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Question 44

Which of these reactions is not a part of breakdown of ozone in stratosphere?

Solution

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Core Concept: Freon-Induced Ozone Depletion Mechanism

In the stratosphere, ultraviolet (UV) radiation breaks down chlorofluorocarbons (like Freons, e.g., CF2Cl2, releasing highly reactive chlorine free radicals (Cl•). These radicals act as catalysts that continuously break down ozone molecules O3 into oxygen O2.

Evaluation of Actual Stratospheric Steps:
  • Initiation Step (Option D):
    $$CF_{2}Cl_2$$(g) —uv→$$Cl^•$$(g) + $$C^{•}F_{2}Cl(g)$$

    This step correctly shows UV light cleaving a C-Cl bond to generate the initial reactive chlorine free radical.

  • Propagation Step 1 (Option C):
    $$Cl^•$$(g) + $$O_3$$(g) → $$O_2$$(g) + $$Cl^•$$O(g)

    The chlorine radical attacks an ozone molecule, destroying it and producing a chlorine monoxide radical ($$Cl^•$$O).

  • Propagation Step 2 (Option B):
    ClO(g) + O(g) → $$O_2$$(g) + Cl(g)

    The chlorine monoxide radical reacts with a free oxygen atom, regenerating the catalytic chlorine radical to continue the cycle.

  • Incorrect Reaction (Option A):
    $$O_3$$(g) + $$O_2$$(g) → $$O_5$$(g) or similar variants

    This hypothetical step does not occur in any atmosphere layer because it is energetically unfavorable and unstable.

Correct Option: A (It is NOT a part of the ozone breakdown process)

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