Problem 44 - Chemistry The Molecular Science

Question

Which of these molecules has a net dipole moment? For each of these polar molecules, indicate the direction of the dipole in the molecule. (a) \(\mathrm{XeF}_{2}\) (b) \(\mathrm{H}_{2} \mathrm{~S}\) (c) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (d) \(\mathrm{HCN}\)

Step-by-Step Solution

Answer

H₂S, CH₂Cl₂, and HCN have a net dipole moment.

Step 1: Understanding the Concept of Dipole Moment

A molecule will have a net dipole moment if the individual bond dipoles don't cancel each other out. In linear and symmetrical molecules, bond dipoles often cancel, resulting in no net dipole moment. If a molecule is asymmetrical, its dipole moments might not cancel, creating a net dipole moment.

Step 2: Analyzing XeF₂

The molecular geometry of XeF₂ is linear with the two fluorine atoms symmetrically arranged around the central xenon atom. Since the bond dipoles are equal and opposite, they cancel each other out, resulting in no net dipole moment.

Step 3: Analyzing H₂S

H₂S has a bent molecular geometry due to the two lone pairs on sulfur. The bond dipoles between hydrogen and sulfur do not cancel out completely because of this bent shape, resulting in a net dipole moment.

Step 4: Analyzing CH₂Cl₂

CH₂Cl₂ is tetrahedral, but asymmetrical because of the presence of two different atoms (H and Cl) bonded to carbon. The bond dipoles of the C-Cl bonds are stronger than C-H and do not cancel out, resulting in a net dipole moment pointing towards the chlorine atoms.

Step 5: Analyzing HCN

HCN is a linear molecule, but the bond dipole between H and CN does not cancel as the CN end is more electronegative. This gives HCN a net dipole moment pointing from the hydrogen towards the nitrogen end.

Key Concepts

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms within a molecule. It is determined by the number of bonding pairs and lone pairs of electrons surrounding the central atom. These interactions dictate the spatial organization and shape of the molecule. For instance, in the case of

• eXeF₂: Linear geometry with equal opposing dipoles, leading to no net dipole moment.
• H₂S: Bent geometry due to the presence of two lone pairs, resulting in an asymmetrical shape.

Understanding the molecular geometry is crucial since it directly affects the distribution of electron density and influences the molecule's dipole moment.

Polar Molecules

Polar molecules arise when there is an uneven distribution of electron density that leads to partial charges within the molecule. This occurs when the molecular geometry does not allow for complete cancellation of bond dipoles. For example:

• In H₂S, the bent shape ensures that the dipole moments do not cancel out, resulting in a polar molecule.
• CH₂Cl₂'s tetrahedral but asymmetrical shape results in a polar molecule with a net dipole moment directed toward the chlorine atoms.

Being aware of whether a molecule is polar is important as it determines how the molecule interacts with other substances, influencing properties like solubility and boiling point.

Bond Dipoles

A bond dipole is created when atoms of differing electronegativities are bonded together, causing an unequal sharing of electron density. The atom with higher electronegativity acquires a partial negative charge, while the other gains a partial positive charge, thus creating a dipole. For instance:

• In HCN, the C-N bond dipole does not cancel out with the H-C dipole due to the linear shape and difference in electronegativity, resulting in a net dipole pointing from hydrogen to nitrogen.
• For CH₂Cl₂, the bond dipoles of C-Cl being stronger than those of C-H are not symmetrically cancelled, producing a net dipole moment.

Recognizing the presence and direction of bond dipoles is vital for predicting molecular behavior, as these dipoles contribute to a molecule's overall polarity and interact with external fields or other molecules.