IB DP Chemistry: SL复习笔记5.3.4 Case Study: Ozone

Case study: Ozone & Bond Strength

  • 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