Problem 70 - A Complete Resource Book in Chemistry for JEE Main

Question

Ozone is formed in the upper layer of the atmosphere from oxygen by the action of (a) cosmic rays (b) ultraviolet rays (c) free radicals (d) nitrogen oxides

Step-by-Step Solution

Answer

Ozone is formed by the action of ultraviolet rays.

Step 1: Understand the Problem

The question asks which factor contributes to the formation of ozone in the upper atmosphere. We need to identify the correct factor that leads to the creation of ozone from oxygen.

Step 2: Recall Ozone Formation Process

Ozone (\(O_3\)) is a molecule made up of three oxygen atoms. In the upper atmosphere, it is primarily formed through a process where oxygen molecules (\(O_2\)) are split by the energy of ultraviolet (UV) rays from the Sun.

Step 3: Analyze the Options

- Option (a) cosmic rays: Cosmic rays are high-energy particles from space, not typically responsible for ozone formation.- Option (b) ultraviolet rays: UV rays cause the dissociation of \(O_2\) into \(O\) atoms, which then combine with \(O_2\) to form ozone \(O_3\).- Option (c) free radicals: Free radicals can play a role in chemical reactions, but are not primarily responsible for ozone formation by themselves.- Option (d) nitrogen oxides: These are involved in ground-level ozone formation, not in the upper atmosphere.

Step 4: Choose the Best Answer

Based on the analysis, the most suitable option is (b) ultraviolet rays, as they directly split oxygen molecules to facilitate the formation of ozone in the upper atmosphere.

Key Concepts

Upper Atmosphere

The upper atmosphere, also known as the stratosphere, is a layer of air that sits above the troposphere and extends up to about 50 km above Earth's surface. This part of the atmosphere has some unique characteristics that are crucial for ozone formation. Unlike the lower atmosphere, the stratosphere contains less turbulence, making it a stable region where chemical reactions occur steadily.
The primary focus here is on the ozone layer, which is vital for protecting the Earth from harmful solar radiation. Ozone molecules are formed in this layer in a dynamic process involving several key factors, primarily the presence of oxygen and ultraviolet rays.
Understanding the upper atmosphere's calm and stable conditions gives insights into why it is an ideal place for ozone to be formed and maintained. The reduced turbulence also means less mixing with other atmospheric layers, keeping the beneficial ozone closer to its formation zone.

Ultraviolet Rays

Ultraviolet (UV) rays are a type of electromagnetic radiation emitted by the Sun. They are split into three categories based on their wavelength: UVA, UVB, and UVC. While the Earth's atmosphere blocks out most UVC and some UVB, a portion of UVB is absorbed, and this is crucial for ozone formation.
UV rays have the right amount of energy to split oxygen molecules ( O_2 ) into individual oxygen atoms ( O ). This process is known as photodissociation. Once the oxygen molecules split, these free oxygen atoms can quickly react with other oxygen molecules to form ozone ( O_3 ).

• UVA: least energy, mostly reaches Earth's surface.
• UVB: moderate energy, helps in forming vitamin D and is partially absorbed by the ozone layer.
• UVC: most energy, fully absorbed by the atmosphere, crucial for the formation of the ozone layer in the upper atmosphere.

In essence, it's these UV rays that facilitate the continuous regeneration of the ozone layer by energizing the splitting of oxygen molecules.

Oxygen Molecules

Oxygen molecules (O_2) are the starting point in the formation of ozone (O_3) in the upper atmosphere. Each oxygen molecule consists of two oxygen atoms bonded together. This stable molecule makes up about 21% of Earth's atmosphere.
For ozone to form, these oxygen molecules need to be split into individual oxygen atoms. This splitting action is primarily driven by the energy supplied from ultraviolet (UV) rays. Without adequate UV energy, the molecules remain bound, and ozone formation would not occur.
Once the oxygen molecule is split, the individual oxygen atoms are highly reactive. These atoms quickly bond with other intact oxygen molecules to form ozone. The equation representing this process is:
\[O_2 + UV ightarrow 2O\]
\[O + O_2 ightarrow O_3\]
This cycle continues across the upper atmosphere, balancing the levels of oxygen and ozone effectively, ensuring a protective shield around Earth.