Problem 19 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The reaction that is NOT involved in the ozone layer depletion mechanism in the stratosphere is: (a) \(\mathrm{CF}_{2} \mathrm{Cl}_{2}(\mathrm{~g}) \stackrel{\text { uv }}{\longrightarrow} \dot{\mathrm{Cl}}(\mathrm{g})+\dot{\mathrm{CF}}_{2} \mathrm{Cl}(\mathrm{g})\) (b) \(\mathrm{ClO}(\mathrm{g})+\mathrm{O}(\mathrm{g}) \longrightarrow \dot{\mathrm{Cl}}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{~g})\) (c) \(\mathrm{CH}_{4}+2 \mathrm{O}_{3} \longrightarrow 3 \mathrm{CH}_{2}=\mathrm{O}+3 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{HOCl}(\mathrm{g}) \stackrel{\mathrm{hv}}{\longrightarrow} \dot{\mathrm{O}} \mathrm{H}(\mathrm{g})+\dot{\mathrm{Cl}}(\mathrm{g})\)

Step-by-Step Solution

Answer

Reaction (c) is not involved in ozone layer depletion.

Step 1: Identify Mechanism of Ozone Depletion

The main mechanism of ozone layer depletion in the stratosphere involves reactions where chlorofluorocarbons (CFCs) release chlorine atoms. These chlorine atoms participate in a catalytic cycle with ozone molecules, resulting in the decomposition of ozone into molecular oxygen (O₂). Thus, any reaction releasing chlorine radicals (\(\dot{\mathrm{Cl}}\)) or utilizing them to convert ozone to oxygen is part of the depletion process.

Step 2: Evaluate Reaction (a)

Reaction \(\mathrm{CF}_{2} \mathrm{Cl}_{2}\, (\mathrm{~g}) \stackrel{\text{uv}}{\longrightarrow} \dot{\mathrm{Cl}}(\mathrm{g})+\dot{\mathrm{CF}}_{2} \mathrm{Cl}(\mathrm{g})\) involves CFCs breaking down under UV light to release chlorine radicals. This reaction contributes to the ozone depletion process.

Step 3: Evaluate Reaction (b)

Reaction \(\mathrm{ClO}(\mathrm{g})+\mathrm{O}(\mathrm{g}) \longrightarrow \dot{\mathrm{Cl}}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{~g})\) involves chlorine monoxide (\mathrm{ClO}) reacting with an oxygen atom to release a chlorine radical and form oxygen molecules. This is part of the ozone depletion cycle.

Step 4: Evaluate Reaction (c)

Reaction \(\mathrm{CH}_{4}+2 \mathrm{O}_{3} \longrightarrow 3 \mathrm{CH}_{2}=\mathrm{O}+3 \mathrm{H}_{2} \mathrm{O}\) involves methane reacting with ozone to form formaldehyde and water. This reaction doesn't involve chlorine radicals and thus does not contribute to the typical mechanism of ozone depletion.

Step 5: Evaluate Reaction (d)

Reaction \(\mathrm{HOCl}(\mathrm{g}) \stackrel{\mathrm{hv}}{\longrightarrow} \dot{\mathrm{O}} \mathrm{H}(\mathrm{g})+\dot{\mathrm{Cl}}(\mathrm{g})\) involves the breakdown of hypochlorous acid under light to release a chlorine radical. This contributes to the ozone depletion process.

Step 6: Conclusion: Identify Non-Contributing Reaction

From the evaluation, reaction (c) does not involve the type of chlorine chemistry responsible for stratospheric ozone depletion. It involves a reaction with methane, which is not part of the chlorofluorocarbon-driven catalytic cycles that deplete ozone.

Key Concepts

Chlorofluorocarbons (CFCs)

Chlorofluorocarbons, more commonly known as CFCs, are man-made compounds containing carbon, chlorine, and fluorine. They were widely used in the past as refrigerants, propellants in aerosol sprays, and solvents. However, their stability and ability to remain in the atmosphere for a long time lead them to drift up to the stratosphere. Here, they are broken down by ultraviolet (UV) radiation, releasing chlorine atoms.
CFCs are a significant environmental concern because when they release chlorine radicals, they start a chain reaction that leads to the depletion of the ozone layer.

• Each chlorine atom released from CFCs can break down thousands of ozone molecules before it is naturally neutralized.
• This process significantly reduces the protective ozone layer, which shields the Earth from harmful UV radiation.

Because of these detrimental effects, the production and use of CFCs have been phased out under international agreements like the Montreal Protocol. Despite this, the CFCs already released continue to affect the ozone layer for many years due to their long atmospheric lifetimes.

Chlorine radicals

Chlorine radicals are highly reactive atoms that play a pivotal role in the destruction of ozone molecules in the atmosphere. When CFCs are exposed to UV light, they break apart, releasing chlorine radicals. These radicals are denoted as \( \dot{\mathrm{Cl}} \).
In the stratosphere, chlorine radicals participate in various reactions that lead to ozone depletion. A key reaction involves a chlorine radical reacting with an ozone molecule \( (\mathrm{O}_3) \), breaking it down into molecular oxygen \( (\mathrm{O}_2) \) and chlorine monoxide \( (\mathrm{ClO}) \).

• The chlorine monoxide can further react with an oxygen atom to release another chlorine radical and form more molecular oxygen.
• This cycle can occur thousands of times, with a single chlorine radical destroying many ozone molecules.

This destructive chain reaction dramatically affects the concentration of ozone in the stratosphere, diminishing the layer that guards against ultraviolet radiation. The persistence and potency of chlorine radicals highlight the essential need for eliminating substances like CFCs from usage.

Stratospheric reactions

The stratosphere is one of the layers of Earth's atmosphere located above the troposphere. It contains the ozone layer, which plays a crucial protective role by absorbing the majority of the Sun's harmful ultraviolet radiation. However, specific chemical reactions in the stratosphere contribute to the depletion of this vital ozone shield.
The reactions primarily involve CFCs and the release of chlorine radicals. When CFCs reach the stratosphere, ultraviolet light causes them to release chlorine atoms, which participate in a series of reactions with ozone molecules.

• These reactions are part of a chain that ends with the conversion of ozone \( (\mathrm{O}_3) \) into molecular oxygen \( (\mathrm{O}_2) \), reducing the effectiveness of the ozone layer.
• Different intermediate compounds, such as chlorine monoxide \( (\mathrm{ClO}) \), form during these processes, perpetuating the cycle of depletion.

The balance in the stratosphere is delicate, as even small changes in the chemical composition can lead to significant ozone loss. To protect this layer, it is crucial to regulate and lower the emission of ozone-depleting substances.