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Chemistry of Colour. Chemical Ideas 6.9. 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.

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chemistry of colour

Chemistry of Colour

Chemical Ideas 6.9

recapping from earlier
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
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.
coloured inorganic compounds
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.
  • E.g. V(+5)  V(+4)  V (+3)  V (+2)

Yellow Blue Green Violet

slide5
For most d-block transition metals, the size of ΔE is such that the light absorbed falls in the visible part of the spectrum.
  • The colour we see is white light minus the frequencies of absorbed light.
coloured inorganic compounds1
Coloured inorganic compounds
  • Changing a ligand in a complex can change the colour - different ligands have different splitting powers.

[Ni(H2O)6]2+(aq) + 6NH3(aq) [Ni(NH3)6]2+(aq) + 6H2O

light green lilac/blue

  • Electrons can move from the ground state in one atom to the excited state in another adjacent atom - electron transfer. Often bright colours, examples include Chrome yellow and Prussian blue.
coloured organic compounds
Coloured organic compounds
  • Often contain unsaturated groups, -C=O, -C=C, -N=N-
  • 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
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.
practise time
Practise Time
  • Now complete CI 6.9 Problems 1 and 2 to check your understanding