Problem 13 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The molecule that has minimum/no role in the formation of photochemical smog, is: (a) \(\mathrm{N}_{2}\) (b) \(\mathrm{CH}_{2}=\mathrm{O}\) (c) \(\mathrm{O}_{3}\) (d) NO

Step-by-Step Solution

Answer

(a) \\mathrm{N}_2\\ has no role in forming photochemical smog.

Step 1: Understanding Photochemical Smog

Photochemical smog is primarily formed from the reaction of hydrocarbons and nitrogen oxides (NOx) in the presence of sunlight. The components that play key roles in this process are nitrogen oxides, volatile organic compounds, and sunlight, which lead to the formation of secondary pollutants like ozone ( O_3 ).

Step 2: Analyzing the Options

We need to identify which molecule among the given options has the least role in the formation of photochemical smog: (a) N_2 - nitrogen gas, (b) CH_2=O - formaldehyde, (c) O_3 - ozone, (d) NO - nitric oxide. Both hydrocarbons like formaldehyde and nitrogen oxides like NO are directly involved in smog formation. Ozone is a secondary component resulting from these reactions.

Step 3: Eliminating Active Molecules

Formaldehyde ( CH_2=O ) and nitric oxide (NO) are known to be active participants in the formation of photochemical smog. Ozone ( O_3 ) is a product of these reactions and does not directly involve in its initial formation.

Step 4: Identifying the Inactive Molecule

N_2 , or nitrogen gas, is largely inert under normal atmospheric conditions and does not participate in the photochemical reactions that lead to smog. Therefore, it has a minimal or no role in the formation of photochemical smog.

Key Concepts

Nitrogen Oxides

Nitrogen oxides, commonly referred to as NOx, are a group of gases that contain different amounts of nitrogen and oxygen. These gases, particularly nitric oxide (NO) and nitrogen dioxide (NO₂), are key contributors to the formation of photochemical smog. They are released into the atmosphere primarily through the combustion of fossil fuels, like those used in vehicles and industrial processes. Once in the atmosphere, nitrogen oxides react in the presence of sunlight, playing a pivotal role in creating harmful air pollutants.
These pollutants include:

• Ozone (O₃), a major component of smog
• Particulate matter, leading to haze

The process begins when nitric oxide reacts with oxygen to form nitrogen dioxide, which then absorbs sunlight, breaking down into nitric oxide and a free oxygen atom. This free oxygen atom can react with oxygen molecules to form ozone, highlighting the importance of nitrogen oxides as precursors in photochemical smog production.

Volatile Organic Compounds

Volatile Organic Compounds (VOCs) are organic chemicals with high vapor pressure at ordinary room temperature. This characteristic allows them to evaporate or sublimate easily from liquid or solid forms into gas. VOCs arise from a variety of sources, including vehicle emissions, industrial processes, and even natural sources such as vegetation.
Their role in smog formation is significant, as they react with nitrogen oxides in the sunlight. Key reactions include:

• Combining with NOx to form ozone
• Facilitating the breakdown of NO₂, releasing more oxygen atoms for ozone production

By engaging in these reactions, VOCs contribute to secondary pollutants in the atmosphere, exacerbating air pollution and health issues associated with smog. Understanding the sources of VOCs and limiting their emissions is crucial in managing air quality and reducing photochemical smog formation.

Ozone Formation

Ozone (O₃) is a secondary pollutant in the context of photochemical smog, meaning it is not emitted directly but forms through chemical reactions in the atmosphere. The formation of ozone is a multistep photochemical process involving nitrogen oxides and volatile organic compounds in the presence of sunlight. Simply put, ozone is not produced directly from sources like cars or factories, but through reactions of precursor pollutants.
Here's how it typically forms:

• NO₂ molecules absorb sunlight and split into NO and atomic oxygen (O)
• The atomic oxygen then reacts with molecular oxygen (O₂) to create ozone (O₃)

This ozone, while beneficial in the upper atmosphere where it protects us from harmful UV radiation, is problematic at ground level. It contributes to air quality issues and poses health risks, especially for individuals with respiratory conditions. Hence, controlling the emission of NOx and VOCs is key to managing ozone levels and reducing smog.