Problem 69 - A Complete Resource Book in Chemistry for JEE Main

Question

About \(20 \mathrm{~km}\) above the earth there is an ozone layer. Which of the following statements about ozone and ozone layer are false? (1) The conversion of ozone to oxygen is an endothermic reaction. (2) Ozone layer is beneficial to us because ozone cuts out the ultraviolet radiation of the sun. (3) Ozone layer is harmful to us because it cuts off radiations useful for photosynthesis. (4) Ozone has a triatomic linear molecule. (a) 1,2 and 3 (b) 1,3 and 4 (c) 2,3 and 4 (d) 3 and 4 only

Step-by-Step Solution

Answer

Option (b) 1, 3, and 4 are false.

Step 1: Analyze Statement 1

Statement 1 claims that the conversion of ozone to oxygen is an endothermic reaction. In reality, the conversion of ozone (\(O_3\)) to oxygen (\(O_2\)) is an exothermic reaction because it releases energy. Therefore, Statement 1 is false.

Step 2: Analyze Statement 2

Statement 2 asserts that the ozone layer is beneficial because it absorbs ultraviolet (UV) radiation. This is true, as the ozone layer protects living organisms on Earth by filtering out harmful UV radiation from the sun. Therefore, Statement 2 is true.

Step 3: Analyze Statement 3

Statement 3 says that the ozone layer is harmful because it cuts off radiations useful for photosynthesis. This is false because the radiation necessary for photosynthesis is in the visible light spectrum, not the UV light spectrum, which the ozone layer absorbs. Thus, Statement 3 is false.

Step 4: Analyze Statement 4

Statement 4 describes ozone as having a triatomic linear molecule. Ozone is indeed a triatomic molecule (\(O_3\)), but it has a bent or angular shape due to the presence of lone pairs on the oxygen atom, not linear. Therefore, Statement 4 is false.

Step 5: Identify the False Statements

Based on the analysis, the false statements are 1, 3, and 4. These correspond to option (b) in the given list.

Key Concepts

Ozone Chemistry

Ozone, with the chemical formula \(O_3\), plays a crucial role in our atmosphere. It is made up of three oxygen atoms. This triatomic molecule is not very stable and tends to convert to the more stable form, oxygen (\(O_2\)). In this conversion process, ozone breaks down into molecular oxygen and free oxygen atoms. What's interesting is that this conversion is an exothermic reaction, meaning it releases energy. That’s the opposite of an endothermic reaction, which requires energy input. Understanding this basic chemistry highlights how ozone acts as an important, albeit unstable, component in protecting our environment.

Ultraviolet Radiation Protection

The ozone layer is like Earth's sunscreen. It protects life on our planet by absorbing and blocking most of the sun's harmful ultraviolet (UV) radiation. There are three types of UV radiation: UV-A, UV-B, and UV-C. The ozone layer efficiently absorbs almost all UV-C and most UV-B, allowing only a small amount to reach the surface. UV-A is least absorbed but is also less harmful compared to the others.

• UV-C: Completely absorbed by the ozone layer and atmosphere, never reaching Earth.
• UV-B: Mostly absorbed, although some still gets through, contributing to skin risks.
• UV-A: Least affected by ozone but generally less harmful.

This protective barrier is essential for preventing potential damage caused by UV rays, such as skin cancer and cataracts.

Photosynthesis and Light Spectrum

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy. This process primarily requires light from the visible spectrum, not the UV spectrum. The visible light spectrum ranges from about 380 nm to 750 nm in wavelength. Chlorophyll, the green pigment in plants, absorbs light most efficiently in the blue and red parts of this spectrum.

• Blue Light: Provides a lot of energy and aids in photosynthesis.
• Red Light: Highly absorbed by chlorophyll and is crucial for the photosynthesis process.
• Green Light: Least absorbed, which is why plants appear green.

Thus, the claim that the ozone layer blocks photosynthesis by preventing helpful radiation is incorrect. The necessary light for photosynthesis is unaffected by the ozone layer's absorption of detrimental UV radiation.

Molecular Geometry of Ozone

The molecular structure of ozone is quite interesting and one that is sometimes misunderstood. Ozone is a triatomic molecule, which means it consists of three oxygen atoms. However, unlike what some might assume, these three oxygen atoms do not form a straight line. Instead, the molecule has a bent structure. This bent shape, with an angle of around 117 degrees, arises due to the presence of lone electron pairs which repel the bonding pairs of electrons, resulting in a non-linear arrangement.
This structure is essential to the molecule's properties and is crucial for understanding why ozone behaves uniformly as it does in atmospheric reactions. Understanding the shape helps in anticipating the chemical behavior of ozone, especially in its role in atmospheric chemistry.