Ozone Depletion
Lajoy Tucker
Teacher
Contents
Introduction & Definitions
Ozone in the upper atmosphere absorbs harmful ultraviolet (UV) radiation, protecting life on Earth.
Ozone is formed and broken down naturally in a reversible cycle:
The presence of chlorofluorocarbons (CFCs) in the stratosphere disrupts this balance, leading to ozone depletion.
Chlorofluorocarbons are organic compounds containing carbon, chlorine, and fluorine historically used as refrigerants and in aerosols.
Role of CFCs in Ozone Depletion
CFCs are unreactive in the lower atmosphere but break down in the upper atmosphere under UV light:
Example:
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A bond undergoes homolytic fission resulting in the formation of two radicals (the covalent bond breaks evenly giving one electron to each to the bonded atoms)
bonds are not broken as they are stronger than bonds
The chlorine radical produced initiates a chain reaction that destroys ozone molecules.
Catalytic Cycle of Ozone Depletion
Chlorine radicals catalyse the breakdown of ozone via a chain reaction:
Step 1:
Step 2:
Overall Reaction:
The chlorine radical is regenerated, allowing one to destroy many ozone molecules.
Environmental Impact and Legislation
The discovery that CFCs deplete ozone led to global concern and international agreements, such as the Montreal Protocol (1987), to phase out CFCs.
Chemists contributed by:
Providing evidence from atmospheric studies.
Developing alternatives to CFCs, such as hydrofluorocarbons , which do not release radicals.
Summary of Key Reactions and Equations
Stage | Reaction | Description |
|---|---|---|
Initiation | UV light breaks bond producing two radicals | |
Propagation 1 | Chlorine radical attacks ozone | |
Propagation 2 | Chlorine oxide radical reacts again | |
Overall | Net loss of ozone molecules |
Practice Questions
Question 1
Write two equations to show how chlorine radicals catalyse the decomposition of ozone
Answer
Question 2
Explain why a single CFC molecule can destroy many ozone molecules
Answer
The chlorine radical () is regenerated during the catalytic cycle, so it can participate in many cycles of ozone destruction.
Give an equation to show the photodissociation of
Answer
A bond is broken by homolytic fission. The bond is stronger and therefore not broken.
Tips
Always show radicals clearly using a dot .
Use UV above the arrow when showing photodissociation.
Don't confuse CFCs with greenhouse gases — this topic focuses on ozone depletion, not climate change.
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