Question

In recent years ozone in the stratosphere has been depleted at an alarmingly fast rate by chlorofluorocarbons (CFCs). A CFC molecule such as \(\mathrm{CFCl}_{3}\) is first decomposed by UV radiation: $$ \mathrm{CFCl}_{3} \longrightarrow \mathrm{CFCl}_{2}+\mathrm{Cl} $$ The chlorine radical then reacts with ozone as follows: $$ \begin{array}{c} \mathrm{Cl}+\mathrm{O}_{3} \longrightarrow \mathrm{ClO}+\mathrm{O}_{2} \\ \mathrm{ClO}+\mathrm{O} \longrightarrow \mathrm{Cl}+\mathrm{O}_{2} \end{array} $$ (a) Write the overall reaction for the last two steps. (b) What are the roles of \(\mathrm{Cl}\) and \(\mathrm{ClO} ?\) (c) Why is the fluorine radical not important in this mechanism? (d) One suggestion to reduce the concentration of chlorine radicals is to add hydrocarbons such as ethane \(\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)\) to the stratosphere. How will this Work?

Short answer

The overall reaction for the last two steps is \(\mathrm{O}_3 + \mathrm{O} \longrightarrow 2 \mathrm{O}_2\). \(\mathrm{Cl}\) acts as a catalyst and \(\mathrm{ClO}\) is an intermediate in these reactions. Fluorine radicals are not important as they don't react readily with ozone. Hydrocarbons like ethane can reduce chlorine radicals by reacting with them to form less reactive compounds.

Step-by-step solution

Combine the reactions

To finalize the overall reaction for the last two steps, simply add the two reactions together and cancel out any terms that appear on both sides of the reaction equations: \(\mathrm{Cl} + \mathrm{O}_3 \longrightarrow \mathrm{ClO} + \mathrm{O}_2\) and \(\mathrm{ClO} + \mathrm{O} \longrightarrow \mathrm{Cl} + \mathrm{O}_2\). Adding these gives \(\mathrm{Cl} + \mathrm{O}_3 + \mathrm{O} \longrightarrow \mathrm{ClO} + \mathrm{O}_2 + \mathrm{Cl} + \mathrm{O}_2\). Canceling out the \(\mathrm{Cl}\) on both sides and the \(\mathrm{O}_2\) on the right gives \(\mathrm{O}_3 + \mathrm{O} \longrightarrow 2 \mathrm{O}_2\).

Roles of \(\mathrm{Cl}\) and \(\mathrm{ClO}\)

\(\mathrm{Cl}\) acts as a catalyst in this sequence of reactions. It is used in the first reaction, and regenerated in the second, allowing it to participate again in the first reaction. \(\mathrm{ClO}\), on the other hand, is an intermediate. It is formed in the first reaction and used up in the second.

The role of fluorine

The fluorine radical is not important in this mechanism because it does not react readily with ozone. This is due to its low reactivity compared to chlorine.

The effect of adding hydrocarbons

Adding hydrocarbons such as ethane could reduce the concentration of chlorine radicals, as these hydrocarbons would react readily with chlorine, forming less reactive compounds and consuming the chlorine radicals in the process.

Key concepts

Stratosphere

The stratosphere is a crucial layer of Earth's atmosphere. It's located above the troposphere and below the mesosphere. This region begins about 10 kilometers (6 miles) above Earth's surface and extends up to 50 kilometers (31 miles). Its importance lies in the presence of the ozone layer.
The ozone layer, found in the lower part of the stratosphere, plays a vital role in protecting life on Earth. It absorbs most of the Sun's harmful ultraviolet (UV) radiation, reducing the risk of skin cancer and other damaging effects on living organisms.

• The stratosphere is characterized by stable atmospheric conditions, with little vertical mixing.
• Temperatures in this layer generally increase with altitude due to ozone absorption of UV radiation.

Understanding the composition and behavior of the stratosphere is essential, particularly when studying phenomena like ozone depletion.

Chlorofluorocarbons

Chlorofluorocarbons (CFCs) are a group of chemical compounds comprising chlorine, fluorine, and carbon. They were once widely used in various products like refrigerants and aerosol propellants due to their stability and non-flammability.
However, these very properties led to a significant environmental issue. When CFCs are released into the atmosphere, they eventually rise up into the stratosphere.

• In the stratosphere, CFCs are broken down by ultraviolet radiation.
• This breakdown releases chlorine atoms, which are extremely effective at depleting ozone molecules.

The environmental impact of CFCs became apparent by the discovery of the ozone hole over Antarctica, prompting global agreements like the Montreal Protocol, aimed at phasing out these harmful substances.

Reaction Mechanism

A chemical reaction mechanism describes the step-by-step process by which reactants are transformed into products. In the context of ozone depletion, it involves several important stages.
When UV radiation hits CFCs, chlorine radicals are released. These radicals catalytically destroy ozone through a sequence of reactions.
This can be shown through the following steps:

• Cl + O extsubscript{3} → ClO + O extsubscript{2}
• ClO + O → Cl + O extsubscript{2}

Upon adding these reactions, the overall reaction can be simplified to: O extsubscript{3} + O → 2O extsubscript{2}.
Chlorine acts as a catalyst, because it is regenerated, meaning a small amount can destroy a large amount of ozone. ClO acts as an intermediate, formed and used up during the reaction sequence.

UV Radiation

Ultraviolet (UV) radiation is a form of energy emitted by the Sun. It is divided into three types: UV-A, UV-B, and UV-C, with UV-C being the most energetic and harmful.
Fortunately, the ozone layer absorbs most UV-B and all UV-C radiation, protecting living organisms on Earth. However, UV radiation also plays a crucial role in chemical reactions in the stratosphere.

• UV radiation breaks down CFC molecules, releasing chlorine radicals.
• It triggers the chain reactions responsible for the depletion of ozone.

With ozone depletion, more UV-B reaches Earth's surface, raising concerns about increased health risks and environmental impacts. Understanding UV radiation and its interactions with atmospheric components is essential in assessing and addressing ozone layer health.