ionic bond

   (b)

   Complex X has a larger d-orbital splitting than complex Y. Compared with Y, the light absorbed by X has

   [1 mark]

   • Aa lower frequency
   • Ba longer wavelength
   • Cthe same frequency, because both contain copper
   • Da higher frequency

   (c)

   Explain why replacing water ligands by ammonia ligands changes the shade of blue.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Complexes of copper(I), such as [CuCl₂]⁻, are colourless, whereas complexes of copper(II), such as [Cu(H₂O)₆]²⁺, are blue. Cobalt(II) forms the pink complex [Co(H₂O)₆]²⁺ and the blue complex [CoCl₄]²⁻.

   (a)

   Explain, with reference to electronic configurations, why copper(II) complexes are coloured but copper(I) complexes are colourless.

   [3 marks]

   (b)

   Write an equation for the formation of [CoCl₄]²⁻ from [Co(H₂O)₆]²⁺ and explain why the colour changes.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student records the colours of some aqueous ions and complexes: Fe²⁺(aq) is pale green, Fe³⁺(aq) is yellow, Ni²⁺(aq) is green, [Ni(NH₃)₆]²⁺ is blue, [Cu(H₂O)₆]²⁺ is blue and [CuCl₄]²⁻ is yellow-green. She is then given solutions containing the ions Ti⁴⁺, V³⁺, Cr³⁺, Mn²⁺, Co²⁺ and Zn²⁺.

   (a)

   Explain how the colours in the data show that colour changes can arise from a change in oxidation number, a change in ligand and a change in coordination number.

   [6 marks]

   (b)

   Predict, with reasons, which of the ions Ti⁴⁺, V³⁺, Cr³⁺, Mn²⁺, Co²⁺ and Zn²⁺ form coloured aqueous solutions.

   [6 marks]

   Total for question 4: 12 marks

End of questions