Problem 41 - Chemistry and Chemical Reactivity

Question

Which of the following molecules is (are) polar? For each polar molecule, indicate the direction of polarity-that is, which is the negative end, and which is the positive end of the molecule. (a) \(\mathrm{BeCl}_{2}\) (b) \(\mathrm{HBF}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{Cl}\) (d) \(\mathrm{SO}_{3}\)

Step-by-Step Solution

Answer

\(\mathrm{HBF}_{2}\) and \(\mathrm{CH}_3\mathrm{Cl}\) are polar. Polarity: \(\mathrm{HBF}_{2}\) toward fluorine, \(\mathrm{CH}_3\mathrm{Cl}\) toward chlorine.

Step 1: Determine Molecular Geometry

Examine the geometry of each molecule to determine whether it's symmetrical or not. For example:- **(a) \(\mathrm{BeCl}_{2}\):** Linear geometry with symmetry.- **(b) \(\mathrm{HBF}_{2}\):** Trigonal planar around boron, less symmetrical.- **(c) \(\mathrm{CH}_3\mathrm{Cl}\):** Tetrahedral shape, asymmetrical with respect to \(\mathrm{Cl}\).- **(d) \(\mathrm{SO}_{3}\):** Trigonal planar and highly symmetrical.

Step 2: Assess Electronegativity Differences

Identify the differences in electronegativity between bonded atoms:- **(a) \(\mathrm{BeCl}_{2}\):** Chlorine (Cl) is more electronegative than beryllium (Be), but they are symmetrically opposed, canceling out polarity.- **(b) \(\mathrm{HBF}_{2}\):** Hydrogen (H) and fluorine (F) have a significant electronegativity difference, resulting in polarity.- **(c) \(\mathrm{CH}_3\mathrm{Cl}\):** Chlorine (Cl) is more electronegative than carbon (C), causing polarity.- **(d) \(\mathrm{SO}_{3}\):** Sulfur (S) and oxygen (O) have differences, but symmetry cancels them out.

Step 3: Determine Overall Molecular Polarity

Using geometry and electronegativity, determine molecular polarity:- **(a) \(\mathrm{BeCl}_{2}\) is nonpolar** due to linear symmetry cancelling the dipole moments.- **(b) \(\mathrm{HBF}_{2}\) is polar** because of asymmetrical dipoles around boron.- **(c) \(\mathrm{CH}_3\mathrm{Cl}\) is polar** because of the asymmetrical lone pair arrangement and presence of \(\mathrm{Cl}\).- **(d) \(\mathrm{SO}_{3}\) is nonpolar** as dipoles cancel out due to symmetry.

Step 4: Identify Direction of Polarity

For polar molecules, indicate the direction of dipoles:- **(b) \(\mathrm{HBF}_{2}\):** The negative end is towards fluorine, and the positive end is towards hydrogen.- **(c) \(\mathrm{CH}_3\mathrm{Cl}\):** The negative end is towards chlorine, and the positive end is near the carbon bonded to hydrogen atoms.

Key Concepts

Molecular Geometry

Molecular geometry is crucial in determining a molecule's polarity. It's the three-dimensional arrangement of atoms in a molecule. Different geometries can lead to symmetrical or asymmetrical structures, affecting the distribution of charge.

• Linear geometry, as observed in BeCl extsubscript{2}, means the molecule's shape is straight. This symmetry can result in the cancellation of dipole moments, making the molecule nonpolar.
• Trigonal planar geometry, seen in HBF extsubscript{2}, often leads to situations where some dipole moments do not cancel, causing polarity. The distribution of atoms and the angles between bonds are key in assessing such symmetry.
• Tetrahedral geometry, like in CH extsubscript{3}Cl, can result in an asymmetrical distribution of charge, leading to a polar molecule. Even small changes in shape can lead to significant differences in polarity.

Understanding geometry helps in predicting whether the molecule will have a net dipole moment, which directly links to molecular polarity.

Electronegativity

Electronegativity refers to an atom's ability to attract electrons towards itself in a chemical bond. The difference in electronegativity between atoms can create polar bonds.

• A bond between two identical atoms is nonpolar, as they share electrons equally. In BeCl extsubscript{2}, while the Be-Cl bonds have some polarity, the symmetrical alignment cancels this out.
• In molecules like HBF extsubscript{2}, the electronegativity difference between hydrogen, boron, and fluorine leads to uneven electron distribution. Fluorine, being highly electronegative, draws electrons more towards itself, causing a polar molecule.
• CH extsubscript{3}Cl is polar due to the significant difference in electronegativity between carbon and chlorine, where chlorine pulls electrons more towards itself.

Electronegativity is a fundamental aspect when evaluating molecular polarity because it determines how electrons are shared or pulled within molecules.

Dipole Moments

Dipole moments occur due to the separation of charges in a molecule. It is a vector quantity, having both a magnitude and direction. Dipole moments help in understanding molecular polarity:

• In BeCl extsubscript{2}, the opposing dipole moments cancel each other due to the linear arrangement, resulting in a nonpolar molecule.
• For HBF extsubscript{2}, the asymmetry created by the arrangement of hydrogen, boron, and fluorine results in a polar molecule with a net dipole moment in the direction from hydrogen to fluorine, making F the negative end.
• CH extsubscript{3}Cl shows a dipole moment directed towards the chlorine atom because of its higher electronegativity compared to carbon, resulting in the chlorine end being negatively charged.

The dipole moment is a clear indicator of molecular polarity, and understanding the direction of dipole moments can help determine the positive and negative ends of a polar molecule.