Problem 49 - A Complete Resource Book in Chemistry for JEE Main

Question

About \(20 \mathrm{~km}\) above the earth, is the ozone layer. Which one of the following statements about ozone and ozone layer is true? (a) conversion of \(\mathrm{O}_{3}\) to \(\mathrm{O}_{2}\) is an endothermic reaction (b) it is beneficial to us as it stops ultraviolet radiation (c) ozone has a triatomic linear molecule (d) it is harmful as it stops useful radiations

Step-by-Step Solution

Answer

Statement (b) is true: the ozone layer is beneficial as it stops ultraviolet radiation.

Step 1: Understand the Role of the Ozone Layer

The ozone layer is crucial because it absorbs the majority of the sun's harmful ultraviolet (UV) radiation. This protects living organisms, including humans, from the damaging effects of UV rays, such as skin cancer and cataracts.

Step 2: Evaluate Each Statement

Let's examine each statement to determine its validity:(a) Conversion of \(\mathrm{O}_{3}\) to \(\mathrm{O}_{2}\) is not an endothermic reaction. It is exothermic, meaning it releases energy.(b) This statement is true. The ozone layer is beneficial as it stops ultraviolet radiation, which can be harmful to life on Earth.(c) Ozone is a triatomic molecule (\(\mathrm{O}_{3}\)), but it is not linear; it has a bent structure.(d) This statement is incorrect. The ozone layer is not harmful; it protects us by blocking harmful ultraviolet radiation, not beneficial radiation.

Step 3: Identify the True Statement

Based on the analysis, statement (b) is the only correct one: the ozone layer is beneficial as it stops ultraviolet radiation.

Key Concepts

Ultraviolet Radiation

Ultraviolet radiation (UV radiation) is a type of electromagnetic radiation that comes from the sun and artificial sources like tanning beds. Although it's invisible to the human eye, UV radiation is important for several reasons, both beneficial and harmful. It has shorter wavelengths than visible light, and this is why it possesses more energy.
Exposure to UV radiation can damage DNA in living organisms, leading to skin cancer, cataracts, and other health issues. However, it's also critical for vitamin D production in humans.
Thankfully, the Earth's ozone layer acts as a protective shield, absorbing and blocking the majority of the sun's UV radiation from reaching the surface.

• High exposure to UV rays can result in sunburn and increase skin cancer risk.
• UV radiation can also affect plants and aquatic ecosystems.
• The ozone layer helps in maintaining the balance, making life secure on Earth.

Ozone Molecule Structure

Ozone is a molecule made up of three oxygen atoms, its chemical formula being \( \mathrm{O}_{3} \). Its molecular structure is distinctly not linear, but rather bent or angular, forming a slight 'V' shape. This configuration arises from the specific bonding and lone pair arrangements in molecular geometry.
Ozone is an allotrope of oxygen, and its unique structure gives it various properties distinct from the more common diatomic oxygen molecule \( \mathrm{O}_{2} \).

• Ozone has a bond angle of approximately 116.8 degrees.
• It is much less stable than \( \mathrm{O}_{2} \) and degrades over time in the atmosphere.
• This bent shape helps it interact with UV radiation more effectively, allowing it to absorb harmful rays before they reach the Earth.

Endothermic vs. Exothermic Reactions

Chemical reactions can either absorb or release energy, leading to two categories: endothermic and exothermic reactions. Understanding these reactions helps in explaining processes occurring in the atmosphere, including those related to the ozone layer.
Exothermic reactions release energy, usually in the form of heat. For instance, converting ozone \( (\mathrm{O}_{3}) \) into molecular oxygen \( (\mathrm{O}_{2}) \) is an exothermic reaction because it releases energy to the surroundings.
In contrast, endothermic reactions absorb energy from the surroundings, typically requiring heat.

• Examples of exothermic reactions include combustion and the mixing of acids with water.
• Photosynthesis in plants is a common example of an endothermic process.
• In terms of the ozone layer, the constant breakdown and formation of ozone molecules involve both endothermic and exothermic reactions, maintaining energetic balance in the atmosphere.