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• why are CFCs unreactive in the troposphere

  • insufficient energy of radiation to break C-Cl bond
• how does chlorine catalyse depletion of the ozone

  1. Cl radical + O3 --> O2 + ClO radical
  2. ClO radical + O radical --> O2 + Cl radical
• bond enthalpy = energy (J) x avogadro's number
• energy (J) = frequency x planck's constant
• wavelength (m) = speed of light / frequency
• removing colour from molecules

  • removing electrons from structure, breaking double bonds
  • disrupting conjugated structure of pi bonds (the repeating single-double bond pattern)
  • means pigment can no longer absorb wavelengths (visible radiation) so does not have a colour
• order of decreasing energy

  • uv or visible
  • infrared
  • microwave
  • radio
• infrared effect on bond

  • bond vibrates more energetically
  • Increases vibrational energy level
• microwave effect on bond

  • bond rotates more energetically
  • increase in rotational energy
• radio effect on bonds

  • molecules move more energetically
  • increase in translational energy level
• effect of uv or visible on bonds

  • ionisation (most energy)
  • homolytic fission / photodissociation
  • emission of a photon (least energy)
• homolytic fission is more common in non-polar solvents and gas phase

  • forms radicals
• radical: molecule with an unpaired electron
• free radical substitution: replacement of one atom by another using radicals
• free radical substitution process

  1. initiation - homolytic fission + photodissociation, molecules to radicals
  2. propagation - multistep, chain reaction, radical to radical
  3. termination - radical to molecule
• formation of ozone

  1. O2 -- uv --> 2 O radicals
  2. O radical + O --> O3
• depletion of ozone

  1. O3 -- uv --> O2 + O
  2. O3 + O --> 2 O2
• radicals that catalyse the depletion of ozone

  • Cl radicals from CFCs
  • NO radicals from aircraft
  • OH radicals from water
• conjugated systems increase the chromophore --> smaller energy between pi electrons, meaning smaller gap, less energy (visible light) needed to promote electrons
• conjugated system: system of repeating double-single bonds