Question

Fluorocarbons are effective scavengers for ozone due to: (a) Photolytic production of oxides of nitrogen (b) Photolytic decomposition of \(\mathrm{O}_{3}\) by \(\mathrm{Cl}\) into \(\mathrm{O}_{2}\) (c) Photolytic decomposition of \(\mathrm{O}_{3}\) producing \(\mathrm{O}_{2}\) (d) Photolytic reaction of \(\mathrm{O}_{2}\) producing \(\mathrm{Cl}\) radicals

Short answer

(b) Photolytic decomposition of \(\mathrm{O}_3\) by \(\mathrm{Cl}\) into \(\mathrm{O}_2\).

Step-by-step solution

Identify the chemical process

Fluorocarbons, such as chlorofluorocarbons (CFCs), can release chlorine atoms in the atmosphere through photolytic reactions. Chlorine is known to react with ozone (\(\mathrm{O}_3\)), breaking it down into molecular oxygen (\(\mathrm{O}_2\)). This process decreases the concentration of ozone in the stratosphere.

Analyze each choice

Review each option by determining if it correctly describes the interaction between fluorocarbons and ozone. - (a) Photolytic production of oxides of nitrogen does not involve fluorocarbons. - (b) Photolytic decomposition of \(\mathrm{O}_3\) by \(\mathrm{Cl}\) into \(\mathrm{O}_2\) is directly related to the ozone depletion by chlorine. - (c) Photolytic decomposition of \(\mathrm{O}_3\) producing \(\mathrm{O}_2\) does not specifically mention the role of \(\mathrm{Cl}\). - (d) Photolytic reaction of \(\mathrm{O}_2\) producing \(\mathrm{Cl}\) radicals refers to the initiation step where \(\mathrm{Cl}\) radicals are produced but not directly related to ozone being a scavenger.

Choose the correct answer

Based on Step 2 analysis, the correct answer is (b). Fluorocarbons generate \(\mathrm{Cl}\) atoms that lead to the decomposition of ozone into \(\mathrm{O}_2\). This specific mechanism identifies how fluorocarbons act as ozone scavengers.

Key concepts

Chlorofluorocarbons (CFCs)

Chlorofluorocarbons, often abbreviated as CFCs, are man-made compounds primarily consisting of chlorine, fluorine, and carbon. They have historically been used in a variety of applications, including refrigeration, air conditioning, and aerosol propellants.

CFCs are colorless, odorless, and non-toxic, making them quite desirable for industrial use. However, their stability is a double-edged sword. Because CFCs do not break down easily, they can drift into the upper atmosphere or stratosphere, lasting up to several decades. This persistence allows them to contribute significantly to ozone depletion over time.

• CFCs were first introduced in the 1930s.
• Their environmental impact was not fully understood until the late 20th century.
• Steps have been taken globally, such as the Montreal Protocol, to phase out their use due to their detrimental impact on the ozone layer.

Photolytic Reactions

Photolytic reactions are chemical reactions initiated by light, particularly ultraviolet (UV) radiation from the sun. In the case of ozone depletion, photolytic reactions play a crucial role by breaking down chlorofluorocarbons (CFCs) in the stratosphere.

When CFCs are exposed to UV radiation, they undergo a photolytic reaction, causing the release of chlorine atoms. These chlorine atoms are highly reactive and can initiate further reactions that lead to the destruction of ozone molecules.

• Photolysis is the first step in a series of reactions that can degrade ozone.
• The UV radiation required for these reactions is most abundant in the stratosphere.
• It only takes a single chlorine atom to destroy thousands of ozone molecules, highlighting the efficiency of photolytic reactions in breaking down ozone.

Stratospheric Chemistry

Stratospheric chemistry involves the study of chemical reactions and processes occurring in the stratosphere, a layer of Earth's atmosphere just above the troposphere, typically from about 10 to 50 kilometers above sea level. This layer is critical as it contains the ozone layer, which protects life on Earth by absorbing the majority of the sun's harmful ultraviolet radiation.

Within the stratosphere, complex chemical interactions occur, involving not just ozone (O\(_3\)) but also various other molecules, including those released from chlorofluorocarbons (CFCs).

• The ozone layer absorbs most UV-B and all UV-C radiation, crucial for life.
• Chemical reactions in the stratosphere are notably influenced by human-made compounds, such as CFCs.
• The introduction of chlorine and bromine atoms from anthropogenic sources can accelerate ozone depletion.

Understanding stratospheric chemistry is essential for comprehending how human actions impact global atmospheric conditions.

Chlorine Atoms

Chlorine atoms are a key player in the process of ozone depletion. Derived mainly from chlorofluorocarbons (CFCs), these atoms are released into the atmosphere through photolytic reactions. Once freed, chlorine atoms react with ozone (O\(_3\)) in a two-step process.

First, a chlorine atom reacts with an ozone molecule, resulting in the formation of chlorine monoxide (ClO) and molecular oxygen (O\(_2\)). The equation for this reaction can be represented as:

\[ \text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2 \]
In the second step, the chlorine monoxide can react with a free oxygen atom, regenerating the chlorine atom and releasing additional molecular oxygen:

\[ \text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2 \]

• This cyclical reaction can repeat thousands of times with each individual chlorine atom.
• The persistence of chlorine in the stratosphere means it can continuously contribute to ozone degradation.
• The impact of chlorine on ozone is why measures have been taken to reduce CFC emissions globally.