Problem 21 - Chemistry: The Central Science

Question

Which member in each pair has the greater dispersion forces? (a) \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{H}_{2} \mathrm{~S}\), (b) \(\mathrm{CO}_{2}\) or \(\mathrm{CO}\), (c) \(\mathrm{SiH}_{4}\) or \(\mathrm{GeH}_{4}\).

Step-by-Step Solution

Answer

In summary, the member with greater dispersion forces for each pair is: (a) H2S, (b) CO2, and (c) GeH4.

Step 1: (a) H2O or H2S

We first compare the two molecules, H2O and H2S, based on size, shape, and number of electrons. Both H2O and H2S are similarly shaped being bent, so their shapes will not be the primary factor in determining which has the greater dispersion forces. However, sulfur is larger and heavier than oxygen, and H2S has more electrons (18) than H2O (10). Thus, H2S will have greater dispersion forces compared to H2O.

Step 2: (b) CO2 or CO

For CO2 and CO, we'll again compare size, shape, and number of electrons. CO2 is linear and larger than CO, which is also linear, and has more electrons (22) than CO (14). This means CO2 will have greater dispersion forces than CO.

Step 3: (c) SiH4 or GeH4

Lastly, we compare SiH4 and GeH4 based on size, shape, and number of electrons. Both SiH4 and GeH4 are tetrahedral in shape, so we need to compare their sizes and electron counts. Germanium is heavier and larger than silicon, and GeH4 has more electrons (41) than SiH4 (32). Therefore, the greater dispersion forces are present in GeH4 as compared to SiH4. In summary, for each pair, the member with the greater dispersion forces is (a) H2S, (b) CO2, and (c) GeH4.

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces that mediate interaction between molecules, including attractions and repulsions between atoms, ions, and molecules. They are weaker than intramolecular forces, such as covalent and ionic bonds.

These forces are critical in understanding the physical properties of substances, such as boiling points, melting points, and solubilities. There are several types of intermolecular forces, with dispersion forces being one of the primary types. Also known as London dispersion forces, these are present in all molecules, whether they are polar or nonpolar.

Dispersion forces originate from the momentary uneven distribution of electrons around a molecule, creating temporary dipoles that can induce dipoles in adjacent molecules. The strength of these forces increases with the number of electrons in a molecule and with an increase in molecular size.

Molecular Polarity

Molecular polarity refers to the distribution of electrical charge around a molecule. When atoms in a molecule share electrons unequally, they create partial electric charges, leading to a dipole moment. This happens in molecules where there is a significant difference in electronegativity between the bonded atoms.

The polarity of a molecule has a substantial effect on its physical and chemical properties. For instance, polar molecules tend to have higher boiling points than nonpolar molecules because of the stronger dipole-dipole intermolecular forces. Polar molecules also tend to dissolve well in polar solvents while nonpolar molecules dissolve in nonpolar solvents.

In the context of dispersion forces, while they are present in both polar and nonpolar molecules, the molecular polarity can contribute to the overall intermolecular forces if dipole-dipole interactions or hydrogen bonding are possible. However, for nonpolar molecules, dispersion forces are the only type of intermolecular force acting.

Van der Waals Forces

Van der Waals forces is a term encompassing a variety of intermolecular forces, including dispersion forces, dipole-dipole interactions, and dipole-induced dipole interactions. These forces are essential for understanding the liquidity, solidity, and gaseous states of substances at different temperatures and pressures.

Dispersion forces are a type of van der Waals forces and are often the weakest, yet they can be significant in larger atoms or molecules with more electrons. In contrast, dipole-dipole interactions are stronger and occur between molecules that have permanent dipole moments.

It's important to note that all these different forces interact with each other in a substance, and their strength can determine the state of matter of the substance under normal conditions. For instance, gases have weak van der Waals forces while solids have stronger forces keeping them together.