Ozone Depletion Chemistry

Ozone depletion is tested in the Cambridge 0620 environmental chemistry section. Candidates need to know what ozone is, how CFCs destroy it, the consequences of ozone depletion, and the alternatives that have replaced CFCs.

What is ozone?

Ozone (O₃) is a molecule made of three oxygen atoms. It is found naturally in the stratosphere (15-30 km above the Earth’s surface), where it forms the ozone layer.

How the ozone layer forms

Ozone is continuously created and destroyed by UV radiation:

Formation: 3O₂ → 2O₃ (UV radiation splits O₂; the free O atoms combine with other O₂ molecules)

Breakdown: 2O₃ → 3O₂ (UV radiation also breaks ozone apart)

Under natural conditions, these two processes are in balance, maintaining a steady ozone concentration.

The role of the ozone layer

The ozone layer absorbs most of the Sun’s harmful ultraviolet (UV-B and UV-C) radiation, preventing it from reaching the Earth’s surface. Without this protection, life on land would be exposed to damaging levels of UV radiation.

CFCs and ozone depletion

What are CFCs?

CFCs (chlorofluorocarbons) are synthetic compounds containing carbon, chlorine, and fluorine. They were widely used as:

• Refrigerants (in fridges and air conditioners)
• Propellants (in aerosol spray cans)
• Solvents (for cleaning electronics)
• Blowing agents (for making expanded polystyrene foam)

CFCs were popular because they are non-toxic, non-flammable, and chemically unreactive at ground level.

How CFCs destroy ozone

1. CFCs are very stable and do not break down in the lower atmosphere
2. They slowly drift up to the stratosphere over several years
3. In the stratosphere, intense UV radiation breaks the C-Cl bond, releasing a free chlorine atom (Cl radical)

CF₂Cl₂ → CF₂Cl + Cl (UV breaks the C-Cl bond)

4. The chlorine atom reacts with ozone:

Cl + O₃ → ClO + O₂

5. The ClO then reacts with another oxygen atom:

ClO + O → Cl + O₂

6. The chlorine atom is regenerated — it is not used up. It acts as a catalyst for ozone destruction. A single chlorine atom can destroy thousands of ozone molecules before it is eventually removed from the stratosphere.

Overall equation: 2O₃ → 3O₂ (catalysed by Cl atoms from CFCs)

Consequences of ozone depletion

Effect	Explanation
Increased skin cancer rates	More UV-B radiation reaches the surface, damaging DNA in skin cells
Increased eye cataracts	UV radiation damages the lens of the eye
Damage to crops	UV inhibits photosynthesis and damages plant tissues
Harm to marine plankton	Plankton near the ocean surface are killed by UV; this disrupts the marine food chain
Damage to materials	UV degrades plastics, paints, and building materials faster

The ozone hole

The most severe ozone depletion occurs over Antarctica each spring (September-October), creating the “ozone hole.” Extreme cold in the polar stratosphere accelerates the chemical reactions that destroy ozone.

Solutions — the Montreal Protocol

In 1987, the Montreal Protocol was signed — an international agreement to phase out the production and use of CFCs. It is one of the most successful environmental treaties:

• CFC production has been reduced by over 99%
• The ozone layer is slowly recovering
• Full recovery is expected by around 2060-2070

Alternatives to CFCs

Alternative	Use	Advantage	Limitation
HFCs (hydrofluorocarbons)	Refrigerants	Do not contain chlorine; do not damage ozone	Some are potent greenhouse gases
HCFCs (hydrochlorofluorocarbons)	Transitional refrigerants	Break down faster in the atmosphere	Still contain chlorine (being phased out)
Hydrocarbons (propane, butane)	Refrigerants, aerosols	No ozone damage, low greenhouse effect	Flammable
CO₂	Some refrigeration	No ozone damage	Greenhouse gas

Ozone depletion vs the greenhouse effect

Students often confuse these two issues:

Feature	Ozone depletion	Greenhouse effect
Where	Stratosphere	Troposphere
Problem	UV radiation reaching Earth	Heat trapped in the atmosphere
Caused by	CFCs (and related compounds)	CO₂, CH₄, and other greenhouse gases
Solution	Montreal Protocol (phase out CFCs)	Reduce fossil fuel use
Effect	Skin cancer, crop damage	Global warming, sea level rise

Worked exam question

Explain how chlorofluorocarbons (CFCs) cause the depletion of the ozone layer. [3]

CFCs are released into the atmosphere and drift up to the stratosphere [1]. UV radiation breaks the C-Cl bond, releasing chlorine atoms/radicals [1]. The chlorine atoms react with ozone molecules (Cl + O₃ → ClO + O₂), destroying them, and the chlorine is regenerated to destroy more ozone — acting as a catalyst [1].

Common exam mistakes

1. Saying CFCs “burn a hole in the ozone layer” — the ozone is destroyed by catalytic chemical reactions, not burning.
2. Confusing ozone depletion with the greenhouse effect — they are separate environmental problems caused by different chemicals.
3. Forgetting that chlorine acts as a catalyst — the key point is that one Cl atom destroys many O₃ molecules because it is regenerated.
4. Saying “ozone causes global warming” — ozone depletion allows UV through; global warming is caused by greenhouse gases trapping heat.