Halogenoalkanes (AQA A-Level Chemistry): Revision Notes

3.3.3 Ozone Depletion

Ozone Depletion: Causes, Chemistry, and Solutions

Ozone ($O₃$) is a naturally occurring molecule in the stratosphere that absorbs harmful ultraviolet (UV) radiation from the Sun.

Human-made chemicals, particularly chlorofluorocarbons (CFCs), have caused significant damage to the ozone layer, resulting in what is commonly referred to as the ozone hole.

Formation of the Ozone Layer

Ozone is formed through reactions involving oxygen molecules ($O₂$) and UV radiation.

• Step 1: UV light breaks down oxygen molecules into oxygen atoms.

$$O_2 \xrightarrow{UV} 2O$$

• Step 2: Oxygen atoms react with $O₂$ molecules to form ozone ($O₃$).

$$O + O_2 \rightarrow O_3$$

Ozone absorbs UV light, maintaining a balance that protects Earth from excessive UV exposure.

Role of CFCs in Ozone Depletion

CFCs are stable compounds used in various products like refrigerants and aerosols.

• While stable in the lower atmosphere, they break down in the stratosphere due to UV radiation, releasing chlorine radicals ($Cl•$).
• These chlorine radicals are highly reactive and act as catalysts, causing the continuous breakdown of ozone. Key Reaction:

$$CFCl_3 \xrightarrow{UV} CFCl_2 + Cl•$$

Mechanism of Ozone Destruction

Chlorine radicals ($Cl•$) react with ozone in a destructive cycle:

• Step 1: Chlorine radicals break down ozone into oxygen and chlorine monoxide ($ClO•$).

$$Cl• + O_3 \rightarrow ClO• + O_2$$

• Step 2: Chlorine monoxide reacts with more ozone, releasing oxygen and regenerating chlorine radicals.

$$ClO• + O_3 \rightarrow 2O_2 + Cl•$$

This cycle allows a single chlorine radical to destroy thousands of ozone molecules, significantly thinning the ozone layer.

Environmental Impacts of Ozone Depletion

Increased UV-B radiation causes:

• Higher risks of skin cancer and cataracts.
• Damage to plant life and marine ecosystems, particularly phytoplankton.
• Faster degradation of materials like plastics.

International Response and Solutions

• The Montreal Protocol (1987) was a key international agreement to phase out CFCs and other ozone-depleting substances.
• Chlorine-free alternatives such as HFCs and HCFCs have been developed. HFCs do not harm the ozone layer, but are potent greenhouse gases.

Key Reactions for Ozone Depletion

The critical reactions involved in the destruction of ozone:

$$Cl• + O_3 \rightarrow ClO• + O_2$$ $$ClO• + O_3 \rightarrow 2O_2 + Cl•$$

These reactions show how chlorine atoms are regenerated and continue to destroy ozone molecules.

The Future of the Ozone Layer

• The ozone layer is slowly recovering due to the reduction of CFCs, but full recovery will take decades.
• Continued monitoring and research into safer alternatives are essential to prevent further ozone depletion.