Question

The normal boiling point of water is unusually high, compared to the boiling points of \(\mathrm{H}_{2} \mathrm{~S}, \mathrm{H}_{2} \mathrm{Se},\) and \(\mathrm{H}_{2} \mathrm{Te}\). Explain this observation in terms of the hydrogen bonding that exists in water, but that does not exist in the other compounds.

Short answer

The high boiling point of water compared to similar compounds, such as \(\mathrm{H}_{2} \mathrm{S}, \mathrm{H}_{2} \mathrm{Se},\) and \(\mathrm{H}_{2} \mathrm{Te}\), can be attributed to the presence of hydrogen bonding in water molecules. The partially positive hydrogen atoms in water form hydrogen bonds with partially negative oxygen atoms in other water molecules, resulting in strong intermolecular attractions and a higher boiling point. In contrast, \(\mathrm{H}_{2} \mathrm{S}, \mathrm{H}_{2} \mathrm{Se},\) and \(\mathrm{H}_{2} \mathrm{Te}\) compounds lack hydrogen bonding due to the less electronegative atoms they are bonded to (sulfur, selenium, and tellurium), leading to weaker intermolecular forces such as Van der Waals or dipole-dipole forces and lower boiling points.

Step-by-step solution

Boiling Point and Intermolecular Forces

Boiling point is the temperature at which the vapor pressure of a liquid equals the external pressure surrounding the liquid. At this temperature, the liquid turns into a gas. The boiling point of a compound depends on the strength of the intermolecular forces between its molecules. Stronger intermolecular forces lead to higher boiling points because more energy (heat) is needed to overcome the forces holding the molecules together.

Types of Intermolecular Forces

There are three main types of intermolecular forces: 1. Van der Waals forces (also called London dispersion forces) 2. Dipole-dipole forces 3. Hydrogen bonding Van der Waals forces are the weakest intermolecular forces and exist between all molecules, including nonpolar molecules. Dipole-dipole forces exist between polar molecules and are stronger than Van der Waals forces. Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom (like oxygen, nitrogen, or fluorine). It is the strongest of the three types of intermolecular forces.

Hydrogen Bonding in Water and Its Absence in the Other Compounds

Water molecules (\(\mathrm{H}_{2} \mathrm{O}\)) are polar due to the difference in electronegativity between oxygen and hydrogen, which results in a partial negative charge on oxygen and a partial positive charge on hydrogen. The partially positive hydrogen atoms in water can form hydrogen bonds with the partially negative oxygen atoms in other water molecules. This strong intermolecular attraction increases the boiling point of water compared to cases where hydrogen bonding is not present and only weaker intermolecular forces act. In contrast, \(\mathrm{H}_{2} \mathrm{S}, \mathrm{H}_{2} \mathrm{Se},\) and \(\mathrm{H}_{2} \mathrm{Te}\), don't exhibit hydrogen bonding. Hydrogen bonding requires a hydrogen atom to be bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine, but in these compounds, hydrogen is bonded to less electronegative atoms (sulfur, selenium, and tellurium, respectively). As a result, only weaker Van der Waals or dipole-dipole forces act between their molecules, leading to lower boiling points compared to water.

Key concepts

Boiling Point

Boiling point is an important concept when studying the phases of matter, particularly how substances transition from liquid to gas. The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure. This means that molecules have enough energy to escape the liquid phase and enter the gaseous phase.

The boiling point is influenced by the intermolecular forces within a liquid. Stronger intermolecular forces require more energy to overcome, resulting in higher boiling points. Thus, substances with strong intermolecular attractions will typically boil at higher temperatures because more heat is needed to separate their molecules.

Intermolecular Forces

Intermolecular forces are the attractions between molecules that determine many physical properties including boiling points. There are three main types of these forces:

• Van der Waals forces: Also known as London dispersion forces, these are the weakest and act between all molecules, whether polar or nonpolar. They result from temporary dipoles that occur when electron clouds shift within molecules.
• Dipole-dipole forces: These occur in polar molecules where permanent dipoles, due to unequal sharing of electrons, lead to positive and negative ends that attract one another.
• Hydrogen bonding: The strongest of the intermolecular forces, hydrogen bonding occurs when hydrogen is covalently bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine. This bond results in a significant dipole where the hydrogen will attract other electronegative atoms in nearby molecules.

Hydrogen bonds are particularly strong, which significantly raises the boiling point of substances like water. In contrast, substances that only exhibit van der Waals or dipole-dipole forces generally have much lower boiling points.

Polarity

Polarity in molecules arises due to the differences in electronegativity between atoms, which leads to unequal sharing of electrons. When one atom is more electronegative, it pulls electrons closer, resulting in a partial negative charge on one end of the molecule and a partial positive charge on the other, creating a dipole.

Polar molecules, like water, have distinct positive and negative ends due to this electron disparity. This polarity makes them capable of stronger interactions like hydrogen bonding. For instance, in water (H₂O), the oxygen atom is more electronegative than the hydrogen atoms, creating a permanent dipole.

On the other hand, molecules like hydrogen sulfide (H₂S) or hydrogen selenide (H₂Se) have atoms with lesser differences in electronegativity, leading to weaker dipole forces and no hydrogen bonding. As a result, these molecules have lower boiling points compared to polar compounds like water, where hydrogen bonding occurs.