Problem 127 - Chemistry Matter and Change

Question

Determine which of the molecules are polar. Explain your answers. $$ \begin{array}{l}{\text { a. } \mathrm{CH}_{3} \mathrm{Cl}} \\ {\text { b. ClF }} \\ {\text { c. } \mathrm{NCl}_{3}} \\ {\text { d. } \mathrm{BF}_{3}} \\\ {\text { e. } \mathrm{CS}_{2}}\end{array} $$

Step-by-Step Solution

Answer

Molecules a. CH₃Cl, b. ClF, and c. NCl₃ are polar. Molecules d. BF₃ and e. CS₂ are nonpolar.

Step 1: Determine Electronegativity Differences and Bond Polarity

To determine the polarity of each molecule, first evaluate the electronegativity of each element in the bond. The greater the difference in electronegativity between two atoms, the more polar the bond. For example, in \( \mathrm{CH}_3 \mathrm{Cl} \), chlorine (3.0) is more electronegative than carbon (2.5), resulting in polar C-Cl bonds. This analysis needs to be repeated for each molecule listed.

Step 2: Assess Molecular Shape and Symmetry

Once you have identified the bond polarities, examine the molecular geometry to determine overall polarity. For \( \mathrm{CH}_3 \mathrm{Cl} \), the structure is tetrahedral with a lone pair on chlorine causing a dipole. Molecules like \( \mathrm{BF}_3 \) and \( \mathrm{CS}_2 \) are symmetrical (trigonal planar and linear, respectively) and hence nonpolar despite having polar bonds.

Step 3: Analyze Overall Dipole Moment

A molecule is polar if it has a net dipole moment resulting from polar bonds that do not cancel out due to symmetry. \( \mathrm{CH}_3 \mathrm{Cl} \), \( \mathrm{ClF} \), and \( \mathrm{NCl}_3 \) have non-canceling dipole moments making them polar, while \( \mathrm{BF}_3 \) and \( \mathrm{CS}_2 \) have dipoles that cancel out, making them nonpolar.

Key Concepts

Electronegativity

Electronegativity is a key concept in determining the molecular polarity of a compound. It measures an atom's ability to attract shared electrons in a bond. By comparing electronegativities of atoms bonded together, we find out if specific bonds are polar. When there’s a significant electronegativity difference between two atoms, electrons are unevenly shared, creating a polar bond. For instance:

• In the molecule \( \text{CH}_3\text{Cl} \), chlorine has an electronegativity of 3.0, while carbon's is 2.5. Chlorine attracts electrons more strongly than carbon, making the C-Cl bond polar.
• Similarly, the bond in \( \text{ClF} \) where the difference between chlorine (3.0) and fluorine (3.98) is significant, causing a polar bond.

Electronegativity helps determine initial bond polarities but does not directly dictate the molecule's overall polarity. To ascertain overall polarity, we must consider the molecule's geometry. Understanding both bond polarity and molecular geometry is crucial for determining if the molecule has a net dipole moment.

Molecular Geometry

Molecular geometry plays a vital role in the polarity of a molecule. It refers to the three-dimensional shape formed by atoms in a molecule. Even if a molecule has polar bonds, it may not be polar overall if its shape allows for the symmetrical distribution of these bonds.Key points about molecular geometry:

• A tetrahedral shape like in \( \text{CH}_3\text{Cl} \) contributes to polarity because one atom (Cl) at the vertex causes an asymmetrical charge distribution.
• In \( \text{BF}_3 \), the trigonal planar structure evens out any polarity because the symmetrically positioned fluorines cause their dipoles to cancel out.
• Linear molecules like \( \text{CS}_2 \) are also nonpolar due to their symmetric geometry despite polar bonds.

By analyzing the symmetry in molecular geometry, you can predict how the molecule will behave in terms of polarity. This understanding helps identify if a molecule's dipole moments will cancel each other out or produce a net dipole moment.

Dipole Moment

The dipole moment is the vector sum of the individual bond dipoles. It indicates the direction and magnitude of a molecule's overall polarity. A molecule with a non-zero dipole moment is polar, meaning it has a positive end and a negative end.Here’s how dipole moments work:

• Molecules like \( \text{CH}_3\text{Cl} \) have a tetrahedral shape. The C-Cl bond dipole doesn’t cancel out, resulting in a net dipole moment and making it polar.
• For \( \text{NCl}_3 \), the trigonal pyramidal shape results in a net dipole moment because the non-bonding electrons at the nitrogen cause an imbalance.
• Molecules such as \( \text{BF}_3 \) and \( \text{CS}_2 \) have dipoles that cancel each other out due to their symmetrical geometry, resulting in a zero dipole moment and being nonpolar.

The overall dipole moment determines how molecules interact with external electric fields and affects their physical properties like solubility and boiling points. Recognizing whether a molecule is polar or nonpolar based on its dipole moment is essential for predicting molecular behavior in various contexts.