Unit 5 Section 3 Formation of Coloured Ions

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Subshell Energy Levels and the Formation of Colour

  • When Ligands come along and bond to ions, some of the orbitals are given more energy than others and this splits the 3d orbital into two different energy levels.
  • Electrons tend to occupy the lower orbitals (the ground state). To jump up to the higher orbitals (excited states) they need to absorb energy equal to the energy gap/change, /E.
  • They absorb this energy from visible light that hits the transition metal ion but the amount of frequency of light absorbed depends on the size of the energy gap.
  • The frequencies of light not absorbed are reflected.These reflected frequencies combine to make the complement of the colour of the absorbed frequencies - this is the colour you see.

The energy absorbed when electrons jump up (excite) can be calculated using:

/E = h x v

  • /E = energy absorbed
  • h = planck's constant (6.63 x 10^-34 js^-1)
  • v = frequency of light absorbed (hertz/Hz)
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Identifying Transition Metal Ions

The Colour of a complex can be altered by any of the factors that can affect the size of the energy gap.

1. Changes in oxidation state:

  • If the oxidation state of a transition metal in a complex ion changes, then the colour of the complex ion may also change.
  • i.e. Pale green [Fe(H2O)6]^2+(aq)  complex >>> Yellow [Fe(H2O)6]^3+(aq) complex

2. Changes in Coordination number:

  • Changes in coordination number may also result in a colour change.
  • This always involves a change of ligand too.
  • i.e. Blue [Cu(H2O)6]^2+(aq) + 4Cl- >>> Yellow [CuCl4]^2-(aq) + 6H2O(aq)

3. Changes in ligand:

  • Changing the ligand can cause a colour change even if the oxidation state and coordination number remain the same.
  • i.e. Pink [Co(H2O)6]^2+(aq) + 6NH3(aq) >>> Straw/Pale Brown [Co(NH3)6]^2+(aq) + 6H2O(aq)
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  • Spectrometry can used to determine the concentration of a solution by measuring how much light it absorbs.
  • White light is shone through a filter, which is chosen to only let the colour of light through that is absorbed by the sample.
  • The light then passes through the sample to a colorimeter, which shows how much light was absorbed by the sample.
  • The more concentrated a coloured solution is, the more light it will absorb.
  • This measurement can be used to work out the concentration of a solution of transition metal ions.
  • Before you can find the unknown concentration of a sample, you have to produce a calibration graph.
  • This involves measuring the absorbances of known concentrations of solutions and plotting the results on a graph.
  • Once you have done this, you can measure the absorbance of your sample and read its concentration off of the graph.

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Spectrometry - Diagram


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