Ozone Depletion

Chemical Inertness: The strength of the C-Cl and C-F bonds makes CFCs extremely stable in the lower atmosphere, allowing them to persist for decades until they diffuse into the stratosphere.

Photodissociation: In the stratosphere, intense UV radiation provides enough energy to break the C-Cl bond (which is weaker than the C-F bond), releasing a highly reactive chlorine radical ($Cl\cdot$).

Catalytic Nature: A single chlorine radical can destroy thousands of ozone molecules because it is regenerated at the end of the reaction cycle, rather than being consumed.

Step 1: Initiation: UV light breaks the C-Cl bond in a CFC molecule: $CCl_3F + h\nu \rightarrow CCl_2F\cdot + Cl\cdot$

Step 2: Propagation A: The chlorine radical reacts with ozone: $Cl\cdot + O_3 \rightarrow ClO\cdot + O_2$

Step 3: Propagation B: The chlorine monoxide radical reacts with another ozone molecule (or oxygen atom): $ClO\cdot + O_3 \rightarrow Cl\cdot + 2O_2$

Overall Equation: The net result of the cycle is the conversion of two ozone molecules into three oxygen molecules: $2O_3 \rightarrow 3O_2$

Radical Notation: Always include the 'dot' ($\cdot$) to represent the unpaired electron on radicals like $Cl\cdot$ and $ClO\cdot$. Omitting this is a frequent cause of lost marks.

The Role of UV: Clearly state that UV light is required for the initial breakdown of the CFC molecule, but it is the chlorine radical that destroys the ozone.

Equation Balancing: Ensure that the chlorine radical appears as both a reactant in the first step and a product in the second step to demonstrate its role as a catalyst.

State the 'Why': When asked why HFCs are better, specifically mention the absence of C-Cl bonds and the resulting lack of chlorine radicals.

Confusing Ozone with Global Warming: While related to atmospheric chemistry, ozone depletion is primarily about UV protection, not the greenhouse effect (though CFCs are also greenhouse gases).

Bond Strength Misunderstanding: Students often think the C-F bond breaks because fluorine is more reactive. In reality, the C-Cl bond is weaker and breaks more easily under UV radiation.

Catalyst Consumption: A common error is suggesting the chlorine is used up. You must emphasize that the $Cl\cdot$ is regenerated, allowing the cycle to continue indefinitely.