Problem 71 - A Complete Resource Book in Chemistry for JEE Main

Question

Pick up the incorrect statements here. (1) Classical smog is good for health but not photochemical smog. (2) During formation of smog the level of ozone in the atmosphere goes down. (3) Classical smog has an oxidizing character while the photochemical smog is reducing in character. (4) Photochemical smog occurs in day time whereas the classical smog occurs in early morning hours. (a) 1,2 and 3 (b) 1,3 and 4 (c) 2 and 3 only (d) 3 and 4 only

Step-by-Step Solution

Answer

The incorrect statements are: (a) 1, 2, and 3.

Step 1: Evaluate Statement 1

Classical smog is primarily composed of sulfur dioxide, which is harmful to health, contradicting the statement that it is good for health. Photochemical smog, indeed, is harmful due to the presence of ozone and other pollutants.

Step 2: Evaluate Statement 2

Photochemical smog involves reactions that produce ozone rather than reduce it. Therefore, it is incorrect to say that the level of ozone goes down during the formation of smog.

Step 3: Evaluate Statement 3

Classical smog is reducing in nature due to the presence of sulfur compounds, while photochemical smog is oxidizing due to the presence of ozone and nitrogen oxides. This statement reverses their correct chemical nature.

Step 4: Evaluate Statement 4

Photochemical smog occurs during the daytime as it requires sunlight for its formation. Classical smog typically forms in the cooler, early morning hours due to temperature inversion. This statement correctly describes the timing of their occurrences.

Step 5: Identify Incorrect Statements

Based on evaluations, the incorrect statements are: 1 (due to health effects), 2 (regarding ozone levels), and 3 (regarding oxidizing and reducing character).

Key Concepts

Ozone Formation

Ozone is a significant component of photochemical smog. It forms through complex reactions involving pollutants like nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. The process begins when NOx are released, primarily from vehicle exhausts and industrial emissions.
When these gases are exposed to sunlight, they participate in a series of reactions that generate ozone. Here's how it happens:

• First, sunlight splits nitrogen dioxide (\(NO_2\)) into nitric oxide (\(NO\)) and an oxygen atom.
• The free oxygen atom then combines with molecular oxygen (\(O_2\)) to form ozone (\(O_3\)).

This ozone is then responsible for the oxidizing character of photochemical smog. It's harmful as it can cause respiratory issues and damage vegetation. This is why photochemical smog is detrimental during sunny, urban environments.
Understanding the role of ozone highlights why statement 2 from the exercise is incorrect as ozone levels increase during photochemical smog formation rather than decrease.

Sulfur Compounds

Sulfur compounds are a major component of classical smog, which is different from photochemical smog. Classical smog, often referred to as London-type smog or industrial smog, is known for its reducing nature due to the presence of sulfur dioxide (\(SO_2\)). Sulfur dioxide originates from the burning of fossil fuels like coal, and in high concentrations, it can lead to severe air quality problems and health issues. Here's what you should know about sulfur compounds:

• They react with water vapor in the atmosphere to form sulfuric acid droplets, contributing to acid rain.
• Sulfur dioxide and the resulting particles can decrease visibility and pose respiratory hazards.

Classical smog mainly occurs under cooler conditions and often in combination with fog, which helps to trap the pollutants at lower levels.
This explains why in statement 3, classical smog is reducing in nature due to these sulfur compounds, contrary to what's suggested there.

Temperature Inversion

Temperature inversion is a critical meteorological condition causing and affecting pollution levels, particularly regarding smog. Under normal circumstances, surface temperatures decrease with altitude, allowing warm air to rise and disperse pollutants. However, during a temperature inversion, a layer of warm air traps cooler air near the surface.
This inversion prevents the rise and dispersion of pollutants, leading to a buildup of contaminants in the lower atmosphere. Let's see how this impacts smog types:

• Classical smog, which primarily consists of water droplets and sulfur compounds, is more likely during temperature inversions. This is because inversions typically occur in cooler morning hours, preventing the dispersal of these pollutants.
• Photochemical smog, despite occurring mainly during the day when sunlight is needed to drive chemical reactions, can also be exacerbated by inversions that trap pollutants emitted in the morning.

The timing described in statement 4 is correct, as classical smog often results from these inversions in the cooler, early hours, whereas photochemical smog appears during daylight due to its dependency on solar energy.