Mechanism of Ozone Depletion Revision Notes | A Level AQA Chemistry

1. The Ozone Layer and Why Chlorine Matters

High in the stratosphere, a thin layer of ozone ($\text{O}_3$) absorbs ultraviolet radiation that would otherwise damage DNA and cause skin cancer. The discovery that human-made chemicals were punching a hole in this shield is one of the most important stories in environmental chemistry.

The culprits are chlorofluorocarbons — CFCs — once used in aerosol sprays and refrigerators. Their chemistry in the stratosphere is a masterclass in radical reactions and catalysis, and it sits squarely in the AQA A Level halogenoalkanes topic.

Here is the central idea: a single chlorine atom can destroy up to 100,000 ozone molecules before it is removed from the stratosphere. That is the power of a catalytic cycle, and understanding it at the mechanistic level is exactly what the specification requires.

2. From CFCs to Chlorine Radicals

CFCs are chemically inert at ground level because the C-F bond is very strong and the C-Cl bond does not encounter sufficient ultraviolet energy there to undergo homolytic fission.

At stratospheric altitudes, high-energy ultraviolet radiation is intense enough to break the C–Cl bond homolytically. Homolytic fission means the bonding electrons split equally between the two atoms, producing two radicals — each with one unpaired electron.

For a CFC such as $\text{CCl}_2\text{F}_2$, ultraviolet radiation causes the reaction: $\text{CCl}_2\text{F}_2 \xrightarrow{\text{UV}} \text{CClF}_2^{\bullet} + \text{Cl}^{\bullet}$. The chlorine radical produced is the species responsible for ozone destruction.

3. The Catalytic Cycle: Two Equations You Must Know

Once a chlorine radical is present in the stratosphere, it enters a two-step cycle that repeatedly destroys ozone. This is the core of the AQA specification requirement, and both equations must be known precisely.

In Step 1, the chlorine radical attacks ozone and removes one oxygen atom, forming the chlorine monoxide radical ($\text{ClO}^{\bullet}$). Ozone has been destroyed, but the chlorine has not been regenerated yet — it is temporarily tied up as $\text{ClO}^{\bullet}$.

In Step 2, the ClO• radical reacts with a second ozone molecule. The chlorine radical is regenerated and two oxygen molecules are formed in total, consistent with the equation ClO• + O3 -> Cl• + 2O2.

Adding the two steps together gives the overall equation: $2\text{O}_3 \rightarrow 3\text{O}_2$. Notice that chlorine does not appear in the overall equation — it is neither created nor destroyed overall. This is the defining feature of a catalyst.

Worked example: Verifying the Catalytic Cycle by Combining Both Steps

Show that combining the two steps of the catalytic cycle gives the overall equation $2\text{O}_3 \rightarrow 3\text{O}_2$, and confirm that $\text{Cl}^{\bullet}$ acts as a catalyst.

1. $\text{Cl}^{\bullet} + \text{O}_3 \rightarrow \text{ClO}^{\bullet} + \text{O}_2$ — Chlorine radical reacts with ozone to give ClO• and O2.
2. $\text{ClO}^{\bullet} + \text{O}_3 \rightarrow 2\text{O}_2 + \text{Cl}^{\bullet}$ — ClO• reacts with a second ozone molecule, regenerating Cl• and producing two O2 molecules.
3. $\text{Cl}^{\bullet} + \text{O}_3 + \text{ClO}^{\bullet} + \text{O}_3 \rightarrow \text{ClO}^{\bullet} + \text{O}_2 + 2\text{O}_2 + \text{Cl}^{\bullet}$ — Write all species on each side before cancelling.
4. $\text{Cl}^{\bullet}$ cancels; $\text{ClO}^{\bullet}$ cancels. Left: $2\text{O}_3$. Right: $\text{O}_2 + 2\text{O}_2 = 3\text{O}_2$ — Cl• and ClO• appear on both sides — they cancel completely.
5. $2\text{O}_3 \rightarrow 3\text{O}_2$ — Chlorine does not appear in the overall equation, confirming it is a catalyst — it is regenerated and not consumed overall.

