Question

Draw the Lewis structure for the chlorofluorocarbon CFC-11, ${\mathrm{CFCl}}_3$. What chemical characteristics of this substance allow it to effectively deplete stratospheric ozone?

Short answer

The Lewis structure for CFC-11, ${\mathrm{CFCl}}_3$, consists of a central carbon atom bonded to a fluorine atom and three chlorine atoms, with each peripheral atom having three lone pairs. The chemical characteristics that enable CFC-11 to effectively deplete stratospheric ozone include the weaker Cl-C bond, allowing UV radiation to dissociate chlorine radicals. These chlorine radicals can then react with ozone to form chlorine monoxide and molecular oxygen. The chlorine radicals continue to destroy ozone molecules by reacting with the oxygen atoms, leading to a severe depletion of stratospheric ozone.

Step-by-step solution

Count the total number of valence electrons

To draw the Lewis structure, we first need to count the total number of valence electrons in the CFC-11 molecule. The number of valence electrons for each atom is as follows: - Carbon (C) has 4 valence electrons - Fluorine (F) has 7 valence electrons - Chlorine (Cl) has 7 valence electrons Considering there are three Cl atoms, the total number of valence electrons in the CFC-11 molecule will be: Total valence electrons = 4 (C) + 7 (F) + 3 * 7 (Cl) = 4 + 7 + 21 = 32

Determine the central atom

We need to determine the central atom in the molecule, which would typically be the atom with the lowest electronegativity (unless it is hydrogen). In CFC-11, the central atom is the Carbon (C).

Draw the skeletal structure

We will now connect the atoms by single bonds to form a skeletal structure. The carbon atom will be at the center, bonded to the fluorine and the three chlorine atoms. Cl | Cl - C - F | Cl

Distribute the remaining electrons

Now, we should distribute the remaining valence electrons as lone pairs to complete the octet for each atom. We used 8 valence electrons in the single bonds (2 electrons for each bond). There are 24 valence electrons left to be distributed. We add 6 electrons (3 lone pairs) to each of the three chlorine atoms and 6 electrons (3 lone pairs) to the fluorine atom. Cl : : : | : : Cl - C - F : : Cl : :

Verify the octet rule

Verify that all atoms satisfy the octet rule; meaning they have 8 electrons in their valence shell. Carbon has 4 bonds (4 * 2 = 8 electrons), Fluorine has 1 bond and 3 lone pairs (2 + 6 = 8 electrons), and each Chlorine atom has 1 bond and 3 lone pairs (2 + 6 = 8 electrons). All atoms satisfy the octet rule.

Chemical characteristics that deplete stratospheric ozone

CFC-11 can effectively deplete ozone in the stratosphere due to the formation of highly reactive chlorine radicals. The Cl-C bond in CFC-11 is weaker than the other C-X (X = halogen) bonds due to the large size and lower electronegativity of chlorine, making it easier for UV radiation to break the bond. When CFC-11 absorbs UV radiation, a high-energy photon causes the dissociation of a chlorine atom from the molecule: ${\mathrm{CFCl}}_3+\text{UV}⟶{\mathrm{CFCl}}_2+{\mathrm{Cl}}^{\ast }$ The chlorine radical (Cl*) reacts with ozone, removing an oxygen atom, and forming chlorine monoxide (ClO) and molecular oxygen (O2). ${\mathrm{Cl}}^{\ast }+{\mathrm{O}}_3⟶\mathrm{ClO}+{\mathrm{O}}_2$ Chlorine monoxide can then react with an oxygen atom, regenerating the chlorine radical and forming molecular oxygen. $\mathrm{ClO}+\mathrm{O}⟶{\mathrm{Cl}}^{\ast }+{\mathrm{O}}_2$ As a result, the chlorine radicals can continuously destroy thousands of ozone molecules before being removed from the stratosphere by other chemical reactions, leading to a severe depletion of stratospheric ozone.