Formation of coloured ionsAQA A-Level Chemistry: Subtopic test

1. 1
   Aqueous copper(II) sulfate is blue and aqueous zinc sulfate is colourless. The copper(II) ion has the electron configuration [Ar]3d⁹ and the zinc ion has the configuration [Ar]3d¹⁰.

   (a)

   Which statement explains why aqueous copper(II) sulfate appears blue?

   [1 mark]

   • AIt absorbs blue light and reflects all other wavelengths
   • BIt absorbs ultraviolet light and emits blue light
   • CIt absorbs some wavelengths of visible light and transmits the remaining wavelengths, which appear blue
   • DIt absorbs all wavelengths of visible light equally

   (b)

   Which change to an aqueous solution of [Cu(H₂O)₆]²⁺ would alter the colour by changing ΔE?

   [1 mark]

   • AReplacing some water ligands with ammonia ligands
   • BDiluting the solution with more water
   • CDoubling the volume of solution in the cuvette
   • DStoring the solution in the dark

   (c)

   Explain why aqueous copper(II) sulfate is coloured but aqueous zinc sulfate is colourless.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A solution of [Ti(H₂O)₆]³⁺ is purple. A student is investigating how the colour of a complex ion is related to the light it absorbs. Use h = 6.63 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹.

   (a)

   Which expression gives the energy difference ΔE for one ion absorbing light of wavelength λ?

   [1 mark]

   • AΔE = hλ / c
   • BΔE = λc / h
   • CΔE = h / (cλ)
   • DΔE = hc / λ

   (b)

   One complex absorbs light of wavelength 500 nm and another absorbs light of wavelength 650 nm. Which statement is correct?

   [1 mark]

   • AThe 650 nm complex has the larger ΔE because the wavelength is longer
   • BThe 500 nm complex has the larger ΔE because the frequency absorbed is higher
   • CBoth complexes have the same ΔE because both absorb visible light
   • DThe 500 nm complex has the smaller ΔE because it absorbs less light

   (c)

   The purple solution absorbs green light with a wavelength of 510 nm. Calculate the energy difference, ΔE, in J for one ion.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student uses a colorimeter to find the concentration of copper(II) ions in a waste solution. Standard solutions of copper(II) sulfate with concentrations of 0.020, 0.040, 0.060 and 0.080 mol dm⁻³ are each treated with excess ammonia solution, and their absorbances at the chosen wavelength are 0.14, 0.28, 0.42 and 0.56 respectively.

   (a)

   Explain why the student adds excess ammonia solution to each solution, and why a coloured filter is used in the colorimeter.

   [3 marks]

   (b)

   The student dilutes 10.0 cm³ of the waste solution to 50.0 cm³, adds excess ammonia solution and measures an absorbance of 0.35. Determine the concentration of copper(II) ions in the original waste solution.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The colours of transition metal solutions depend on the ions present. Aqueous cobalt(II) ions, [Co(H₂O)₆]²⁺, are pink, but adding concentrated hydrochloric acid gives blue [CoCl₄]²⁻. Aqueous iron(II) ions are pale green and aqueous iron(III) ions are yellow-brown. Pale blue [Cu(H₂O)₆]²⁺ absorbs light most strongly at a wavelength of 800 nm, and deep blue [Cu(NH₃)₄(H₂O)₂]²⁺ absorbs most strongly at 600 nm. Use h = 6.63 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹ and L = 6.02 × 10²³ mol⁻¹.

   (a)

   Explain why transition metal complex ions are coloured, and why the colour changes when the ligand, the co-ordination number or the oxidation state changes.

   [6 marks]

   (b)

   Calculate ΔE in kJ mol⁻¹ for each of the two copper(II) complexes and state how replacing water ligands by ammonia ligands changes ΔE.

   [6 marks]

   Total for question 4: 12 marks

End of questions