Problem 15 - Chemistry

Question

Which member of each of the following pairs of substances would you expect to have a higher boiling point? (a) \(\mathrm{O}_{2}\) and \(\mathrm{Cl}_{2},\) (b) \(\mathrm{SO}_{2}\) and \(\mathrm{CO}_{2}\), (c) \(\mathrm{HF}\) and \(\mathrm{HI}\)

Step-by-Step Solution

Answer

The substances with higher boiling points are: (a) \(\mathrm{Cl}_{2}\), (b) \(\mathrm{SO}_{2}\), and (c) \(\mathrm{HF}\)

Step 1: Consider pair (a) \(\mathrm{O}_{2}\) and \(\mathrm{Cl}_{2}\)

Consider the molecule sizes and electron configurations of these two substances. Oxygen is a smaller molecule than Chlorine. Larger molecules and those with more electrons produce stronger London dispersion forces, which are the primary intermolecular forces for nonpolar molecules like these. Thus, \(\mathrm{Cl}_{2}\) has higher boiling point because it has stronger London dispersion forces.

Step 2: Think over pair (b) \(\mathrm{SO}_{2}\) and \(\mathrm{CO}_{2}\)

Although \(\mathrm{SO}_{2}\) and \(\mathrm{CO}_{2}\) are both linear molecules consisting of two double bonds, \(\mathrm{SO}_{2}\) is a polar molecule and \(\mathrm{CO}_{2}\) is nonpolar. Polar molecules experience dipole-dipole interactions along with London dispersion forces, while nonpolar molecules only experience London dispersion forces. Therefore, \(\mathrm{SO}_{2}\) has a higher boiling point.

Step 3: Evaluate pair (c) \(\mathrm{HF}\) and \(\mathrm{HI}\)

Both \(\mathrm{HF}\) and \(\mathrm{HI}\) are polar molecules that exhibit dipole-dipole intermolecular forces. However, \(\mathrm{HF}\) participates in hydrogen bonding, a special kind of dipole-dipole interaction. The presence of hydrogen bonding in \(\mathrm{HF}\) significantly increases its boiling point over \(\mathrm{HI}\). Thus, \(\mathrm{HF}\) has a higher boiling point.

Key Concepts

Boiling Point

The boiling point of a substance is the temperature at which it changes from a liquid to a gas. It is significantly influenced by the strength of the intermolecular forces present between the molecules. The stronger these forces are, the higher the boiling point will be. This is because more energy is needed to break the intermolecular forces holding the liquid together.

Different intermolecular forces, such as London dispersion forces, dipole-dipole interactions, and hydrogen bonding, affect the boiling point in unique ways. A substance with only weak London dispersion forces will generally have a lower boiling point compared to one with strong hydrogen bonds. Knowing which intermolecular forces are at play can help predict and compare the boiling points of different substances.

London Dispersion Forces

London dispersion forces are the weakest type of intermolecular force. They arise due to temporary dipoles that occur when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles. All molecules, whether polar or nonpolar, exhibit these forces.

However, they are the only type of intermolecular force that exists between nonpolar molecules like \(\mathrm{O}_{2}\) and \(\mathrm{Cl}_{2}\). In these cases, larger molecules or those with more electrons can create stronger dispersion forces. As a result, \(\mathrm{Cl}_{2}\), being larger with more electrons than \(\mathrm{O}_{2}\), has stronger London dispersion forces and, therefore, a higher boiling point.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules where there's a permanent dipole moment. These interactions are stronger than London dispersion forces and affect the boiling points of polar substances more significantly.

For example, \(\mathrm{SO}_{2}\) and \(\mathrm{CO}_{2}\) are both linear molecules, but \(\mathrm{SO}_{2}\) is polar while \(\mathrm{CO}_{2}\) is nonpolar. \(\mathrm{SO}_{2}\)'s polar nature enables it to engage in dipole-dipole interactions, leading to a higher boiling point than nonpolar \(\mathrm{CO}_{2}\), which only experiences weaker London dispersion forces. Hence, when comparing boiling points, it’s crucial to identify which molecules are capable of these interactions.

Hydrogen Bonding

Hydrogen bonding is a special, stronger kind of dipole-dipole interaction that occurs when hydrogen is directly bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. This bonding significantly increases the boiling point of the substances involved.

In our example, \(\mathrm{HF}\) can form hydrogen bonds due to hydrogen being bonded to fluorine, whereas \(\mathrm{HI}\) does not exhibit hydrogen bonding despite being polar, as iodine is less electronegative than fluorine. Therefore, \(\mathrm{HF}\) has a much higher boiling point than \(\mathrm{HI}\). The ability to form hydrogen bonds makes \(\mathrm{HF}\) stand out, demonstrating how these strong interactions crucially influence boiling points.