3.2.5 Transition metals (A-level only)

• • The 3d block contains 10 elements, all of which are metals. Unlike the metals in Groups 1 and 2, the transition metals Ti to Cu form coloured compounds and compounds where the transition metal exists in different oxidation states. Some of these metals are familiar as catalysts. The properties of these elements are studied in this section with opportunities for a wide range of practical investigations.
• Transition metal characteristics of elements Ti–Cu arise from an incomplete d sub-level in atoms or ions.
• The characteristic properties include:

  • complex formation

  • formation of coloured ions

  • variable oxidation state

  • catalytic activity.
• A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons.
• A complex is a central metal atom or ion surrounded by ligands.
• Co-ordination number is number of co-ordinate bonds to the central metal atom or ion.
• The water supply near a vineyard can be tested for copper(II) ions by forming a blue aqueous complex with EDTA4– ions. The concentration of this complex can be determined using a colorimeter.

  Outline the practical steps that you would follow, using colorimetry, to determine the concentration of this complex in a sample of water.

  • Calibrate a colorimeter/produce a calibration curve.
  • By testing the colorimeter with solutions of copper-EDTA complex of known concentration.
  • Add excess EDTA salt to the sample.
• Explain what is meant by the term bidentate ligand

  • TWO ATOMS that each donate a lone pair (of electrons) / coordinate bonds from TWO ATOMS
• Explain why complexes formed from transition metal ions are coloured.

  •    M1 absorb some wavelengths / frequencies of visible light
  • M2     to promote/excite electrons in d-orbitals 1
  • Must have 'd'

  M3    remaining/complementary wavelengths/frequencies/colours/energies of visible light reflected/transmitted to give colour seen
• Describe how a calibration graph is produced and used to find the concentration of the iron(III) complex.

  • M1     MEASURE absorbance for (a range of) known concentrations
  • M2 plot graph absorbance v concentration
  • M3  read value of concentration for the measured absorbance from this graph