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Difficulty: MediumClimate Change, Global Warming and Ozone Layer Depletion

During stratospheric ozone layer depletion, chlorofluorocarbons (CFCs) release free chlorine radicals (Cl∙\text{Cl}^\bullet) upon photolysis by solar ultraviolet radiation. Which of the following statements correctly describes how a chlorine radical acts catalytically to destroy ozone molecules without being consumed in the net reaction?

  1. It reacts with an ozone molecule (O3\text{O}_3) to form chlorine monoxide (ClO\text{ClO}) and oxygen (O2\text{O}_2), and is regenerated when ClO\text{ClO} reacts with a free oxygen atom (O\text{O}).Answer
  2. B
    It absorbs terrestrial longwave infrared radiation, raising local stratospheric temperatures high enough to induce thermal dissociation of ozone.
  3. C
    It reacts irreversibly with stratospheric moisture to produce hydrochloric acid (HCl\text{HCl}), consuming one molecule of ozone per chlorine atom in a single stoichiometric reaction.
  4. D
    It drives the photolytic oxidation of atmospheric nitrogen into nitrous oxide (N2O\text{N}_2\text{O}), which subsequently blocks the formation of oxygen radicals.

Answer

A chlorine radical destroys ozone by reacting with an ozone molecule (O3\text{O}_3) to produce chlorine monoxide (ClO\text{ClO}) and molecular oxygen (O2\text{O}_2). It is subsequently regenerated when ClO\text{ClO} reacts with a free atomic oxygen atom (O\text{O}), allowing the cycle to repeat.
The catalytic cycle of ozone depletion by chlorine free radicals consists of two key elementary reactions: 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. Because the free chlorine radical (Cl∙\text{Cl}^\bullet) is regenerated in the second step, it is not consumed in the overall reaction (O3+O→2O2\text{O}_3 + \text{O} \rightarrow 2\text{O}_2), enabling a single chlorine radical to destroy up to 10510^5 ozone molecules in the stratosphere.

Step-by-Step Solution

1
Identify the primary reaction of the chlorine radical with ozone.
Cl∙+O3→ClO∙+O2\text{Cl}^\bullet + \text{O}_3 \rightarrow \text{ClO}^\bullet + \text{O}_2
The chlorine radical abstracts an oxygen atom from ozone, creating chlorine monoxide.
2
Determine the regeneration step of the catalyst.
ClO∙+O→Cl∙+O2\text{ClO}^\bullet + \text{O} \rightarrow \text{Cl}^\bullet + \text{O}_2
Chlorine monoxide reacts with atomic oxygen in the stratosphere, liberating the chlorine radical.
3
Sum the two steps to get the net chemical equation.
Net reaction: O3+O→2O2\text{O}_3 + \text{O} \rightarrow 2\text{O}_2
The chlorine radical (Cl∙\text{Cl}^\bullet) acts purely as a catalyst and does not appear in the net reaction.

Key Concept

Catalytic destruction of stratospheric ozone by halogen radicals
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Topics

Climate Change, Global Warming and Ozone Layer Depletion