Question

Ozone in the upper atmosphere can be destroyed by the following two-step mechanism: $$ \begin{gathered} \mathrm{Cl}(g)+\mathrm{O}_{3}(g) \longrightarrow \mathrm{ClO}(g)+\mathrm{O}_{2}(g) \\ \mathrm{ClO}(g)+\mathrm{O}(g) \longrightarrow \mathrm{Cl}(g)+\mathrm{O}_{2}(g) \end{gathered} $$ (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?

Short answer

The overall equation for the ozone destruction process is \(O_{3}(g) + O(g) \longrightarrow 2O_{2}(g)\). The catalyst in this reaction is the Cl (Chlorine) atom, as it participates in the reaction steps but remains chemically unchanged at the end. The intermediate is ClO (Chlorine monoxide), as it forms during the reaction and gets consumed later in another step. The main difference between the catalyst and the intermediate is their roles in the reaction: the catalyst speeds up the reaction, while the intermediate is a temporary species that forms and gets consumed during the reaction process.

Step-by-step solution

Determine the overall equation

To find the overall equation for this two-step mechanism, we can add the equations of both steps and then simplify by cancelling out common species that appear on both sides of the reaction. Step 1: \(Cl(g) + O_{3}(g) \longrightarrow ClO(g) + O_{2}(g) \) Step 2: \(ClO(g) + O(g) \longrightarrow Cl(g) + O_{2}(g) \) Adding the equations together, we get: \(Cl(g) + O_{3}(g) + ClO(g) + O(g) \longrightarrow ClO(g) + O_{2}(g) + Cl(g) + O_{2}(g) \) Now, we can simplify by cancelling out the common species (Cl and ClO) that appear on both sides of the reaction: \(O_{3}(g) + O(g) \longrightarrow 2O_{2}(g) \) So, the overall equation for this process is: \(O_{3}(g) + O(g) \longrightarrow 2O_{2}(g) \)

Identify the catalyst

A catalyst is a substance that takes part in a reaction and speeds up the rate of the reaction but remains chemically unchanged at the end of the reaction. If we look at the two-step mechanism and the overall equation, we can see that the Cl (Chlorine) atom is participating in both steps of the reaction, but it is not consumed in the overall reaction. Therefore, Cl (Chlorine) is the catalyst in this reaction.

Identify the intermediate

An intermediate is a species that is formed during a reaction but does not appear in the overall equation as it gets consumed during the reaction. In the two-step mechanism, we can see that ClO (Chlorine monoxide) is formed in the first step and then consumed in the second step. It does not appear in the overall equation. Therefore, ClO (Chlorine monoxide) is the intermediate in this reaction.

Distinguish between the catalyst and the intermediate

The main difference between the catalyst and the intermediate in this reaction is their roles and behavior during the reaction: 1. Catalyst (Cl) - It participates in the reaction steps but remains chemically unchanged at the end of the reaction. It helps to speed up the reaction by lowering the activation energy. 2. Intermediate (ClO) - It is formed during one of the reaction steps and is consumed later in another step. It does not appear in the overall equation as it is not a part of the initial reactants or final products. In this ozone destruction reaction, the catalyst (Cl - Chlorine) is responsible for speeding up the reaction, while the intermediate (ClO - Chlorine monoxide) is a temporary species that forms and later gets consumed during the reaction process.