Problem 86 - Chemistry The Central Science In Si Units

Question

The substance chlorine monoxide, \(\mathrm{ClO}(g)\), is important in atmospheric processes that lead to depletion of the ozone layer. The ClO molecule has an experimental dipole moment of \(1.24 \mathrm{D},\) and the \(\mathrm{Cl}-\mathrm{O}\) bond length is \(160 \mathrm{pm} .(\mathbf{a})\) Determine the magnitude of the charges on the \(\mathrm{Cl}\) and \(\mathrm{O}\) atoms in units of the electronic charge, \(e .(\mathbf{b})\) Based on the electronegativities of the elements, which atom would you expect to have a partial negative charge in the ClO molecule? (c) Using formal charges as a guide, propose the dominant Lewis structure for the molecule. (d) The anion \(\mathrm{ClO}^{-}\) exists. What is the formal charge on the Cl for the best Lewis structure for \(\mathrm{ClO}^{-}\) ?

Step-by-Step Solution

Answer

(a) ±0.16e; (b) Oxygen; (c) Single bond with O having lone pairs; (d) 0.

Step 1: Convert the bond length to meters

The bond length given is 160 pm, which we need to convert to meters. Since 1 pm = 1 x 10^{-12} m, the bond length in meters is: \( 160 \times 10^{-12} \text{ m} \) or \( 1.60 \times 10^{-10} \text{ m} \).

Step 2: Calculate the charge using the dipole moment

The dipole moment \( \mu \) given is 1.24 D. 1 Debye (D) = \( 3.34 \times 10^{-30} \text{ C\cdot m} \). The dipole moment in C·m is \( 1.24 \times 3.34 \times 10^{-30} \text{ C\cdot m} = 4.1416 \times 10^{-30} \text{ C\cdot m} \). The dipole moment \( \mu = q \times d \), thus \( q = \frac{\mu}{d} \). Therefore, \( q = \frac{4.1416 \times 10^{-30}}{1.60 \times 10^{-10}} \). This gives \( q \approx 2.5885 \times 10^{-20} \text{ C} \).

Step 3: Convert the charge to units of electronic charge

The electronic charge \( e = 1.602 \times 10^{-19} \text{ C} \). The charge on each atom in units of \( e \) is calculated as: \( \frac{2.5885 \times 10^{-20}}{1.602 \times 10^{-19}} \approx 0.1616 \). Thus, each atom has a charge of approximately \( \pm 0.16 e \).

Step 4: Determine which atom is partially negative

Electronegativities: Cl (3.16) and O (3.44) according to the Pauling scale. Oxygen is more electronegative, therefore, it will have the partial negative charge in the ClO molecule.

Step 5: Propose the dominant Lewis structure for ClO

The formal charges guide our Lewis structure. ClO likely has a single bond between Cl and O, with three lone pairs on oxygen and two on chlorine. This structure provides Cl with a formal charge of 0 and O with -1, since oxygen is more electronegative. This minimizes formal charges and gives the more electronegative atom (oxygen) a negative formal charge.

Step 6: Find the formal charge on Cl in ClO⁻ anion

For \( \text{ClO}^- \), the best Lewis structure involves extending the bonding, potentially forming a double bond to minimize charges. Cl would have one formal charge: \[ \text{Formal charge on Cl} = \text{valence electrons} - (\text{non-bonding electrons} + \frac{1}{2} \text{bonding electrons}) \]. Typically for ClO⁻, it has 7 valence electrons originally, \( \text{Formal charge on Cl} = 7 - (6 + 1) = 0 \).

Key Concepts

Ozone Depletion

The thinning of the Earth's ozone layer is a significant environmental concern. Ozone, a molecule made of three oxygen atoms, acts as a shield by absorbing the majority of the sun's harmful ultraviolet radiation. Chlorine from chlorine monoxide (\( \text{ClO} \)) can participate in reactions that decompose ozone, leading to ozone depletion. This occurs mainly in Earth's stratosphere, where the presence of chlorofluorocarbons (CFCs) and their photodissociation leads to the release of chlorine atoms.
Once free, these chlorine atoms can catalytically destroy large amounts of ozone by cycling through reactions involving ozone and molecular oxygen, without being consumed in the process.

• This depletion process is especially prominent in colder regions, contributing to the formation of ozone holes, notably over Antarctica.
• Efforts have been made globally to curb substances that release ozone-depleting chemicals, fostering recovery of the ozone layer over time.

Dipole Moment

The dipole moment is a measure of the polarity of a molecule, indicating the separation of positive and negative charges. In the case of \( \text{ClO} \), the dipole moment is experimentally determined to be 1.24 D (debye).
This value helps in understanding the charge distribution within the molecule. The dipole moment (\( \mu \)) is calculated by the product of the magnitude of charge (\( q \)) and the distance (\( d \)) between the charges:\[ \mu = q \times d \]The sizable dipole moment of chlorine monoxide suggests an uneven distribution of electrons between the chlorine and oxygen atoms, with oxygen pulling more electron density towards itself due to its higher electronegativity.

• Molecules with significant dipole moments tend to interact strongly with polar solvents, like water.
• These interactions greatly influence the physical properties and reactivity of the substance.

Lewis Structure

The Lewis structure is a diagrammatic approach to representing the bonds, lone pairs, and electron distribution within a molecule. For \( \text{ClO} \), drawing its Lewis structure requires understanding valence electrons and electronegativity.
Chlorine has 7 valence electrons, while oxygen also has 6 valence electrons. The best Lewis structure for this molecule is where a single bond connects Cl and O, while each atom retains appropriate lone pairs.

• Typically, chlorine will have one lone pair, and oxygen will have three, ensuring that the formal charges are minimized.
• Formal charges guide modifications in the structure, aiming to reflect the most stable electronic arrangement by placing negative formal charges on more electronegative atoms.

The resulting configuration helps explain the observed dipole moment and the chemical behavior of \( \text{ClO} \).

Formal Charge

Formal charge is a theoretical charge assigned to atoms within a molecule, assuming that electrons in all chemical bonds are shared equally. To calculate the formal charge, follow the formula:\[ \text{Formal charge} = \text{valence electrons} - (\text{non-bonding electrons} + \frac{1}{2} \text{bonding electrons}) \]
In \( \text{ClO} \), we're particularly concerned with keeping the charges minimal. The formal charge assignment often aids in predicting the most stable structure:

• For \( \text{ClO} \), oxygen has a formal charge of -1, while chlorine has a formal charge of 0 in the molecule's neutral form.
• In the ion \( \text{ClO}^{-} \), charges adjust to maintain the anion's overall negative charge, with the possible presence of a double bond between Cl and O to balance electron sharing.

Understanding formal charges can be critical in predicting the likelihood of reactions and molecular interactions for different chemical species.