5.3.4 Case Study: Ozone

• 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