Free Science Lessons: Ozone Layer and Its Depletion

Overview

• Objective: Understand the role of the ozone layer and the impact of chlorofluorocarbons (CFCs) and nitrogen oxide radicals on its destruction.
• Relevant for: OCR and AQA specifications.

The Ozone Layer

• Location: Stratosphere, 20 to 40 km above Earth's surface.
• Composition: High concentration of ozone (O₃).
• Function: Absorbs ultraviolet (UV) radiation from the sun, protecting living organisms from UV damage.

Formation and Breakdown of Ozone

1. Formation:

   • UV radiation breaks the double bond in oxygen molecules (O₂) → Forms two oxygen radicals (O).
   • Oxygen radical reacts with O₂ → Forms ozone (O₃).

2. Breakdown:

   • Ozone absorbs UV light → Breaks down into an oxygen radical and an oxygen molecule.
   • Balance: Ozone formation and breakdown occur at the same rate, keeping the ozone amount constant.

Role of Chlorofluorocarbons (CFCs) in Ozone Depletion

• Properties: Haloalkanes, highly stable due to strong carbon-halogen bonds, non-toxic.
• Uses: Previously used in fridges, freezers.
• Impact:
  • CFCs reach the stratosphere → UV radiation breaks C-Cl bond (photodissociation).
  • Produces chlorine radicals (highly reactive).
  • Propagation Steps:
    • Step 1: Cl radical + O₃ → O₂ + ClO radical.
    • Step 2: ClO radical + O radical → Cl radical + O₂.
  • Cycle: Continues destructively, one CFC can destroy thousands of ozone molecules.

Impact of Nitrogen Oxide Radicals

• Sources: Formed by lightning and aircraft.
• Destructive Cycle:
  • NO radical + O₃ → NO₂ radical + O₂.
  • NO₂ radical + O radical → NO radical + O₂.
  • Similar cycle as with CFCs, leading to ozone depletion.

Conclusion

• Key Concept: Radicals such as chlorine and nitrogen oxides catalyze the breakdown of ozone, leading to depletion of the ozone layer.
• Scientific Response: Evidence led to the phasing out of CFCs and development of less harmful alternatives.