Problem 50 - 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

The true statement is (b): ozone layer is beneficial as it stops ultraviolet radiation.

Step 1: Understand the role of ozone

The ozone (O_3) layer plays a critical role in protecting Earth from the Sun's harmful ultraviolet (UV) radiation. Without it, life on Earth's surface could not thrive due to increased UV exposure.

Step 2: Analyze statement (a)

Statement (a) suggests that the conversion of \( ext{O}_3\) to \( ext{O}_2\) is an endothermic reaction. However, this conversion releases energy as it forms a more stable molecule, suggesting it is an exothermic reaction.

Step 3: Analyze statement (b)

Statement (b) claims that the ozone layer is beneficial because it stops ultraviolet radiation. This is true as the ozone layer absorbs and scatters the solar UV radiation, preventing most of it from reaching the Earth's surface.

Step 4: Analyze statement (c)

Statement (c) describes ozone as having a triatomic linear molecule. However, the structure of ozone is bent, because it is made up of three oxygen atoms with bond angles less than 180°.

Step 5: Analyze statement (d)

Statement (d) states that the ozone layer is harmful as it stops useful radiations. This is false because the ozone layer protects living organisms by absorbing harmful UV radiation, which is not typically considered 'useful' in high doses.

Step 6: Conclusion: Identify the true statement

Reviewing the statements, the true statement about the ozone layer is that it is beneficial as it stops ultraviolet radiation from reaching the Earth, which corresponds to statement (b).

Key Concepts

Ultraviolet Radiation Protection

The ozone layer is like Earth's sunscreen, protecting us from harmful ultraviolet (UV) rays from the sun. It is located roughly 20 kilometers above the Earth and consists of ozone (O_3), a molecule composed of three oxygen atoms. This layer is crucial because it absorbs most of the sun's harmful UV radiation.
UV rays are a type of electromagnetic radiation. Though we don’t see them, they can cause skin cancer, cataracts, and can harm animals and plants. Without the ozone layer absorbing these rays, life on Earth would be significantly impacted. This protection is a natural shield, reducing UV exposure and allowing life to thrive on the planet.

• The ozone layer absorbs and blocks the most dangerous wavelengths of UV radiation.
• It allows only a small amount of naturally beneficial UV radiation to reach the Earth's surface.

Thus, preserving the ozone layer is vital for safeguarding life on our planet.

Ozone Molecule Structure

The ozone molecule might not look as you expect. Often, people think of it as linear due to its three oxygen atoms, but that's not the case. Ozone's structure is actually bent or angular.
This bent structure arises because of the specific way its atoms are bonded. It consists of three oxygen atoms held together with two bonds that are not perfectly straight, which gives the molecule a V-like shape. The bond angle in ozone is approximately 117°—much less than a straight 180°, which causes this curvature.

• Ozone (O_3) is an allotrope of oxygen and has a higher energy state than normal oxygen (O_2).
• The bent shape of ozone plays a critical role in its ability to absorb UV radiation efficiently.

Understanding this structure is essential since it contributes to the molecule's unique physical and chemical properties.

Endothermic vs Exothermic Reactions

Chemistry often revolves around understanding how energy changes in reactions. There are two main types of reactions based on energy flow: endothermic and exothermic reactions.
An endothermic reaction requires energy input to occur. You can think of it as the reaction needing a little extra energy kick to proceed. For example, photosynthesis is endothermic because it uses sunlight energy to transform carbon dioxide and water into glucose and oxygen.
On the other hand, exothermic reactions release energy, making them generally more spontaneous. The conversion of ozone (O_3) to oxygen (O_2) is an example of an exothermic reaction. When ozone breaks down, it forms a more stable oxygen molecule, releasing energy in the process.

• Endothermic reactions absorb heat, resulting in a temperature drop.
• Exothermic reactions release heat, often increasing the surrounding temperature.

Recognizing whether a reaction is endothermic or exothermic is crucial for understanding how energy is transferred in chemical processes.