3.2.5.4 Formation of coloured ions

• • Transition metal ions can be identified by their colour.
  • Colour arises when some of the wavelengths of visible light are absorbed
  • and the remaining wavelengths of light are transmitted or reflected.
• d electrons move from the ground state to an excited state when light is absorbed.
• The energy difference between the ground state and the excited state of the d electrons is given by:

  ∆E = hν = hc/λ
• Changes in oxidation state, co-ordination number and ligand alter ∆E and this leads to a change in colour
• • The absorption of visible light is used in spectroscopy.
  • A simple colorimeter can be used to determine the concentration of coloured ions in solution.
• A simple colorimeter can be used to determine the concentration of coloured ions in solution.

  Suggest why it is important that the container for each sample has the same dimensions. [1 mark]

  • Absorption depends on proportional to path length / distance travelled through solution
• Suggest why the coloured filter is used in a colorimter [1 mark]

  • To select the colour / frequency / wavelength that is (most strongly) absorbed (by the sample)
  • ACCEPT the filter is chosen to complement the colour of the solution
• Suggest one reason why a colorimetric method might be chosen in preference to titration [1 mark]

  • Quicker to analyse extracted samples than by titration / uses smaller volumes of solution
• By considering the properties of the reactants and products, state why it is possible to use a spectrometer to measure the concentration of the manganate(VII) ions in this reaction mixture.

  • MANGANATE(VII) ions are COLOURED! (purple)
  • All other reactants and products are NOT coloured (or too faintly coloured to detect)