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?

Short answer

(a) The overall equation for this process is: \(\mathrm{O}_{3}(g)+\mathrm{O}(g) \longrightarrow 2\mathrm{O}_{2}(g)\) (b) The catalyst in the reaction is \(\mathrm{Cl}(g)\) (chlorine). (c) The intermediate in the reaction is \(\mathrm{ClO}(g)\) (chlorine monoxide).

Step-by-step solution

Add both reactions together to find the overall equation

Add both reactions together to determine the overall equation. You can sum up the reactions horizontally by adding the reactants and the products separately on both sides of the equation. $$ \begin{aligned} \mathrm{Cl}(g)+\mathrm{O}_{3}(g) & \longrightarrow \mathrm{ClO}(g)+\mathrm{O}_{2}(g) \\ \mathrm{ClO}(g)+\mathrm{O}(g) & \longrightarrow \mathrm{Cl}(g)+\mathrm{O}_{2}(g) \\ \hline \mathrm{Cl}(g)+\mathrm{O}_{3}(g)+\mathrm{O}(g)+\mathrm{ClO}(g) & \longrightarrow \mathrm{ClO}(g)+\mathrm{O}_{2}(g)+\mathrm{Cl}(g)+\mathrm{O}_{2}(g) \end{aligned} $$ Now, eliminate the common terms on the left and right sides of the arrow, which are Cl and ClO: $$ \begin{aligned} \mathrm{O}_{3}(g)+\mathrm{O}(g) & \longrightarrow \mathrm{O}_{2}(g)+\mathrm{O}_{2}(g) \end{aligned} $$ Combine the two O₂(g) on the right side: $$ \begin{aligned} \mathrm{O}_{3}(g)+\mathrm{O}(g) & \longrightarrow 2\mathrm{O}_{2}(g) \end{aligned} $$ This equation shows the overall process. (a) The overall equation for this process is: $$ \begin{aligned} \mathrm{O}_{3}(g)+\mathrm{O}(g) & \longrightarrow 2\mathrm{O}_{2}(g) \end{aligned} $$

Identify the catalyst

The catalyst will be present on both sides of the overall equation because it is not consumed in the reaction. - In this case, \(\mathrm{Cl}(g)\) and \(\mathrm{ClO}(g)\) are present in both sides of the overall equation, leading us to believe that they are the catalysts. (b) The catalyst in the reaction is \(\mathrm{Cl}(g)\) (chlorine).

Determine the intermediate in the reaction

The intermediate is a substance that appears in both reactions but not in the overall equation. - In this case, \(\mathrm{ClO}(g)\) is present in both reactions but not in the overall equation, so it is the intermediate. (c) The intermediate in the reaction is \(\mathrm{ClO}(g)\) (chlorine monoxide).

Key concepts

Catalyst

In the realm of chemical reactions, a catalyst plays a vital role. It speeds up a reaction without being consumed in the process. Essentially, a catalyst provides an alternative energy pathway for the reaction, which requires lower activation energy. This means reactions happen faster or more efficiently.

In atmospheric chemistry, catalysts often participate in cycles that influence significant environmental processes. In our given reaction involving ozone depletion,

• Chlorine, represented as \( \mathrm{Cl}(g) \), acts as the catalyst.
• Although it partakes in each reaction step, it does not appear in the net equation. This tells us it isn't depleted nor permanently altered.

By serving multiple reaction cycles, a single chlorine atom can facilitate the decomposition of many ozone molecules, making it particularly harmful when released into the atmosphere.

Chemical intermediate

While studying chemical reactions, we come across compounds called intermediates. These are short-lived species formed during a reaction process, appearing only in the intermediate steps but not in the final or overall reaction equation. They are essential in bridging the reactants to the products.

In the two-step mechanism for ozone destruction,

• Chlorine monoxide, \( \mathrm{ClO}(g) \), is a chemical intermediate.
• It appears during the reaction to temporarily bind with oxygen atoms, facilitating the breakup of ozone molecules.

ClO does not appear in the overall equation and highlights how intermediates are crucial transient species in complex reaction mechanisms.

Atmospheric chemistry

Atmospheric chemistry is the study of chemical processes that occur in the Earth's atmosphere. It encompasses all chemical reactions, interactions, and the cycles they fuel between atmospheric particles. One critical concern in this field is ozone depletion.

The ozone layer, situated in the Earth's stratosphere, plays an essential role by absorbing harmful ultraviolet radiation. However, human activities release chemicals such as chlorofluorocarbons (CFCs) that release chlorine atoms—catalysts in the depletion of ozone. These reactions are:

• Initiated by the release of chlorine atoms into the atmosphere.
• Chlorine atoms then react with ozone \( \mathrm{O}_3 \), albeit not used in the overall destruction reaction.

Understanding these reactions and the role of various substances can aid in forming policies and technologies that protect our ozone layer and maintain life-sustaining atmospheric conditions.