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.

Step 1: Analyze Statement 1

Statement 1 claims that the conversion of ozone (\(\text{O}_3\)) to oxygen (\(\text{O}_2\)) is an endothermic reaction. This is incorrect, as the conversion process releases energy, making it an exothermic reaction.

Step 2: Evaluate Statement 2

Statement 2 states that the ozone layer is beneficial because it filters out the sun's ultraviolet radiation. This is true, as the ozone layer absorbs significant amounts of UV-B and UV-C rays, protecting living organisms from harmful effects.

Step 3: Consider Statement 3

Statement 3 claims the ozone layer is harmful since it blocks radiations essential for photosynthesis. This is false, as the ozone layer mainly filters harmful UV radiation and does not affect the visible light necessary for photosynthesis.

Step 4: Review Statement 4

Statement 4 says ozone has a triatomic linear molecule. This is false because the ozone molecule is triatomic but has a bent shape with an angle of about 117 degrees, not linear.

Step 5: Determine the False Statements

After evaluating, statements 1, 3, and 4 are false. Statement 2 is true. Therefore, options that include these incorrect statements are (b) 1, 3, and 4.

Key Concepts

Endothermic vs Exothermic Reactions

Chemical reactions can either absorb or release energy, generally in the form of heat. Understanding the difference between these two types of reactions is crucial in chemistry.

An **endothermic reaction** absorbs energy from its surroundings. This means it takes in heat during the reaction, causing the environment to cool down. Think of melting ice or photosynthesis; these are typical endothermic processes where energy (heat or sunlight) is necessary for the reaction to proceed.

• Examples: Photosynthesis, melting of ice.
• Energy is a reactant in the equation.
• Environmental temperature drops because heat is absorbed.

On the other hand, an **exothermic reaction** releases energy. During these reactions, energy is given off, often in the form of heat, making the surrounding environment feel warmer. Combustion and the conversion of ozone to oxygen are examples of exothermic reactions. For instance, when ozone (\(\text{O}_3\)) breaks down into oxygen (\(\text{O}_2\)), energy is released, a characteristic of exothermic reactions.

• Examples: Combustion, dissolution of sodium hydroxide.
• Energy is a product in the equation.
• Surrounding temperature rises due to heat release.

Ultraviolet Radiation

Ultraviolet (UV) radiation is a type of energy emitted by the sun. It is a component of the electromagnetic spectrum with shorter wavelengths than visible light, making it invisible to the human eye.

There are three types of UV radiation: UV-A, UV-B, and UV-C.

• **UV-A**: Has the longest wavelength and can penetrate the skin, leading to skin aging and DNA damage over time.
• **UV-B**: Partially absorbed by the ozone layer, it's stronger than UV-A and is responsible for sunburn and a higher risk of skin cancer.
• **UV-C**: Most energetic and entirely absorbed by the ozone layer, not reaching the Earth's surface.

The **ozone layer** plays a crucial role by absorbing a significant amount of UV-B and all of UV-C. Hence, it prevents these harmful radiations from reaching living organisms on Earth, shielding us from their damaging effects. Without this shield, life on Earth would face severe consequences due to increased exposure to UV radiation, including higher skin cancer rates and environmental impacts.

Molecular Geometry

Molecular geometry is essential for understanding the shape of a molecule, which affects its properties and reactions. Let's consider the **ozone molecule** as an example.

Although ozone (\(\text{O}_3\)) contains three oxygen atoms, it does not form a straight line. Instead, it has a **bent shape** due to the presence of electron pairs on the central oxygen atom, creating an angular arrangement.

• Ozone's bond angle is approximately 117 degrees.
• The bent structure results from electron repulsion, which means the lone pairs push the bonds away from each other.
• This geometry affects ozone's chemical reactivity and its role in absorbing UV radiation.

Molecular geometry can impact a molecule's polarity, reactivity, phase of matter, color, magnetism, biological activity, and more. Understanding the shape of molecules like ozone helps chemists predict and explain these various properties and behaviors in different substances.