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

Under the influence of ultraviolet radiation in the stratosphere, chlorofluorocarbons (CFCs) undergo photolysis to yield reactive chlorine free radicals (Cl∙\text{Cl}^\bullet). Which of the following chemical equations correctly represents the propagation step in which the chlorine radical directly reacts with ozone to cause its depletion?

  1. Cl∙+O3→ClO∙+O2\text{Cl}^\bullet + \text{O}_3 \rightarrow \text{ClO}^\bullet + \text{O}_2Answer
  2. B
    CF2Cl2+hν→CF2Cl∙+Cl∙\text{CF}_2\text{Cl}_2 + h\nu \rightarrow \text{CF}_2\text{Cl}^\bullet + \text{Cl}^\bullet
  3. C
    CO2+O3→CO3+O2\text{CO}_2 + \text{O}_3 \rightarrow \text{CO}_3 + \text{O}_2
  4. D
    CO2+hν→CO+O∙\text{CO}_2 + h\nu \rightarrow \text{CO} + \text{O}^\bullet

Answer

Cl∙+O3→ClO∙+O2\text{Cl}^\bullet + \text{O}_3 \rightarrow \text{ClO}^\bullet + \text{O}_2
In the catalytic cycle of ozone destruction, the reactive chlorine free radical (Cl∙\text{Cl}^\bullet) collides with an ozone molecule (O3\text{O}_3), abstracts an oxygen atom to form a chlorine monoxide radical (ClO∙\text{ClO}^\bullet), and releases diatomic oxygen (O2\text{O}_2).

Step-by-Step Solution

1
Identify the initiation reaction of CFCs in the stratosphere
UV radiation breaks a C-Cl bond in CFCs: CF2Cl2+hν→CF2Cl∙+Cl∙\text{CF}_2\text{Cl}_2 + h\nu \rightarrow \text{CF}_2\text{Cl}^\bullet + \text{Cl}^\bullet.
High-energy UV photons supply sufficient energy to homolytically cleave the carbon-chlorine bond.
2
Determine the propagation step involving ozone
The produced chlorine radical acts as a homogeneous catalyst, reacting with ozone: Cl∙+O3→ClO∙+O2\text{Cl}^\bullet + \text{O}_3 \rightarrow \text{ClO}^\bullet + \text{O}_2.
This specific step directly consumes atmospheric ozone while maintaining the catalytic cycle.

Key Concept

Catalytic mechanism of stratospheric ozone depletion by chlorine free radicals
Estimated Time:1m 0s
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Topics

Greenhouse Effect, Global Warming, and Ozone Layer Depletion

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