Question

Loss of atmospheric ozone has led to an ozone "hole" over Antarctica. The loss occurs in part through three consecutive steps: (1) Chlorine atoms react with ozone \(\left(\mathrm{O}_{3}\right)\) to form chlorine monoxide and molecular oxygen. (2) Chlorine monoxide forms \(\mathrm{ClOOCl}\). (3) \(\mathrm{ClOOCl}\) absorbs sunlight and breaks into chlorine atoms and molecular oxygen. (a) Write a balanced equation for each step. (b) Write an overall balanced equation for the sequence.

Short answer

Step 1: \( \text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2 \)Step 2: \( 2 \text{ClO} \rightarrow \text{ClOOCl} \)Step 3: \( \text{ClOOCl} \rightarrow 2 \text{Cl} + \text{O}_2 \)Overall: \( \text{O}_3 \rightarrow 2 \text{O}_2 \)

Step-by-step solution

- Write the equation for Step 1

In the first step, chlorine atoms react with ozone to form chlorine monoxide and molecular oxygen. The reaction is: \[ \text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2 \]

- Write the equation for Step 2

In the second step, chlorine monoxide reacts to form \text{ClOOCl}. The reaction is: \[ 2 \text{ClO} \rightarrow \text{ClOOCl} \]

- Write the equation for Step 3

In the third step, \text{ClOOCl} absorbs sunlight and breaks into chlorine atoms and molecular oxygen. The reaction is: \[ \text{ClOOCl} \rightarrow 2 \text{Cl} + \text{O}_2 \]

- Write the overall balanced equation

To find the overall balanced equation, combine the equations from steps 1, 2, and 3. Notice that \text{ClO} and \text{ClOOCl} are intermediates and cancel out in the overall reaction. Add the equations together: \[ \text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2 \] \[ 2 \text{ClO} \rightarrow \text{ClOOCl} \] \[ \text{ClOOCl} \rightarrow 2 \text{Cl} + \text{O}_2 \] Combining them, we get: \[ \text{Cl} + \text{O}_3 \rightarrow \text{Cl} + 2 \text{O}_2 \] The chlorine atoms cancel out on both sides, leaving the overall balanced equation: \[ \text{O}_3 \rightarrow \text{O}_2 + \text{O}_2 \] or simply: \[ \text{O}_3 \rightarrow 2 \text{O}_2 \]

Key concepts

Balanced Chemical Equations

Balanced chemical equations are crucial for understanding and representing chemical reactions. They ensure the same number of each type of atom on both sides of the equation, following the Law of Conservation of Mass. In the context of ozone depletion, each step involves individual reactions that contribute to the overall process. For example, in the first reaction:
\[ \text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2 \]
We start with one chlorine atom (Cl) and one ozone molecule (\( \text{O}_3 \)). The products are chlorine monoxide (\( \text{ClO} \)) and molecular oxygen (\( \text{O}_2 \)). This reaction is balanced because the number and types of atoms on each side are equal.
Balancing each step individually is fundamental before combining them into an overall balanced equation. By ensuring balance in each step, especially when dealing with intermediates and final products, we can understand the chemical changes occurring in complex processes like ozone depletion.

Chlorine-Catalyzed Ozone Depletion

Chlorine-catalyzed ozone depletion involves a series of chemical reactions primarily driven by the presence of chlorine atoms in the atmosphere. Chlorine atoms, often originating from chlorofluorocarbons (CFCs), play a significant role in the breakdown of ozone (\( \text{O}_3 \)).
Here is how the process works, step-by-step:

• In Step 1, a chlorine atom reacts with an ozone molecule to produce chlorine monoxide (\( \text{ClO} \)) and molecular oxygen (\( \text{O}_2 \)).
• In Step 2, two \( \text{ClO} \) molecules combine to form \( \text{ClOOCl} \) through a intermediary reaction.
• In Step 3, \( \text{ClOOCl} \) absorbs sunlight and decomposes, releasing two chlorine atoms and one more molecule of oxygen (\( \text{O}_2 \)).

The chlorine atoms act as a catalyst because they are not consumed in the overall reaction, allowing them to continuously break down ozone molecules. The net result of this series of reactions is the conversion of ozone into molecular oxygen, as shown in the overall balanced equation:
\[ \text{O}_3 \rightarrow 2 \text{O}_2 \]
This cycle can continue, leading to significant ozone depletion in the stratosphere, particularly over regions like Antarctica.

Ozone Hole Over Antarctica

The ozone hole over Antarctica is a dramatic example of the consequences of chlorine-catalyzed ozone depletion. The phenomenon is particularly severe over Antarctica due to specific atmospheric conditions.
Here are key points to understand why the ozone hole forms:

• Extremely cold temperatures in the Antarctic stratosphere during the winter lead to the formation of polar stratospheric clouds. These clouds provide a surface for the chemical reactions that release chlorine atoms from compounds like CFCs.
• With the return of sunlight in the spring, the reactions accelerate, increasing chlorine atom concentration, which then breaks down ozone.
• The recurring destruction of ozone creates a significant 'hole' in the ozone layer, resulting in reduced ozone levels above the continent.

The decrease in ozone allows more harmful ultraviolet (UV) radiation to reach the Earth's surface, contributing to health risks like skin cancer and cataracts, as well as environmental damage.