Question

Draw the Lewis structure for the chlorofluorocarbon CFC-11, \(\mathrm{CFCl}_{2}\). What chemical characteristics of this substance allow it to effectively deplete stratospheric ozone?

Short answer

The Lewis structure for CFC-11 (\(\mathrm{CFCl}_{2}\)) has carbon in the center, surrounded by fluorine and two chlorine atoms, with single bonds between them. The molecule effectively depletes stratospheric ozone due to its stability, ability to reach high altitudes, and its capacity for releasing chlorine atoms. The strong carbon-fluorine and carbon-chlorine bonds contribute to its stability and, upon exposure to ultraviolet radiation, the carbon-chlorine bond breaks, releasing chlorine atoms which catalyze the breakdown of ozone molecules.

Step-by-step solution

Determine the total number of valence electrons

To draw the Lewis structure, we first need to determine the total number of valence electrons in the molecule. For this, we'll look at the valence electrons of each atom: - Carbon (C) has 4 valence electrons, - Fluorine (F) has 7 valence electrons, - Chlorine (Cl) has 7 valence electrons, and there are 2 chlorine atoms. The total number of valence electrons is: \(4 + 7 + 2 \times 7 = 25\).

Arrange the atoms and place single bonds

We'll arrange the atoms with carbon in the center, surrounded by fluorine and the 2 chlorine atoms. This is because carbon can form four bonds, while the other atoms can only form single bonds in this case. Place single bonds between the carbon and each of the surrounding atoms (F, Cl, Cl).

Distribute the remaining valence electrons

We have used 6 valence electrons so far for the single bonds. We now have 19 remaining valence electrons to distribute. Fill the octets for the surrounding atoms (F, Cl, Cl) by adding pairs of electrons around each of them. Each pair of electrons counts as 2.

Check the octets and the Lewis structure

After filling the octets, check if all atoms have full octets. In this case, all atoms have full octets: - Carbon has 4 bonds (1 to F and 2 to Cl atoms), - Fluorine has 1 bond (to C) and 6 non-bonding electrons, - Both Chlorine atoms have 1 bond (to C) and 6 non-bonding electrons. Now, we can see the Lewis structure for CFC-11 (\(\mathrm{CFCl}_{2}\)): ``` F | Cl - C - Cl ```

Determine the chemical characteristics that deplete stratospheric ozone

CFC-11 is an efficient ozone depleter due to its stability, ability to reach high altitudes, and its capacity for releasing chlorine atoms. The key features that contribute to these characteristics are: 1. Carbon-fluorine and carbon-chlorine bonds: These bonds are quite strong and make CFC-11 stable. This means it can survive long enough to reach the stratosphere and take part in ozone depletion reactions. 2. Releasing chlorine atoms: Ultraviolet radiation in the stratosphere can break the carbon-chlorine bond in CFC-11, releasing chlorine atoms which can catalyze the breakdown of ozone molecules. One chlorine atom can break down thousands of ozone molecules, making CFC-11 highly effective in depleting stratospheric ozone.

Key concepts

Valence Electrons

Valence electrons play a pivotal role in the chemical bonding and structure of molecules. These electrons are located in the outermost shell of an atom and determine how atoms connect with each other to form molecules. In our exercise, we focused on determining the total number of valence electrons in the molecule CFC-11 (CFCl_2). Carbon has 4 valence electrons, fluorine has 7, and chlorine (with two atoms present) contributes 14 valence electrons. The summation results in 25 valence electrons to be used in bonding and filling the octets for a stable molecule.

To understand how these valence electrons interact, arranging them correctly is crucial. We start by forming bonds—each bond contains two electrons—following the 'octet rule', which stipulates that each atom prefers to have eight valence electrons to resemble the electron arrangement of noble gases. This rule guides us to draw a stable Lewis structure, where electrons are shared or distributed as lone pairs around the atoms, ensuring full outer shells.

Molecular Geometry

The molecular geometry defines the spatial arrangement of atoms in a molecule and significantly influences the physical and chemical properties of the substance. After drawing the Lewis structure of CFC-11, the next step is to interpret the molecular geometry. In this particular case, with a central carbon atom bonded to two chlorine atoms and one fluorine atom, the molecular shape is trigonal planar. This geometry results from the carbon atom forming three single bonds and not having any lone pairs, distributing the atoms at roughly 120-degree angles around it.

The Significance of Electron Pair Repulsion

The VSEPR (Valence Shell Electron Pair Repulsion) theory explains that electron pairs around a central atom will arrange themselves to minimize repulsion. In the CFC-11 molecule, there are no remaining electron pairs on the central carbon to repulse the bonding pairs, hence maintaining a flat molecular structure. It is this geometry that indirectly bears relevance to stratospheric ozone depletion, as the structural stability is key in allowing CFC compounds to diffuse into the upper atmosphere.

Stratospheric Ozone Depletion

Stratospheric ozone depletion is a crucial environmental issue, with chlorofluorocarbons (CFCs) like CFC-11 being integral to this process. Ozone depletion refers to the thinning of the ozone layer in Earth's stratosphere, primarily due to the release of chlorine atoms from stable compounds such as CFC-11.

When CFC-11 rises to the stratosphere, ultraviolet (UV) radiation cleaves the carbon-chlorine bond, liberating atomic chlorine. These chlorine atoms act as catalysts, initiating a chain reaction that destroys ozone molecules. A single chlorine atom can break down thousands of ozone molecules, significantly depleting this vital layer that protects life on Earth from harmful UV radiation.

Environmental Impact

The role of CFC-11 as an ozone depleter is further accentuated by its stability and longevity in the atmosphere, which allows many of these molecules to make their way into the upper layers where the ozone is present. The reduction of the ozone layer leads to increased UV radiation reaching the Earth's surface, which can cause health issues like skin cancer and also has damaging effects on ecosystems.