Question

If an average \(\mathrm{O}_{3}\) molecule "lives" only \(100-200\) seconds in the stratosphere before undergoing dissociation, how can \(\mathrm{O}_{3}\) offer any protection from ultraviolet radiation?

Short answer

Even though individual ozone molecules have a short lifetime of 100-200 seconds in the stratosphere, the ozone layer still effectively protects Earth from harmful ultraviolet (UV) radiation because the formation and dissociation of ozone occur simultaneously. The absorption of UV-C and UV-B radiation by ozone filters out most of the dangerous wavelengths from reaching the Earth's surface and maintains the ozone layer through a continuous process of ozone molecule formation and dissociation. This ensures the ozone layer remains dense enough to shield life on Earth from damaging UV radiation.

Step-by-step solution

Understand ozone formation and dissociation

Ozone is formed in the stratosphere as a result of the interactions between oxygen molecules (O2) and ultraviolet (UV) radiation from the Sun. When high-energy UV-C radiation (100-280 nm) encounters O2, it causes the O2 molecule to dissociate into two separate oxygen atoms (O). These oxygen atoms can then react with other O2 molecules to form ozone (O3). This process can be represented by the following equations: 1. \(O_2 + UV-C \rightarrow 2O\) 2. \(O + O_2 \rightarrow O_3\) Ozone can also dissociate back into oxygen molecules and atoms when it absorbs UV-C or UV-B radiation (280-320 nm), as shown in the following reaction: 3. \(O_3 + UV-C / UV-B \rightarrow O_2 + O\)

Explain the protective role of ozone

Ozone in the stratosphere is crucial for life on Earth, as it plays a major role in protecting living organisms from the harmful effects of ultraviolet radiation. The absorption of UV-C and UV-B radiation by ozone effectively filters out most of these dangerous wavelengths from reaching the Earth's surface. When an ozone molecule absorbs UV-C or UV-B radiation, it undergoes dissociation, as described in reaction 3 above. This process not only helps reduce the amount of harmful UV radiation that reaches the Earth, but also releases an oxygen atom, which can then go on to react with other O2 molecules to create more O3, thus maintaining the ozone layer.

Discuss how ozone's short lifetime doesn't hinder its protective function

While it's true that individual ozone molecules have a relatively short lifetime in the stratosphere (100-200 seconds), this does not imply that the ozone layer as a whole is ineffective in providing protection from UV radiation. The continuous formation and dissociation of ozone occur simultaneously in the stratosphere. While individual ozone molecules may have a short lifetime, new ozone molecules are constantly being formed as a result of reactions between oxygen atoms and O2 molecules, enabled by the absorption of UV-C radiation. This ongoing process ensures that the ozone layer remains sufficiently dense to absorb and filter out harmful UV radiation, providing crucial protection for life on Earth. In summary, even though individual ozone molecules have short lifetimes, the ozone layer as a whole continues to serve as an effective shield against harmful ultraviolet radiation. The ongoing formation and dissociation of ozone molecules in the stratosphere ensures that the ozone layer remains intact and able to protect living organisms from the damaging effects of UV radiation.

Key concepts

Ozone Formation

The formation of ozone in the stratosphere is a fascinating process that begins with the interaction between sunlight and oxygen molecules. When high-energy UV-C radiation from the Sun strikes a molecule of oxygen, it splits the molecule into two separate oxygen atoms. This is the first step in forming ozone. The equation for this reaction is:

• \(O_2 + \text{UV-C} \rightarrow 2O\)

After these oxygen atoms are created, they find and react with intact oxygen molecules. The Magic of Chemistry allows them to form ozone, scientifically noted as \(O_3\). This second step of the process can be described by the equation:

• \(O + O_2 \rightarrow O_3\)

This formation process is crucial as it continuously replenishes the ozone layer, even though each individual ozone molecule doesn't last long. That's why despite their brief existence, ozone molecules collectively manage to provide significant protection from UV radiation.

UV Radiation Protection

Ozone plays an essential role in protecting life on Earth by filtering out harmful ultraviolet radiation from the Sun. This protective function is critical because certain UV rays, like UV-C and UV-B, can cause serious health issues such as skin cancer and cataracts, and can damage the DNA of living organisms. When ozone molecules absorb these UV rays, they prevent them from reaching the Earth's surface. This absorption not only protects the Earth but also contributes to the natural cycle of ozone formation and dissociation:

• UV rays break apart ozone molecules into oxygen and a free oxygen atom.
• The free oxygen atom often later reforms with an \(O_2\) molecule, creating ozone again.

This cyclical process ensures that plenty of ozone is always present in the stratosphere to continue shielding the planet from excessive UV exposure.

Stratospheric Chemistry

The chemistry of the stratosphere, where the ozone layer resides, is complex and dynamic. It’s dominated by the constant interplay between ozone formation and dissociation. This means that while individual ozone molecules might be short-lived, the ozone layer maintains a stable concentration due to its continuous replenishment. In the stratosphere:

• Ozone is steadily formed and destroyed, balancing the overall amount.
• This balance is maintained largely because of sunlight-driven reactions.
• Various factors, such as atmospheric pollutants, can affect these reactions.

Understanding this chemistry helps us appreciate how crucial the ozone layer is in maintaining life on Earth. It draws attention to the need for protecting it against harmful substances, like chlorofluorocarbons, which can disrupt these critical reactions.

Ozone Dissociation

Although ozone molecules don't last long—each only surviving about 100 to 200 seconds—the process of dissociation is what makes the ozone layer resilient. Ozone dissociates into an oxygen molecule and an oxygen atom when it absorbs UV-C or UV-B radiation. This is represented by the equation:

• \(O_3 + \text{UV-C or UV-B} \rightarrow O_2 + O\)

The dissociation might seem destructive, but it’s actually a necessary part of how the ozone layer functions. The free oxygen atom can quickly react with another \(O_2\) to form a new ozone molecule. This process is happening continuously, ensuring that the ozone layer recycles itself and remains effective as a UV shield. It is this symbiotic cycle of creation and destruction, driven by sunlight, that allows the ozone to consistently protect the planet despite the fleeting existence of its individual molecules.