Problem 122 - Chemistry Matter and Change

Question

Identify which molecule is polar. (Chapter 8 ) $$\begin{array}{ll}{\text { a. } \operatorname{SiH}_{4}} & {\text { c. } H_{2} S} \\ {\text { b. } N O_{2}} & {\text { d. } N C l_{3}}\end{array}$$

Step-by-Step Solution

Answer

The molecule \(H_2S\) is polar.

Step 1: Determine the molecule's geometry

First, we need to identify the molecular geometry of each molecule, as the shape of the molecule affects its polarity. - For \( ext{SiH}_4\), the geometry is tetrahedral.- For \( ext{NO}_2\), the geometry is bent due to the presence of a lone electron pair.- For \( ext{H}_2S\), the geometry is also bent because of two lone pairs on sulfur.- For \( ext{NCl}_3\), the geometry is trigonal pyramidal due to one lone pair on nitrogen.

Step 2: Analyze electronegativity difference

Next, we evaluate the electronegativity difference between the central atom and the surrounding atoms to estimate bond polarity.- In \( ext{SiH}_4\), silicon and hydrogen have a small electronegativity difference, making the bonds nonpolar.- In \( ext{NO}_2\), nitrogen and oxygen have a significant electronegativity difference, making the bonds polar.- In \( ext{H}_2S\), sulfur and hydrogen have moderate electronegativity differences, leading to polar bonds.- In \( ext{NCl}_3\), nitrogen and chlorine have a moderate electronegativity difference, making the bonds polar.

Step 3: Evaluate net dipole moment

For a molecule to be polar, it must have an asymmetrical shape or unequal charge distribution resulting in a net dipole moment.- \( ext{SiH}_4\) is symmetrical, resulting in no net dipole moment.- \( ext{NO}_2\), with a bent shape and polar bonds, has a net dipole moment.- \( ext{H}_2S\) is asymmetrical due to its bent shape and has a net dipole moment.- \( ext{NCl}_3\)'s trigonal pyramidal shape also leads to an asymmetric charge distribution, resulting in a net dipole moment.

Step 4: Identify which molecules are polar

From the analysis:- \( ext{SiH}_4\) is nonpolar.- \( ext{NO}_2\) is polar.- \( ext{H}_2S\) is polar.- \( ext{NCl}_3\) is polar. Since multiple molecules are polar, choose one particular polar molecule. In this case, we'll say \( ext{H}_2S\) is polar due to its bent shape and polar bonds.

Key Concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. To determine a molecule's polarity, understanding its geometry is crucial. The shape of a molecule affects how evenly charge is distributed across it, which in turn determines if the molecule is polar or nonpolar.

For example, in the molecule \(\text{SiH}_4\), which has a tetrahedral geometry, the symmetrical distribution of identical hydrogen atoms around silicon means any dipole moments cancel out, resulting in a nonpolar molecule.

In contrast, both \(\text{H}_2S\) and \(\text{NO}_2\) exhibit bent geometries due to the presence of lone pairs of electrons on the central atoms. This bending results in an asymmetrical distribution of charge, making these molecules polar. Similarly, \(\text{NCl}_3\) has a trigonal pyramidal shape due to one lone pair on nitrogen, leading to an uneven charge distribution and therefore polarity. Recognizing these shapes helps identify the potential for a molecule to be polar.

Electronegativity Difference

Electronegativity is a measure of an atom's ability to attract electrons in a chemical bond. The difference in electronegativity between atoms in a bond dictates the polarity of that bond.

If the difference in electronegativity between bonded atoms is significant, the bond is polar. This means that the electron density shifts toward the more electronegative atom, causing a partial negative charge on it and a partial positive charge on the less electronegative atom.

• In \(\text{NO}_2\), the difference between nitrogen and oxygen is large, resulting in polar bonds.
• In \(\text{H}_2S\), even though the difference is moderate between sulfur and hydrogen, the bonds are polar.
• In \(\text{NCl}_3\), nitrogen and chlorine also have a moderate difference, creating polar bonds.
• However, in \(\text{SiH}_4\), the difference is minimal, rendering the bonds nonpolar.

Understanding the electronegativity difference is essential when evaluating whether or not a molecule is polar.

Net Dipole Moment

A net dipole moment occurs in molecules where there is an uneven distribution of electrical charge, leading to a region of positive and a region of negative charge. For a molecule to have a net dipole moment and thus be polar, the molecule must have an asymmetrical shape or differing bond polarities that do not cancel out.

As seen in molecules like \(\text{H}_2S\), the asymmetric bent shape and partial charges do not cancel, resulting in a net dipole moment. Similarly, \(\text{NO}_2\) and \(\text{NCl}_3\) also have net dipole moments due to their asymmetric shapes and polar bonds.

Conversely, in \(\text{SiH}_4\), despite having polar bonds, the symmetric geometric configuration cancels out the dipole moments, resulting in no net dipole moment and a nonpolar molecule. Recognizing the importance of shape and symmetry alongside bond polarity is critical in predicting molecular polarity.

Polar Bonds

Polar bonds are central to understanding molecular polarity. They occur when two atoms in a molecule have different electronegativities, leading to an unequal sharing of electrons. This creates a dipole, with a positive and negative end.

In molecules such as \(\text{H}_2S\), \(\text{NO}_2\), and \(\text{NCl}_3\), each bond is polar due to the differences in electronegativity between the central atoms and surrounding atoms. In these cases, polar bonds combined with molecular geometry result in an uneven charge distribution, making these molecules polar overall.

However, in molecules like \(\text{SiH}_4\), although there may be small electronegativity differences, the geometrical and symmetrical arrangement causes any potential dipoles to cancel out, leading to nonpolar bonds. Understanding whether a bond is polar and if they align to create an overall dipole is key to determining molecular polarity.