6.9 Chemistry of Colour

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6.9 Chemistry of Colour
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Recapping from earlier

• Coloured substances absorb radiation in the
  visible region of the EM spectrum.
• Absorb energy - outermost electrons promoted to
  excited state.
• Same electrons involved in bonding or lone pairs.
• Innermost electrons more tightly held by
  attraction to the nucleus - more energy needed
  for excitation.

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Pulling theory together

• Some transitions needing less energy are brought
  about by visible light
• Excitation energy is greater when UV light is
  absorbed.
• Compounds absorbing UV radiation appear
  colourless.

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Coloured organic compounds

• Often contain unsaturated groups, -CO, -CC,
  -NN-
• Usually part of extended delocalised electron
  system called the chromophore.
• Electrons in double bonds more spread out -
  require less energy to excite than those in
  single bonds particularly in conjugated system.
• Absorption of radiation in visible region.

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Groups, energy and colour

• -OH, -NH2 , or NR2 attached to chromophores to
  enhance or modify the colours.
• Lone pair electrons become involved in the
  delocalised system.
• Small changes change the energy of light absorbed
  and therefore the colour.
• Dyes often different colours in acids and alkalis
  - useful indicators.
• Methyl orange bond to H at pH 3.5 and below -
  red, above pH3.5 there is no H bonded and the
  dye is yellow.

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Coloured inorganic compounds

• Often contain transition metals.
• Ligands cause 5 d orbitals to split into two
  levels
• Energy needed to excite to a higher level depends
  on the oxidation state of the metal and the type
  of ligand.
• Redox reactions often accompanied by colour
  changes.

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• Changing a ligand in a complex can change the
  colour - different ligands have different
  splitting powers.
• Electrons can move from the ground state in one
  atom to the excited state in another atom -
  electron transfer. Often bright colours, Chrome
  yellow and Prussian blue.