Problem 20 - Chemistry: The Central Science

Question

Draw the Lewis structure for the chlorofluorocarbon CFC-11, CFCl \(_{3}\). What chemical characteristics of this substance allow it to effectively deplete stratospheric ozone?

Step-by-Step Solution

Answer

The Lewis structure for CFC-11 (CFCl\(_{3}\)) is: Cl | Cl - C - Cl | F CFC-11 has the following chemical characteristics that allow it to effectively deplete stratospheric ozone: 1. Stability: CFCs are incredibly stable molecules because of the strong carbon-halogen bonds, which allows them to reach the ozone layer. 2. UV absorption: In the stratosphere, CFC-11 absorbs high-energy ultraviolet (UV-C) radiation and undergoes photodissociation, releasing chlorine radicals. 3. Catalytic ozone destruction: The chlorine radicals released in the photodissociation of CFC-11 act as catalysts for ozone depletion, destroying thousands of ozone molecules, leading to the depletion of the ozone layer.

Step 1: Count valence electrons

CFC-11 contains one carbon atom, three chlorine atoms, and one fluorine atom. To find the total number of valence electrons, add the valence electrons for each element. Carbon has 4 valence electrons, chlorine has 7 valence electrons, and fluorine has 7 valence electrons. Total valence electrons = (1 Carbon x 4) + (3 Chlorine x 7) + (1 Fluorine x 7) = 4 + 21 + 7 = 32 valence electrons

Step 2: Identify central atom

Carbon is less electronegative than both chlorine and fluorine, so it will be the central atom in the molecule.

Step 3: Arrange peripheral atoms

Position the three chlorine atoms and one fluorine atom around the central carbon atom. Each chlorine and fluorine atom will form a single bond with carbon, and this will help to fulfill the octet rule.

Step 4: Complete the octet rule

Draw the single bonds between carbon and the three chlorine atoms, and carbon and the fluorine atom. This will require 8 valence electrons (4 bonds with 2 electrons each). There are 24 valence electrons remaining (32 - 8 = 24). Place them as lone pairs on the chlorine and fluorine atoms to complete the octet rule. Each chlorine and fluorine atom needs 6 additional electrons in non-bonding pairs to complete its octet (8 electrons per atom).

Step 5: Draw the Lewis structure

The Lewis structure for CFC-11 is: Cl | Cl - C - Cl | F Each chlorine atom has three lone pairs, and the fluorine atom has three lone pairs as well.

Step 6: Identify the chemical characteristics

CFC-11 has the following chemical characteristics that allow it to effectively deplete stratospheric ozone: 1. Stability: CFCs are incredibly stable molecules because of the strong carbon-halogen bonds. This stability allows them to persist in the atmosphere for an extended period and reach the ozone layer. 2. UV absorption: Upon reaching the stratosphere, the molecule absorbs high-energy ultraviolet (UV-C) radiation and undergoes photodissociation, which leads to the release of chlorine radicals: CFCl\(_{3}\) + UV-C → CFCl\(_{2}\) + Cl\(^{•}\) 3. Catalytic ozone destruction: The chlorine radicals that foram as a result of the photodissociation of CFC-11 act as catalysts for ozone depletion by undergoing the following reaction with ozone: Cl\(^{•}\) + O\(_{3}\) → ClO\(^{•}\) + O\(_{2}\) The chlorine monoxide radical (ClO\(^{•}\)) can then react with atomic oxygen, generating a chlorine radical again and propagating the cycle: ClO\(^{•}\) + O → Cl\(^{•}\) + O\(_{2}\) This results in the overall destruction of ozone: O\(_{3}\) + O → 2 O\(_{2}\) Each chlorine radical can destroy thousands of ozone molecules, which contributes to the depletion of the ozone layer.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom, which play a crucial role in chemical bonding and reactions.
They determine how an atom will interact with others, and are vital in drawing Lewis structures.
In the case of chlorofluorocarbon (CFC-11), knowing the valence electrons of carbon, chlorine, and fluorine helps in constructing its Lewis structure.

• **Carbon:** Carbon has 4 valence electrons.
• **Chlorine:** Each chlorine atom contributes 7 valence electrons.
• **Fluorine:** Fluorine also adds 7 valence electrons.

To find the total, sum the contributions: Carbon (1 x 4) + Chlorine (3 x 7) + Fluorine (1 x 7) = 32 valence electrons.
These electrons are distributed to form bonds and lone pairs, following key rules like the octet rule.

Octet Rule

The octet rule is a guiding principle in chemistry, governing how atoms bond.
It states that atoms tend to form bonds until they are surrounded by eight valence electrons, resembling the electron configuration of a noble gas.
This rule is key in drawing Lewis structures. In the case of CFC-11:

• **Central Atom:** Carbon acts as the central atom because it is less electronegative than chlorine and fluorine.
• **Bonding:** Each chlorine and fluorine atom forms a single bond with carbon, using one pair of electrons per bond.
• **Lone Pairs:** After bonding, the remaining electrons (24 in total for CFC-11) are arranged as lone pairs on the peripheral atoms—chlorine and fluorine— to ensure each has 8 electrons in their valence shell.

While the octet rule is a powerful tool, there are exceptions, particularly involving elements with larger d-orbitals. However, for CFCs, the rule holds, leading to stable configurations.

Ozone Depletion

Ozone depletion refers to the thinning of the Earth's ozone layer caused by chemical reactions. This layer is crucial for protecting life from harmful ultraviolet (UV) radiation.
Chlorofluorocarbons like CFC-11 are significant contributors to this phenomenon due to their stable structure allowing them to drift up to the stratosphere.

• **Persistence:** CFCs do not degrade easily, allowing them to accumulate in the atmosphere over time.
• **Reactivity:** When exposed to UV light, CFCs release chlorine atoms, which catalyze the breakdown of ozone.

This process involves the repetitive destruction of ozone molecules, significantly impacting the ozone layer and, subsequently, increasing UV exposure on Earth.
Mitigating ozone depletion involves regulating substances like CFCs to prevent further damage.

Photodissociation

Photodissociation is the process by which a chemical compound is broken down by photons.
This is a key step in the atmospheric breakdown of substances like CFCs, enabling ozone layer depletion. In the atmospheric context:

• **UV Interaction:** When CFCs like CFCl eographically _{3} are exposed to UV-C radiation, they absorb this energy.
• **Molecular Breakdown:** The energy causes a bond in the molecule to break, releasing a chlorine radical ( Cl^{•} ).
• **Chain Reaction:** This radical is highly reactive and plays a central role in ozone depletion by engaging in further reactions.

Photodissociation demonstrates how stable molecules like CFCs can transform under specific conditions, leading to significant environmental impacts. Understanding this process highlights the importance of protecting the stratospheric ozone against such reactive substances.