• A study of?bond enthalpy?can explain why ozone and oxygen in the atmosphere play very different roles in the flow of energy
• These processes have a profound effect on the amount of solar radiation reaching ground level
• The structure of oxygen and ozone molecules influences the amount of energy needed to break their bonds:

The structure of oxygen and ozone

• The double bond in oxygen is stronger than the?delocalised π bonds?in ozone
  • We say the?bond order?of oxygen is?2?and the?bond order?of ozone is?1.5
  • Both bonds are broken by ultraviolet radiation but the bond in oxygen requires radiation of higher energy and shorter wavelength than the bond in ozone
• High energy UV radiation in the stratosphere breaks the oxygen-oxygen double bond creating oxygen atoms

O₂?(g)??→?O??(g)?+ ?O??(g)????????????H +ve, UV light, λ < 242 nm

• These oxygen atoms have unpaired electrons- they are known as?free radicals
• The?free radicals?are highly reactive and quickly attack oxygen molecules forming ozone in an?exothermic?reaction, which raises the temperature of the stratosphere

OZONE FORMATION?? ? ? ? ? ? ? ? O??(g)?+ ?O₂?(g)??→?O₃?(g)??????????????H - ve

• Ozone requires less energy to break than oxygen
• It produces an oxygen molecule and an oxygen free radical:

OZONE DEPLETION? ? ? ? ? ? ? ? ?O₃?(g)???→???O??(g)?+ ?O₂?(g)? ???????????H +ve, UV light,?λ< 330 nm

• The radical reacts with another ozone molecule making two molecules of oxygen in an?exothermic?reaction

OZONE DEPLETION? ? ? ? ? ? ? ? ?O₃?(g)??+?O??(g)?→???2O₂?(g)? ???????????H - ve

• The temperature in the stratosphere is maintained by the balance of ozone formation and ozone depletion in a process known as the Chapman Cycle
• It is not a closed system as matter and energy flow in and out, but it is what is called a?steady state

The Chapman cycle

• Unfortunately, chemicals we have introduced into the atmosphere have interfered with this steady state resulting in ozone depleting at a faster rate than it is replaced
• Amongst these chemicals are?chlorofluorocarbons (CFCs)?found in refrigerants, propellants and solvents
• CFCs?are greatly damaging to stratospheric ozone and have been largely replaced by safer alternatives following the 1985 Montreal Protocol
• The depletion of ozone has allowed greater amounts of harmful?UV light?to reach the surface of the Earth
• UV light?has been linked to greater incidence of skin cancer and cataracts as well as the destruction of phytoplankton and reduced plant growth