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Difficulty: MediumGreenhouse Effect, Global Warming, and Ozone Layer Depletion

Chlorofluorocarbons (CFCs) released into the atmosphere diffuse into the stratosphere, where they are decomposed by intense solar ultraviolet radiation. Which active reactive species generated during this photolysis process directly catalyzes the catalytic breakdown of ozone (O3\text{O}_3) molecules?

  1. A
    Carbon(IV) oxide molecule (CO2\text{CO}_2)
  2. Chlorine free radical (Cl∙\text{Cl}^\bullet)Answer
  3. C
    Methane molecule (CH4\text{CH}_4)
  4. D
    Dinitrogen(I) oxide molecule (N2O\text{N}_2\text{O})

Answer

The chlorine free radical (Cl∙\text{Cl}^\bullet) released from the photolysis of chlorofluorocarbons directly catalyzes the depletion of stratospheric ozone molecules.
Under stratospheric ultraviolet (UV) irradiation, chlorofluorocarbon (CFC) molecules undergo homolytic cleavage to produce reactive chlorine free radicals (Cl∙\text{Cl}^\bullet). A chlorine radical attacks an ozone molecule (O3\text{O}_3) to form a chlorine monoxide radical (ClO∙\text{ClO}^\bullet) and a diatomic oxygen molecule (O2\text{O}_2). The chlorine monoxide radical subsequently reacts with atomic oxygen to regenerate the free chlorine radical (Cl∙\text{Cl}^\bullet). Because the radical is regenerated, it acts as a catalyst in the continuous breakdown of the protective ozone layer.

Step-by-Step Solution

1
Identify the photolysis reaction of CFCs in the stratosphere.
Ultraviolet radiation breaks a C-Cl\text{C-Cl} bond in CFCs (e.g., CF2Cl2+hν→CF2Cl∙+Cl∙\text{CF}_2\text{Cl}_2 + h\nu \rightarrow \text{CF}_2\text{Cl}^\bullet + \text{Cl}^\bullet), releasing reactive chlorine free radicals.
High-energy UV photons possess sufficient energy to cleave the chemical bond in the tropospheric-stable CFC molecules upon reaching the stratosphere.
2
Analyze the catalytic cycle of ozone destruction.
Cl∙+O3→ClO∙+O2\text{Cl}^\bullet + \text{O}_3 \rightarrow \text{ClO}^\bullet + \text{O}_2 followed by ClO∙+O→Cl∙+O2\text{ClO}^\bullet + \text{O} \rightarrow \text{Cl}^\bullet + \text{O}_2.
The chlorine free radical acts as a true catalyst because it is regenerated at the end of the reaction cycle, allowing a single radical to destroy up to 100,000100,000 ozone molecules.
3
Distinguish between greenhouse gases and ozone-depleting catalysts.
CO2\text{CO}_2 and CH4\text{CH}_4 trap infrared heat (greenhouse effect), whereas Cl∙\text{Cl}^\bullet radicals chemically decompose ozone (ozone layer depletion).
Conflating global warming mechanisms with ozone depletion mechanisms is a common conceptual error.

Key Concept

Photolysis of CFCs and Catalytic Cycle of Ozone Layer Depletion
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Topics

Greenhouse Effect, Global Warming, and Ozone Layer Depletion

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