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

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

Step 1: Understand the Question

The problem is asking about the formation process of photochemical oxidants, specifically PAN (Peroxyacetyl nitrate) and PBN. We need to identify which chemical interaction leads to their formation.

Step 2: Analyzing the Options

We have four options that describe potential chemical interactions involving hydrogen sulphide, carbon dioxide, or nitrogen oxides with hydrocarbons, all in the presence of sunlight. Since PAN and PBN are both products of photochemical reactions, understanding their common formation pathway is key.

Step 3: Identifying the Correct Chemical Reaction

Photochemical oxidants like PAN are typically formed in the atmosphere from reactions involving nitrogen oxides (NOx) and volatile organic compounds (VOCs, often hydrocarbons) in the presence of sunlight. This is part of the complex reactions leading to tropospheric ozone and other pollutants. None of the other options are associated with the formation of photochemical oxidants in this context.

Step 4: Select the Correct Answer

Based on the analysis, the correct answer is (c) because PAN and similar oxidants are products of reactions between nitrogen oxides and hydrocarbons in sunlight.

Key Concepts

Photochemical Oxidants

Photochemical oxidants are a group of secondary pollutants. They are not emitted directly into the atmosphere but form in the air through chemical reactions. One of the most common photochemical oxidants is ozone, along with peroxyacetyl nitrate (PAN). The formation of these pollutants involves a series of complex reactions. These reactions involve precursor chemicals and are driven by sunlight. Photochemical oxidants can have harmful effects on human health, vegetation, and the environment. They are a significant component of photochemical smog.

Nitrogen Oxides

Nitrogen oxides, often referred to as NOx, are a group of gases that play a critical role in atmospheric chemistry. The most common are nitric oxide (NO) and nitrogen dioxide (NO2). These gases are primarily produced during the combustion of fossil fuels. When released into the atmosphere, nitrogen oxides undergo further reactions. These reactions can lead to the formation of photochemical oxidants like ozone and PAN. NOx emissions are a major concern due to their contribution to air pollution and their adverse effects on human health and ecosystems.

Hydrocarbons

Hydrocarbons are organic compounds consisting exclusively of hydrogen and carbon. They are a crucial component in the formation of photochemical oxidants. In the context of air pollution, volatile organic compounds (VOCs) are typically referred to as hydrocarbons. When hydrocarbons are released into the atmosphere, they can react with nitrogen oxides in the presence of sunlight. This reaction leads to the creation of secondary pollutants, including photochemical oxidants. Reducing hydrocarbons in the atmosphere can help minimize air pollution and improve air quality.

Peroxyacetyl Nitrate

Peroxyacetyl nitrate (PAN) is a specific type of photochemical oxidant. It forms in the atmosphere through reactions involving nitrogen oxides and volatile organic compounds. PAN is particularly significant in urban and industrial areas where emissions are high. It is known for its ability to travel long distances, making it a key player in regional pollution. PAN is harmful to plants, causing damage to forest ecosystems and crops. It also irritates the eyes and respiratory system in humans, contributing to the adverse health effects associated with air pollution.

Sunlight Interaction

The presence of sunlight is a critical factor in photochemical reactions. Sunlight provides the energy necessary for these reactions to occur. When sunlight interacts with nitrogen oxides and hydrocarbons, it leads to the formation of photochemical oxidants. These reactions are more pronounced during sunny, warm days, particularly in urban areas. Reducing sunlight-exposed emissions of NOx and VOCs can help reduce the formation of photochemical oxidants. Understanding the role of sunlight in these processes is vital for developing strategies to manage air quality and limit pollution.