Question

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

Step-by-step solution

Combine the given reactions to find the overall equation

We have the following two reactions: \( \begin{array}{c}{\mathrm{Cl}(g)+\mathrm{O}_{3}(g) \longrightarrow \mathrm{ClO}(g)+\mathrm{O}_{2}(g)} \\\ {\mathrm{ClO}(g)+\mathrm{O}(g) \longrightarrow \mathrm{Cl}(g)+\mathrm{O}_{2}(g)}\end{array} \) Add the reactions together to find the overall equation: \( \begin{array}{c} \mathrm{Cl}(g)+\mathrm{O}_{3}(g) \longrightarrow \mathrm{ClO}(g)+\mathrm{O}_{2}(g) \\ +\text{ } \mathrm{ClO}(g)+\mathrm{O}(g) \longrightarrow \mathrm{Cl}(g)+\mathrm{O}_{2}(g) \\ \hline \mathrm{O}_{3}(g) + \mathrm{O}(g) \longrightarrow 2\mathrm{O}_{2}(g) \end{array} \) The overall equation for the process is \(\mathrm{O}_{3}(g) + \mathrm{O}(g) \longrightarrow 2\mathrm{O}_{2}(g)\).

Identify the catalyst

A catalyst is a substance that participates in a reaction, but is not consumed during the process. In this case, we need to look for a substance that appears in both reactions but does not appear as a reactant or product in the overall equation. Looking at the two steps of the reaction, we can see that the substance "Cl" appears in both steps and is not present in the overall equation. Thus, the catalyst for this reaction is Chlorine (\(\mathrm{Cl}\)).

Identify the intermediate

An intermediate is a substance that is produced in one step of a reaction, but is consumed in a subsequent step. Again, it is not present as a reactant or product in the overall equation. Looking at the two steps of the reaction, we can see that the substance "ClO" appears in both steps and is not present in the overall equation. Thus, the intermediate for this reaction is Chlorine Monoxide (\(\mathrm{ClO}\)).

Results

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 Chlorine (\(\mathrm{Cl}\)). c) The intermediate in the reaction is Chlorine Monoxide (\(\mathrm{ClO}\)).

Key concepts

Catalytic Decomposition of Ozone

The upper atmosphere contains an essential layer of ozone (O₃) which protects life on Earth from harmful ultraviolet (UV) radiation. However, certain chemical processes can lead to the decomposition of this ozone, a phenomenon often referred to as ozone depletion. One such process involves a catalytic cycle where certain substances accelerate the breakdown of ozone without being consumed in the overall reaction.

Let's consider the simple yet illustrative two-step reaction provided in the exercise. Initially, a chlorine atom (Cl) reacts with ozone (O₃) to produce chlorine monoxide (ClO) and oxygen (O₂). Subsequently, the ClO reacts with a single oxygen atom (O) to regenerate the chlorine atom and form additional O₂. When these two reactions are combined to discern the overall effect on ozone, we find that the net result is simply the conversion of a molecule of O₃ and an atom of O into two molecules of O₂, with the chlorine acting as a catalyst.

This sequence exemplifies the catalytic decomposition of ozone—a single chlorine atom can destroy many ozone molecules due to its regeneration cycle and this detachment from the overall equation.

Chlorine as a Catalyst

In the context of ozone layer chemistry, chlorine is a notorious catalyst. A catalyst is a substance that speeds up a chemical reaction but remains unchanged at the end. It effectively lowers the energy barrier for the reaction, making it easier for other substances to react.

Chlorine atoms are particularly effective catalysts for ozone decomposition because they can react with ozone to initiate the breakdown, and yet chlorine itself is reformed by the end of the cycle, ready to commence the process anew. This cyclic nature allows for a single chlorine atom to destroy a vast number of ozone molecules over time.

The source of atmospheric chlorine is primarily from man-made chlorofluorocarbons (CFCs), once used in refrigeration and aerosol applications, which release chlorine atoms when broken down by UV light—a reminder of the profound impact human activity can have on environmentally critical chemistry.

Reaction Intermediates in Ozone Layer

During the catalytic decomposition of ozone, certain molecules are created and destroyed within the reaction cycle; these are known as intermediates. In the exercise, we identify chlorine monoxide (ClO) as an intermediate.

Intermediates such as ClO are crucial for the overall reaction to proceed. After chlorine (Cl) reacts with ozone (O₃) to form ClO, this molecule subsequently reacts with a single oxygen atom (O) to regenerate the chlorine atom and create more oxygen (O₂). Though vital for the transition from reactants to final products, intermediates like ClO do not appear in the overall reaction equation because they are not present at the start or finish—they exist transiently.

Understanding the role of intermediates is essential for comprehending complex reaction mechanisms in atmospheric chemistry, which often include several sequential steps involving numerous transitional species.