Answer: $2\text{O}_3 \rightarrow 3\text{O}_2$. Cl• is a catalyst: it is consumed in Step 1 and regenerated in Step 2, so it does not appear in the overall equation.

4. Chlorine as a Catalyst: Conceptual Analysis

The word 'catalyst' has a precise meaning here. Chlorine atoms provide an alternative reaction pathway for the decomposition of ozone — one with a lower activation energy than the uncatalysed route. Because Cl• is regenerated at the end of each cycle, a single chlorine atom can destroy a vast number of ozone molecules.

Estimates suggest one chlorine radical can destroy up to 100,000 ozone molecules before it is eventually removed from the stratosphere by other reactions. This amplification effect is why even small quantities of CFCs released at ground level can cause disproportionately large damage to the ozone layer.

The reaction profile above illustrates the key point: the catalytic pathway (via Cl• and ClO•) has a lower activation energy than the direct decomposition of ozone. The overall enthalpy change is the same — the catalyst does not alter the thermodynamics, only the kinetics.

5. Equation Balancing and Atom Counting

AQA examiners regularly ask students to complete or balance the equations for the catalytic cycle. A systematic atom count is the safest approach — do not rely on memory alone when under exam pressure.

Worked example: Atom Count Check: Verifying Both Steps Are Balanced

Confirm that both steps of the catalytic cycle are correctly balanced by counting atoms and checking radical electrons.

1. Left: 1 Cl, 3 O. Right: 1 Cl (in ClO•), 1 O (in ClO•) + 2 O (in O2) = 1 Cl, 3 O — Atoms balance: 1 Cl and 3 O on each side. Both Cl• (left) and ClO• (right) carry one unpaired electron, so the radical count also balances.
2. Left: 1 Cl (in ClO•), 1 O (in ClO•) + 3 O (in O3) = 1 Cl, 4 O. Right: 1 Cl (in Cl•), 4 O (in 2O2) = 1 Cl, 4 O — Atoms balance: 1 Cl and 4 O on each side. The radical electron is carried by Cl• on the right, consistent with the ClO• on the left.
3. Step 1 + Step 2 → 2O3 → 3O2. Left: 2 × 3 = 6 O atoms. Right: 3 × 2 = 6 O atoms. Cl• and ClO• cancel from both sides. — 6 oxygen atoms on each side. Cl• and ClO• appear on both sides of the combined equation and cancel, confirming they are not consumed overall.

Answer: Both individual steps and the overall equation $2\text{O}_3 \rightarrow 3\text{O}_2$ are correctly balanced.

6. Environmental Significance and Wider Context

The ozone hole — most pronounced over Antarctica — is a direct consequence of this catalytic chemistry. Stratospheric conditions, particularly the very low temperatures over polar regions, promote the formation of polar stratospheric clouds. These cloud surfaces accelerate the release of reactive chlorine species from reservoir molecules, intensifying the catalytic destruction of ozone.

The Montreal Protocol (1987) phased out the production of CFCs internationally. Because CFCs persist in the atmosphere for decades, recovery of the ozone layer is slow — but measurable improvements have been observed since the late 1990s. This is a rare example of global environmental policy producing a scientifically verifiable outcome.

From a chemistry perspective, the ozone depletion story reinforces a broader principle about halogenoalkanes: the strength of the C–F bond makes fluorine substituents essentially inert, while the weaker C–Cl bond is the site of reactivity — both in nucleophilic substitution reactions at A Level and in the radical chemistry of the stratosphere.

7. Exam Technique and Common Mistakes

Questions on this topic appear in two main forms: writing or completing the equations for the catalytic cycle, and explaining why chlorine acts as a catalyst. Both require precise language and accurate equations — the mark scheme is unforgiving on missing radicals or unbalanced equations.