Problem 108 - Chemistry The Central Science In Si Units

Question

Ozone in the upper atmosphere can be destroyed by the following two-step mechanism: $$ \begin{aligned} \mathrm{Cl}(g)+\mathrm{O}_{3}(g) & \longrightarrow \mathrm{ClO}(g)+\mathrm{O}_{2}(g) \\ \mathrm{ClO}(g)+\mathrm{O}(g) & \longrightarrow \mathrm{Cl}(g)+\mathrm{O}_{2}(g) \end{aligned} $$ (a) What is the overall equation for this process? (b) What is the catalyst in the reaction? (c) What is the intermediate in the reaction?

Step-by-Step Solution

Answer

(a) \( \text{O}_3(g) + \text{O}(g) \rightarrow 2\text{O}_2(g) \); (b) \( \text{Cl} \); (c) \( \text{ClO} \).

Step 1: Analyze the given reactions

We are given two reactions involving chlorine, ozone, and other species. The reactions are: 1. \( \text{Cl}(g) + \text{O}_3(g) \rightarrow \text{ClO}(g) + \text{O}_2(g) \) 2. \( \text{ClO}(g) + \text{O}(g) \rightarrow \text{Cl}(g) + \text{O}_2(g) \). Each reaction represents a step in the overall process.

Step 2: Determine the overall chemical equation

To find the overall reaction, we add the two reactions together, allowing cancellation of species that appear on both sides. \( \text{Cl}(g) + \text{O}_3(g) + \text{ClO}(g) + \text{O}(g) \rightarrow \text{ClO}(g) + \text{O}_2(g) + \text{Cl}(g) + \text{O}_2(g) \). After canceling \( \text{Cl}(g) \) and \( \text{ClO}(g) \), the overall reaction is: \( \text{O}_3(g) + \text{O}(g) \rightarrow 2\text{O}_2(g) \).

Step 3: Identify the catalyst

A catalyst is a substance that speeds up a reaction without being consumed. In the given reactions, \( \text{Cl} \) starts as a reactant in the first step and is regenerated in the second step without being consumed. Therefore, \( \text{Cl} \) is the catalyst.

Step 4: Identify the intermediate

An intermediate is a species that is formed in one step and consumed in another. From the given reactions, \( \text{ClO} \) is produced in the first reaction and consumed in the second reaction, making it the intermediate.

Key Concepts

Ozone Depletion

Ozone depletion refers to the thinning of the ozone layer in the Earth's stratosphere. This layer plays a crucial role by protecting living organisms from harmful ultraviolet (UV) radiation from the sun. However, certain chemical reactions can lead to its depletion. One major factor is the presence of chlorine (Cl) atoms released from man-made compounds like chlorofluorocarbons (CFCs). These chlorine atoms can catalyze reactions leading to the breakdown of ozone molecules.Let's consider a two-step mechanism illustrating this:

• First, chlorine reacts with ozone (\( \text{O}_3 \)) to form chlorine monoxide (\( \text{ClO} \)) and oxygen (\( \text{O}_2 \)).
• Second, \( \text{ClO} \) can react with atomic oxygen (\( \text{O} \)) to regenerate chlorine and produce more oxygen.

Combined, these steps reduce the concentration of ozone, contributing to its overall depletion. The understanding of these reactions is key in crafting regulations to limit substances like CFCs that instigate these processes.

Catalysis

Catalysis involves substances called catalysts that increase the rate of a chemical reaction without being consumed in the process. In the context of ozone depletion, chlorine (\( \text{Cl} \)) acts as a catalyst.Here's how:

• In the first step of the mechanism, chlorine reacts with ozone to form \( \text{ClO} \).
• In the second step, the chlorine monoxide \( \text{ClO} \) then reacts with an oxygen atom, and the chlorine is regenerated.

Through this process, chlorine continuously participates in reactions, enabling the conversion of ozone into oxygen without being consumed. This exemplifies the role of catalysts in enabling reactions to proceed rapidly and repeatedly with minimal presence, thus making them extremely influential in both natural and industrial chemical processes.

Reaction Intermediates

Reaction intermediates are often transitory species formed during a chemical reaction that are not present in the final product. They play a critical role in the step-by-step progression of a chemical process.In the ozone depletion mechanism, \( \text{ClO} \) (chlorine monoxide) acts as a reaction intermediate. Here's a breakdown of its role:

• In the initial reaction, chlorine interacts with ozone, producing \( \text{ClO} \) as an intermediate.
• This intermediate then participates in the subsequent reaction with free oxygen, leading to the re-formation of chlorine and the production of \( \text{O}_2 \).

Despite its brief existence, \( \text{ClO} \) is essential in facilitating the decomposition of ozone by providing a pathway that permits the regeneration of the chlorine catalyst. Recognizing the importance of intermediates aids in understanding complex reaction mechanisms and can help in devising strategies to mitigate processes like ozone depletion.