Problem 17 - Chemistry and Chemical Reactivity

Question

Which member of each of the following pairs of compounds has the higher boiling point? (a) \(\mathrm{O}_{2}\) or \(\mathrm{N}_{2}\) (c) HF or HI (b) \(\mathrm{SO}_{2}\) or \(\mathrm{CO}_{2}\) (d) \(\operatorname{SiH}_{4}\) or \(\operatorname{Ge} \mathrm{H}_{4}\)

Step-by-Step Solution

Answer

(a) \\( O_{2}\\) (b) \\( SO_{2}\\) (c) HF (d) \\( \ GeH_{4}\\)

Step 1: Understanding Boiling Points

Boiling point is influenced by intermolecular forces among compounds. The stronger the intermolecular forces, the higher the boiling point. Common forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces.

Step 2: Comparison in Oxygen and Nitrogen

(a) Both \(O_{2}\) and \(N_{2}\) are diatomic molecules without permanent dipole moments, so they rely on London dispersion forces. Since both molecules are similar in size, they have similar dispersion forces. However, \(O_{2}\) is slightly more polarizable than \(N_{2}\), giving it a marginally higher boiling point.

Step 3: Analyzing HF and HI

(c) \(HF\) exhibits hydrogen bonding, which is a strong form of dipole-dipole interaction, while \(HI\) relies mostly on London dispersion forces. Hydrogen bonds in \(HF\) are significantly stronger than the dispersion forces in \(HI\), resulting in \(HF\) having a higher boiling point.

Step 4: Evaluating \\( SO_{2}\\) and \\( \ CO_{2}\\)

(b) \(SO_{2}\) is a polar molecule with dipole-dipole interactions, while \(CO_{2}\) is non-polar with only London dispersion forces. The dipole-dipole interactions in \(SO_{2}\) are stronger than the forces in \(CO_{2}\), leading to a higher boiling point for \(SO_{2}\).

Step 5: Comparing \\( \ SiH_{4}\\) and \\( \ GeH_{4}\\)

(d) Both \( \SiH_{4}\) and \( \GeH_{4}\) are tetrahedral and have similar structures, relying on London dispersion forces for bonding. However, \( \GeH_{4}\) has a greater molar mass and more electrons, leading to stronger dispersion forces and thus a higher boiling point than \( \SiH_{4}\).

Key Concepts

Intermolecular Forces

Intermolecular forces are attractions that occur between molecules. These forces are responsible for many physical properties, such as boiling points, melting points, and solubility.
While individual forces are generally weak compared to intramolecular forces, they significantly affect how substances behave. They become particularly important when discussing the boiling points of compounds. Here, the strength and nature of intermolecular forces can deeply influence which compound will boast a higher boiling point.
These forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Each type has its own characteristics and affects compounds differently based on their molecular structure and electron distribution.

London Dispersion Forces

London dispersion forces, also known as Van der Waals forces, are weak intermolecular attractions that arise from temporary dipoles formed when electrons shift temporarily in electron clouds. These forces exist between all molecules, whether they are polar or nonpolar.
They are the only intermolecular forces present in nonpolar molecules. In the case of similar sized molecules, the more polarizable molecule (one which has cloudier, more easily shifted electron clouds) will experience stronger dispersion forces. This often results in a higher boiling point.
For example, both oxygen ( O_{2} ) and nitrogen ( N_{2} ) are nonpolar molecules that rely on London dispersion forces. Since O_{2} is slightly more polarizable than N_{2} , it has a slightly higher boiling point.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that specifically involves hydrogen atoms bonded to highly electronegative elements such as fluorine, oxygen, or nitrogen.
This bonding occurs because the hydrogen becomes significantly positive due to the large difference in electronegativity. This then attracts neighboring electronegative atoms, forming an attraction that is stronger than typical dipole-dipole interactions.
One such example is hydrofluoric acid ( HF ), which exhibits strong hydrogen bonding due to the presence of hydrogen and fluorine. This strong interaction significantly elevates its boiling point when compared to compounds like hydrogen iodide ( HI ), which only exhibits weaker dispersion forces.

Dipole-Dipole Interactions

Dipole-dipole interactions occur in polar molecules that possess permanent dipoles due to differences in electronegativity of the atoms within molecules. These interactions are attractions between the positive end of one molecule and the negative end of another.
The strength of dipole-dipole interactions depends largely on the magnitude of the dipoles and their orientation. Molecules with strong permanent dipoles generally have higher boiling points than those relying on weak dispersion forces.
A good example can be seen in sulfur dioxide ( SO_{2} ). Being a polar molecule with a significant dipole moment, it experiences strong dipole-dipole interactions, which give it a higher boiling point compared to nonpolar carbon dioxide ( CO_{2} ), which relies solely on London dispersion forces.