6 9chemistry of colour

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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 absorbed • Compounds absorbing UV radiation appear colourless 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 • -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 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 • 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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