Problem 113 - Chemistry The Central Science

Question

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

Step-by-Step Solution

Answer

(a) The overall equation for this process is: \(O_3 (g) + O (g) \longrightarrow 2 O_2 (g)\). (b) The catalyst in the reaction is Chlorine (Cl). We know this because it is present in both steps of the mechanism but not consumed in the overall reaction. (c) The intermediate in the reaction is Chlorine monoxide (ClO). We distinguish it from the catalyst because it is formed and consumed within the reaction steps playing the role of a bridge, while the catalyst is not consumed during the whole process.

Step 1: Combine the two equations

To find the overall equation, we need to add the two given equations. Firstly, let's write them down: 1) \(Cl (g) + O_3 (g) \longrightarrow ClO (g) + O_2 (g)\) 2) \(ClO (g) + O (g) \longrightarrow Cl (g) + O_2 (g)\) Now, let's sum the equations: \(Cl (g) + O_3 (g) + ClO (g) + O (g) \longrightarrow ClO (g) + O_2 (g) + Cl (g) + O_2 (g)\)

Step 2: Simplify the overall equation

Now, let's eliminate equal substances from both sides of the combined equation: - \(ClO (g)\) appears on both sides, so we can eliminate it. - Also, \(Cl (g)\) appears on both sides, so we can eliminate it too. Now, the simplified overall equation is: \(O_3 (g) + O (g) \longrightarrow 2 O_2 (g)\) (a) So, the overall equation for this process is: \(O_3 (g) + O (g) \longrightarrow 2 O_2 (g)\).

Step 3: Identify the catalyst

A catalyst is a substance that is not consumed in the overall reaction and increases the reaction rate. From the overall equation, we can see that Chlorine (Cl) appears in both steps but is not part of the overall reaction. (b) Therefore, the catalyst in the reaction is Chlorine (Cl). We know this because it is present in both steps of the mechanism, but it is not consumed in the overall reaction.

Step 4: Identify the intermediate

An intermediate is a substance that is formed in one step and consumed in another step. In this mechanism, Chlorine monoxide (ClO) is formed in the first step and consumed in the second step. (c) So, the intermediate in the reaction is Chlorine monoxide (ClO). We distinguish it from the catalyst because it is formed and consumed within the reaction steps playing the role of a bridge, while the catalyst is not consumed during the whole process.

Key Concepts

Catalyst Identification

In the world of chemical reactions, a catalyst plays a special role as a helper. It speeds up the reaction without being used up itself. In the case of ozone depletion, we must identify which substance acts as the catalyst in this multi-step process.
When we look at the two-step reaction mechanism:

• Step 1: \( \mathrm{Cl}(g) + \mathrm{O}_3(g) \longrightarrow \mathrm{ClO}(g) + \mathrm{O}_2(g) \)
• Step 2: \( \mathrm{ClO}(g) + \mathrm{O}(g) \longrightarrow \mathrm{Cl}(g) + \mathrm{O}_2(g) \)

Chlorine (Cl) appears at the beginning and also reappears at the end of the second step without being consumed.
This means that Chlorine is not part of the end result of the overall chemical reaction. Such behavior is characteristic of a catalyst. It's effectively recycled, allowing it to facilitate the transformation of ozone without being used up, making it the catalyst of the reaction.

Reaction Intermediate

In multi-step chemical reactions, intermediates often act as bridges between the starting reactants and the final products. They appear briefly during the reaction but do not stick around.
The reaction intermediate differs from the catalyst mainly because it is both created and consumed during the process, often in consecutive steps.

In this specific sequence:

• In Step 1: Chlorine monoxide (ClO) is produced.
• In Step 2: The same ClO is used up in a reaction that regenerates Chlorine (Cl) and forms \(\mathrm{O}_2\).

Thus, Chlorine monoxide is our reaction intermediate. It doesn't appear in the overall chemical equation but plays a crucial role within the steps of the reaction process. It connects the transformation stages, by being formed only to be immediately used in the subsequent reaction.

Overall Chemical Equation

To understand the overall impact of the reactions involved in ozone depletion, it is essential to derive the overall chemical equation. This equation provides a summary, showing how reactants transform into products in a simplified form without additional complexities such as intermediates or catalysts.

From the combination of two steps we have:

• Step 1: \(\mathrm{Cl}(g) + \mathrm{O}_3(g) \rightarrow \mathrm{ClO}(g) + \mathrm{O}_2(g)\)
• Step 2: \(\mathrm{ClO}(g) + \mathrm{O}(g) \rightarrow \mathrm{Cl}(g) + \mathrm{O}_2(g)\)

When these steps are combined, the Chlorine and Chlorine monoxide molecules cancel each other out because they do not appear in the final result; they restore themselves, indicating they are not used up in the reaction.
This leaves us with a simplified final equation:
\[\mathrm{O}_3(g) + \mathrm{O}(g) \longrightarrow 2 \mathrm{O}_2(g)\]This end equation shows the true transformation occurring in the process, displaying simply how ozone and oxygen atoms contribute to form oxygen molecules. It gives a cleaner perspective of the overall effect of the reaction mechanism.