Problem 25 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following statements about the depletion of ozone layer is correct? (a) The problem of ozone depletion is less serious at poles because \(\mathrm{NO}_{2}\) solidifies and is not available for consuming CIO' radicals. (b) The problem of ozone depletion is more serious at poles because ice crystals in the clouds over poles act as catalyst for photochemical reactions involving the decomposition of ozone of \(\mathrm{Cl}^{\circ}\) and \(\mathrm{CIO}^{\circ}\) radicals. (c) Freons, chlorofluorocarbons, are inert. Chemically, they do not react with ozone in stratosphere. (d) Oxides of nitrogen also do not react with ozone in stratosphere.

Step-by-Step Solution

Answer

Statement (b) is correct.

Step 1: Identifying the Ozone Depletion Mechanism

The problem of ozone depletion primarily involves chlorine and bromine atoms that catalyze the breakdown of ozone molecules in the stratosphere. In particular, reactions with chlorofluorocarbons (CFCs) broken down by UV light release chlorine atoms, which aggressively break down ozone (O₃) into oxygen (O₂).

Step 2: Analyzing Each Given Statement

We have been given four statements, each representing a possible condition or cause related to ozone depletion. We will evaluate them based on established scientific knowledge about ozone depletion.

Step 3: Evaluating Statement (a)

Statement (a) claims ozone depletion is less serious at poles because \(\mathrm{NO}_{2}\) solidifies and is not available for reaction. This is incorrect because there is a higher concern for ozone depletion at poles due to the presence of polar stratospheric clouds that contribute to ozone depletion.

Step 4: Evaluating Statement (b)

Statement (b) suggests problems are more serious at the poles due to ice crystals acting as catalysts. This is accurate; ice crystals in polar clouds provide a surface for reactions that release active chlorine radicals, causing severe ozone depletion, observed especially as the ozone hole over Antarctica.

Step 5: Evaluating Statement (c)

Statement (c) states that freons (CFCs) are inert and do not react with ozone. This is false. Freons are inert in the lower atmosphere but release chlorine atoms in the stratosphere which then participate in ozone destruction.

Step 6: Evaluating Statement (d)

Statement (d) mentions oxides of nitrogen do not react in the stratosphere. This is not correct; nitrogen oxides can participate in ozone layer chemistry by cycling and moderating the concentration of ozone depleting radicals.

Step 7: Conclusion and Correct Answer

Based on the evaluations, statement (b) is correct. Ozone depletion is indeed more serious at the poles due to the catalytic effect of ice crystals on photochemical reactions that deplete ozone.

Key Concepts

Chlorofluorocarbons (CFCs)

Chlorofluorocarbons, commonly known as CFCs, are a class of human-made compounds that contain chlorine, fluorine, and carbon. They were widely used in various applications such as refrigerants, propellants in aerosol sprays, and as solvents in industrial cleaning due to their low toxicity and chemical stability.
However, this stability becomes problematic when CFCs reach the stratosphere. Here, ultraviolet (UV) radiation is strong enough to break the bonds in CFC molecules.
This process releases chlorine atoms, which are highly reactive with ozone (O₃) molecules. A single chlorine atom can destroy thousands of ozone molecules before it is removed from the stratosphere. This reaction proceeds as follows:

• UV light breaks down CFCs, releasing chlorine (^•Cl) radicals.
• These radicals interact with ozone, converting O₃ into molecular oxygen (O₂) and chlorine monoxide (ClO).

Given their longevity in the atmosphere, CFCs can continue affecting the ozone layer for up to a century, making their impact significant and long-lasting.

Polar Stratospheric Clouds

Polar Stratospheric Clouds, or PSCs, form in the winter months in the polar regions of the Earth, particularly over Antarctica and the Arctic. These clouds form at very high altitudes where the stratosphere is extremely cold.
PSCs play a crucial role in ozone depletion because they provide the surfaces upon which chemical reactions occur that release active chlorine radicals from otherwise inert chlorine-containing compounds (like hydrochloric acid and chlorine nitrate).

• During the winter, these clouds form a surface on which chlorine compounds can chemically transform into active chlorine once exposed to sunlight.
• As sunlight returns to the polar regions in spring, these reactions accelerate, leading to a sharp increase in chlorine radicals, which in turn results in the dramatic breakdown of ozone.

This process explains why the most severe ozone depletion occurs in late winter to early spring, when PSCs are most prevalent.

Chlorine Radicals

Chlorine radicals are highly reactive and are one of the primary culprits in ozone layer depletion. Once released into the stratosphere, they participate in a series of reactions that break down ozone.
The main reactions involve chlorine radicals breaking down ozone into oxygen, while simultaneously regenerating the chlorine radical, allowing it to continue reacting with more ozone molecules:

• Chlorine radicals react with ozone to form chlorine monoxide (ClO) and oxygen.
• ClO further reacts to release the chlorine radical, which can go on to destroy more ozone.

This cyclical destruction process means a single chlorine atom can deplete a vast number of ozone molecules over time. The presence of chloride radicals is heightened by certain conditions like those found in polar regions, where Polar Stratospheric Clouds help release more chlorine radicals from other atmospheric compounds.