Problem 63 - A Complete Resource Book in Chemistry for JEE Main

Question

Photochemical oxidants such as PAN and PBN are formed (a) by the action of hydrogen sulphide on hydrocarbons in the presence of sunlight (b) by the action of carbon dioxide on hydrocarbons in the presence of sunlight (c) by the action of nitrogen oxides on hydrocarbons in the presence of sunlight (d) none of these

Step-by-Step Solution

Answer

Option (c) by the action of nitrogen oxides on hydrocarbons in the presence of sunlight.

Step 1: Understand What Photochemical Oxidants Are

Photochemical oxidants, such as PAN (Peroxyacetyl nitrate) and PBN (Peroxybenzoyl nitrate), are a group of air pollutants formed by complex chemical reactions in the atmosphere. They are part of photochemical smog.

Step 2: Identify Key Ingredients and Conditions

To form photochemical oxidants, typically, you need hydrocarbons and nitrogen oxides ( ext{NO}_x), such as ext{NO} and ext{NO}_2, in the presence of sunlight. Sunlight provides the energy required for the chemical reactions.

Step 3: Analyze Each Option

Review each given option: (a) involves hydrogen sulphide, which does not contribute to photochemical oxidants formation in this context. (b) involves carbon dioxide, which is not reactive in forming photochemical oxidants with hydrocarbons and sunlight. (c) involves nitrogen oxides, which are known precursors for photochemical oxidants when reacting with hydrocarbons in sunlight.

Step 4: Select the Correct Answer

Given that photochemical oxidants are formed by the reaction of nitrogen oxides with hydrocarbons in the presence of sunlight, option (c) is the correct choice.

Key Concepts

Photochemical Smog

Photochemical smog, often referred to as "summer smog," is a type of air pollution that is primarily associated with urban areas. It emerges when sunlight interacts with pollutants such as nitrogen oxides and volatile organic compounds, commonly known as hydrocarbons. The term "photochemical" signifies a chemical reaction that is initiated or accelerated by light. In this case, solar radiation is the catalyst that triggers the reaction.
During this process, new pollutants called photochemical oxidants are formed, which include compounds like ozone and peroxyacetyl nitrate (PAN). These oxidants are significant contributors to the hazy skies often seen over populated cities. Notably, photochemical smog is more prevalent in the hotter months when sunlight is more intense, hence the nickname "summer smog."
The impact of photochemical smog reaches beyond mere visibility issues. It can cause respiratory problems in humans and significant harm to plant life. Efforts to minimize the harmful effects of photochemical smog often focus on reducing emissions of its precursor chemicals, namely hydrocarbons and nitrogen oxides.

Hydrocarbons

Hydrocarbons, a primary component of photochemical smog, are organic compounds composed entirely of hydrogen and carbon. They can be found in various forms, from simple structures like methane to more complex molecules like benzene. In the atmosphere, they act as volatile organic compounds (VOCs).
These VOCs are released into the environment through several human activities. Common sources include:

• Vehicle exhausts
• Industrial emissions
• Evaporation of fuels and solvents

When in the air, these hydrocarbons don't just float around harmlessly. In the presence of sunlight, they react with nitrogen oxides to form photochemical oxidants and contribute to the creation of smog. This is a part of what makes hydrocarbons a focus in the battle against air pollution. Reducing VOC emissions can significantly lessen the formation rate of photochemical smog, emphasizing the importance of controlling hydrocarbon output in urban environments.

Nitrogen Oxides

Nitrogen oxides (NOx), specifically nitric oxide (NO) and nitrogen dioxide (NO2), are key contributors to air pollution and the formation of photochemical smog. These compounds are primarily produced from burning fossil fuels, such as in vehicles and power plants.
Once released into the atmosphere, nitrogen oxides undergo various reactions, especially when they come in contact with sunlight. Sunlight photolyzes NO2 back into NO and atomic oxygen, the latter of which combines with diatomic oxygen (O2) to form ozone (O3). This reaction is a fundamental step in the formation of photochemical smog.
Efforts to reduce the presence of photochemical smog often target lower NOx emissions. Strategies include promoting cleaner vehicle technologies, improving fuel quality, and enforcing stricter regulations on industrial outputs. By controlling nitrogen oxide emissions, cities can experience significant reductions in the formation of harmful photochemical smog and an improvement in air quality.

Atmospheric Chemistry

Atmospheric chemistry is the study of chemical processes that occur in the Earth's atmosphere. It plays a vital role in understanding air pollution and the mechanisms behind phenomena like photochemical smog.
In the case of photochemical smog, atmospheric chemistry helps us understand how environmental conditions and chemical components interact. The presence of sunlight not only drives the reactions between nitrogen oxides and hydrocarbons but also influences the formation of secondary pollutants like ozone and PAN.
Understanding atmospheric chemistry is critical for developing strategies to combat pollution. It informs the development of computer models that can predict smog formation, providing policymakers with tools to manage air quality effectively. As our understanding deepens, it becomes possible to develop more sophisticated technologies and approaches to limit the harmful impacts of pollution on human health and the environment.