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