Problem 55 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is mainly responsible for depletion of ozone layer? (a) water (b) carbon dioxide (c) chlorofluorocarbons (d) methane

Step-by-Step Solution

Answer

Chlorofluorocarbons (CFCs) are mainly responsible for depletion of the ozone layer.

Step 1: Understanding the Ozone Layer Depletion

The ozone layer is a part of Earth's atmosphere that absorbs most of the Sun's harmful ultraviolet radiation. Certain chemicals released into the atmosphere can destroy ozone molecules, leading to the depletion of this protective layer.

Step 2: Identifying Chlorofluorocarbons (CFCs)

Chlorofluorocarbons (CFCs) are compounds that contain carbon, chlorine, and fluorine. They were commonly used in air conditioning, refrigeration, and as propellants in aerosol sprays. When CFCs are emitted into the atmosphere, they rise to the stratosphere where the ozone layer is located.

Step 3: Chemical Reaction of CFCs with Ozone

In the stratosphere, CFCs are broken down by UV radiation, releasing chlorine atoms. These chlorine atoms react with ozone (O_3), turning it into oxygen (O_2) and reducing the amount of ozone. This reaction significantly contributes to the depletion of the ozone layer.

Step 4: Comparing Effects of Other Options

Evaluate the impact of the other options: water, carbon dioxide, and methane. Water is part of the natural hydro-logic cycle, and it does not significantly affect the ozone. Carbon dioxide and methane are greenhouse gases contributing to global warming, but they don't directly lead to ozone layer depletion.

Key Concepts

Chlorofluorocarbons (CFCs)

Chlorofluorocarbons, commonly known as CFCs, are a group of chemical compounds that contain carbon, chlorine, and fluorine. They were once widely used in various industrial applications, such as refrigerants in air conditioning and refrigeration systems, as propellants in aerosol sprays, and as solvents in the cleaning of electronic components. The popularity of CFCs stemmed from their stability and non-flammable characteristics.

However, that same stability is a problem when CFCs are released into the atmosphere. These compounds can ascend to the stratosphere without decomposing, where the vital ozone layer exists.

• In the stratosphere, ultraviolet (UV) radiation is strong enough to break down CFC molecules.
• This process releases chlorine atoms into the atmosphere.
• Chlorine atoms are extremely reactive and can destroy ozone molecules, leading to harmful ozone depletion.

CFCs have been largely phased out through international agreements like the Montreal Protocol due to their destructive effects on the ozone layer.

Ultraviolet Radiation

Ultraviolet (UV) radiation is a type of energy emitted by the sun. It is part of the electromagnetic spectrum and is invisible to the human eye. Despite being invisible, UV radiation plays a significant role in affecting environmental and human health.

UV radiation is primarily categorized into three types based on their wavelengths:

• UV-A: Has the longest wavelength and can penetrate into the deeper layers of skin.
• UV-B: Has slightly shorter wavelengths and is mostly absorbed by the ozone layer, preventing much of its radiation from reaching the Earth's surface.
• UV-C: Has the shortest wavelength and is almost completely absorbed by the ozone layer, never reaching the Earth.

Thanks to the ozone layer's protective capability, most harmful UV radiation is filtered out before it can reach us. Without the ozone layer, Earth would be subjected to much higher UV radiation levels, increasing risks like skin cancer and cataracts.

Additionally, excessive UV radiation can negatively impact ecosystems, affecting the growth of phytoplankton and other critical components of the marine food web. Protecting the ozone layer is essential to safeguarding our planet from these potential dangers.

Stratosphere

The stratosphere is one of the four main layers of the Earth's atmosphere, sitting above the troposphere and below the mesosphere.

It stretches from about 10 kilometers to 50 kilometers above the Earth’s surface. One notable feature of the stratosphere is the presence of the ozone layer, which is instrumental in shielding the Earth from harmful ultraviolet radiation.

• This layer contains a high concentration of ozone (O_3) molecules, which absorb and scatter the solar ultraviolet light.
• The absorption of UV radiation by ozone molecules also warms the stratosphere, which is why this atmospheric layer is warmer than the layers directly above and below it.
• Because air in the stratosphere is stable, substances like CFCs can remain there for a long time.

When introduced into the stratosphere, certain chemicals like CFCs can cause ozone depletion.

The stability of the stratosphere means that pollutants have trouble escaping, allowing reactions that lead to ozone loss to occur repeatedly before they are eventually eliminated. Protecting this atmospheric layer helps maintain a balance in the Earth's radiative balance and overall climate system.