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6.9 Chemistry of Colour


Innermost electrons more tightly held by attraction to the nucleus - more energy ... Often bright colours, Chrome yellow and Prussian blue. ... – PowerPoint PPT presentation

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Title: 6.9 Chemistry of Colour

6.9 Chemistry of Colour
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.

Pulling theory together
  • Some transitions needing less energy are brought
    about by visible light
  • Excitation energy is greater when UV light is
  • Compounds absorbing UV radiation appear

Coloured organic compounds
  • Often contain unsaturated groups, -CO, -CC,
  • 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.

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.

Coloured inorganic compounds
  • Often contain transition metals.
  • Ligands cause 5 d orbitals to split into two
  • 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

  • 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.
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