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: $$ \begin{array}{l} \mathrm{Cl}+\mathrm{O}_{3} \longrightarrow \mathrm{ClO}+\mathrm{O}_{2} \\ \mathrm{ClO}+\mathrm{O} \longrightarrow \mathrm{Cl}+\mathrm{O}_{2} \end{array} $$ (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? (e) Draw potential-energy versus reaction progress diagrams for the uncatalyzed and catalyzed (by Cl) destruction of ozone: \(\mathrm{O}_{3}+\mathrm{O} \longrightarrow 2 \mathrm{O}_{2} .\) Use the thermodynamic data in Appendix 2 to determine whether the reaction is exothermic or endothermic.

Short answer

(a) \( \mathrm{O}_{3} + \mathrm{O} \rightarrow 2\mathrm{O}_{2} \). (b) Cl is a catalyst; ClO is an intermediate. (c) Fluorine radicals are not effective here. (d) Ethane forms \(\mathrm{HCl}\), reducing \(\mathrm{Cl}\) radicals. (e) Reaction is exothermic, catalyzed by Cl.

Step-by-step solution

Combine Steps (a) Reaction of Cl and ClO with Ozone

To write the overall reaction, combine the two given reactions: 1. \( \mathrm{Cl} + \mathrm{O}_{3} \rightarrow \mathrm{ClO} + \mathrm{O}_{2} \)2. \( \mathrm{ClO} + \mathrm{O} \rightarrow \mathrm{Cl} + \mathrm{O}_{2} \)Adding these two reactions cancels out the intermediates (\(\mathrm{Cl}\) and \(\mathrm{ClO}\)) to give the overall reaction:\[ \mathrm{O}_{3} + \mathrm{O} \rightarrow 2\mathrm{O}_{2} \]

Define Roles (b) Roles of Cl and ClO

The \(\mathrm{Cl}\) atom is a catalyst: it is regenerated in the reaction process and is not consumed over the course of the reaction. \(\mathrm{ClO}\) acts as a reactive intermediate; it is formed and then consumed in the steps leading to the overall reaction.

Explain Irrelevance (c) Fluorine Radical

The fluorine radical does not appear in these reactions because it does not effectively catalyze ozone depletion. Chlorine radicals specifically have a high reactivity with ozone, which fluorine radicals do not demonstrate in this context.

Describe Mechanism (d) Role of Ethane

Adding hydrocarbons like ethane \((\mathrm{C}_{2}\mathrm{H}_{6})\) provides hydrogen atoms which can react with \(\mathrm{Cl}\) radicals to form \(\mathrm{HCl}\). This reaction decreases the concentration of free \(\mathrm{Cl}\) radicals available to deplete ozone, thereby slowing the destruction of ozone.

Draw and Analyze (e) Energy Diagrams and Reaction Type

Using thermodynamic data, find that the reaction \(\mathrm{O}_{3} + \mathrm{O} \rightarrow 2\mathrm{O}_{2}\) is exothermic. Draw energy diagrams with reaction progress on the x-axis and potential energy on the y-axis. The uncatalyzed reaction has a higher activation energy compared to when the process is catalyzed by \(\mathrm{Cl}\). The catalyzed path shows a lower peak energy, indicating reduced activation energy.

Key concepts

Chlorofluorocarbons (CFCs)

Chlorofluorocarbons, or CFCs, are compounds that consist of chlorine, fluorine, and carbon. Initially popular as refrigerants and aerosol propellants, CFCs are now infamous for their role in ozone layer depletion. When these compounds reach the stratosphere, they are broken down by ultraviolet (UV) radiation. This process releases chlorine radicals, active agents in ozone destruction. Notably, a CFC molecule like \(\text{CFCl}_3\), under UV light, decomposes into \(\text{CFCl}_2\) and a chlorine radical (Cl).

• CFCs are stable until exposed to UV light.
• The chlorine radicals they release are highly reactive.
• They have been largely phased out due to measures like the Montreal Protocol aimed at protecting the ozone layer.

Despite their historical utility, CFCs are now considered detrimental due to their lasting impact on ozone integrity.

Catalysis

In the context of ozone depletion, catalysis is a critical concept. It refers to the acceleration of a chemical reaction through the involvement of a catalyst. In this case, chlorine atoms act as catalysts in the breakdown of ozone molecules.
The decomposition of ozone involves intermediate steps where chlorine is temporarily transformed into chlorine monoxide ( ClO ). However, in the end, chlorine is regenerated, allowing it to proceed to catalyze further ozone destruction without being consumed.

• Chlorine serves as a catalyst, immensely speeding up ozone breakdown.
• The Cl atom is involved in a cycle of continuous ozone degradation, emphasizing its catalytic role.
• The involvement of a catalyst dramatically lowers the energy barrier needed to degrade ozone.

Through catalysis, even a small amount of catalyst, like chlorine, can wreak significant damage on ozone over extended periods.

Reaction Kinetics

Reaction kinetics provides insight into the rates at which chemical reactions occur. In the case of ozone degradation, understanding these kinetics is pivotal. The sequence of reactions involving chlorine radicals happens at a molecular level and defines how quickly and effectively ozone is depleted.
The process is initiated when chlorine radicals react with ozone ( O_3 ) to form chlorine monoxide ( ClO ) and oxygen. Next, this ClO reacts with an oxygen atom to regenerate Cl and produce more oxygen.

• Kinetics explains the reaction speed and pathways.
• These reactions are fast and efficient under the conditions in the stratosphere.
• Understanding kinetics helps in devising strategies to mitigate ozone depletion.

By studying reaction kinetics, we can appreciate how swiftly chemical reactions such as these can transform environmental conditions.

Stratosphere Chemistry

The stratosphere is the second major layer of Earth's atmosphere, and it is where the ozone layer resides. Stratospheric chemistry involves understanding how various compounds, including chlorofluorocarbons (CFCs), interact and undergo transformations in this region.
In the stratosphere, the high UV radiation causes CFCs to dissociate, forming active products like chlorine radicals. These radicals then participate in reactions that break down ozone ( O_3 ) molecules, reducing the effectiveness of the ozone layer, which serves as Earth's shield against harmful UV radiation.

• The stratosphere offers unique conditions - high UV levels, low density, and specific temperature ranges.
• Chemistry here is vital for understanding processes like ozone formation and depletion.
• Human actions, including the release of CFCs, have shown how easily stratospheric chemistry can be disrupted.

Understanding stratosphere chemistry is crucial for policies aimed at protecting and potentially replenishing the ozone layer.