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 \(\mathrm{H}_{2}\) 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 unusually high boiling point of water compared to other group 16 hydrides, such as \(\mathrm{H}_{2}\mathrm{S}, \mathrm{H}_{2}\mathrm{Se},\) and \(\mathrm{H}_{2}\mathrm{Te}\), can be explained by the presence of hydrogen bonding in water. These hydrogen bonds lead to strong intermolecular forces in water, requiring more energy to overcome causing the high boiling point. In contrast, the absence of hydrogen bonding in other group 16 hydrides leads to weaker intermolecular forces and lower boiling points.

Step-by-step solution

Compare Boiling Points

Observe and compare the boiling points of water and the other group 16 hydrides: - \(\mathrm{H}_{2}\mathrm{O}\) - Water: \(100^{\circ} \mathrm{C}\) - \(\mathrm{H}_{2}\mathrm{S}\) - Hydrogen Sulfide: \(-60^{\circ} \mathrm{C}\) - \(\mathrm{H}_{2}\mathrm{Se}\) - Hydrogen Selenide: \(-41^{\circ} \mathrm{C}\) - \(\mathrm{H}_{2}\mathrm{Te}\) - Hydrogen Telluride: \(-2^{\circ} \mathrm{C}\) As we can see, the boiling point of water is significantly higher than the boiling points of other group 16 hydrides.

Understand Hydrogen Bonding

Hydrogen bonding is a type of intermolecular force that occurs when a hydrogen atom is bonded to a highly electronegative atom (such as fluorine, oxygen, or nitrogen) and is attracted to another electronegative atom. This creates a strong dipole-dipole interaction, known as a hydrogen bond, which is stronger than other types of van der Waals forces. The presence of hydrogen bonding significantly increases the strength of intermolecular forces between molecules, leading to an increase in boiling point as more energy is required to break these stronger bonds.

Analyze Hydrogen Bonding in Water

In a water molecule, \(\mathrm{H}_{2}\mathrm{O}\), the oxygen atom is highly electronegative, leading to polarity in the molecule. The hydrogen atoms form partial positive charges while the oxygen atom forms a partial negative charge. These polar molecules can attract each other, forming hydrogen bonds between the partially positive hydrogen atom in one water molecule and the partially negative oxygen atom in another water molecule. These hydrogen bonds are responsible for the strong intermolecular forces in water which in turn results in the high boiling point of water.

Analyze Absence of Hydrogen Bonding in Other Compounds

Now, let's analyze the other group 16 hydrides, \(\mathrm{H}_{2}\mathrm{S}, \mathrm{H}_{2}\mathrm{Se},\) and \(\mathrm{H}_{2}\mathrm{Te}\). None of the mentioned elements (sulfur, selenium, or tellurium) are as electronegative as oxygen, resulting in weaker polarities and hence weak dipole-dipole interactions. As a result, these compounds do not experience hydrogen bonding, leading to weaker intermolecular forces and lower boiling points compared to water.

Conclude

In conclusion, the unusually high boiling point of water compared to other group 16 hydrides such as \(\mathrm{H}_{2}\mathrm{S}, \mathrm{H}_{2}\mathrm{Se},\) and \(\mathrm{H}_{2}\mathrm{Te}\) can be explained by the presence of hydrogen bonding in water. These hydrogen bonds lead to strong intermolecular forces in water, requiring more energy to overcome and causing the high boiling point. In contrast, the absence of hydrogen bonding in other group 16 hydrides leads to weaker intermolecular forces and lower boiling points.

Key concepts

Intermolecular Forces

Intermolecular forces are attractions that occur between molecules, which dictate many of the physical properties of a substance, such as boiling point and melting point. There are several types of intermolecular forces, with varying strengths, and one of the strongest among these is hydrogen bonding.

What sets these interactions apart is their exceptional ability to influence a substance's state. For example, stronger intermolecular forces will generally lead to higher boiling and melting points because more energy is required to overcome these forces during phase changes.

Let's consider molecules like those found in water, which have a bent shape due to their molecular geometry, and feature highly electronegative oxygen atoms. These oxygen atoms pull electron density from the hydrogen atoms, creating dipoles within the molecule. The positively charged end of one water molecule is powerfully attracted to the negatively charged end of another, forming a stable hydrogen bond.

In substances lacking this ability to form hydrogen bonds, only weaker interactions such as London dispersion forces or dipole-dipole interactions will be at play. These weaker forces don't require as much energy to break, leading to lower boiling and melting points. Thus, understanding the type and strength of intermolecular forces is key to predicting and explaining the physical properties of substances.

Boiling Points

The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the surrounding pressure, allowing the liquid to turn into a gas. It's a uniquely important physical property that provides insight into the intermolecular forces present in a substance.

Generally, the higher the intermolecular forces within a liquid, the higher the boiling point, as more thermal energy, in the form of heat, is needed to break the bonds holding the molecules together in the liquid state. The unusually high boiling point of water, at 100 degrees Celsius, is a classic demonstration of the impact of hydrogen bonding, an intermolecular force present in water that is significantly stronger than forces present in many other liquids.

These strong hydrogen bonds in water contribute to a much higher boiling point compared to other group 16 hydrides such as which do not possess such strong intermolecular bonds. Recognizing the correlation between intermolecular forces and boiling points helps us understand why substances like water can exist as a liquid over a wide range of temperatures on Earth, supporting life.

Electronegativity

Electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons (or electron density) towards itself within a chemical bond. The greater an atom's electronegativity, the more strongly it can pull electrons towards itself when involved in a chemical bond.

Oxygen, for instance, has a high electronegativity, which enables it to form polar covalent bonds with hydrogen atoms in water. The difference in electronegativity between oxygen and hydrogen leads to a separation of charges within the molecule, making it polar. The highly electronegative oxygen atom acquires a partial negative charge, while the hydrogens have partial positive charges.

This difference in charge within the molecule is the foundation for hydrogen bonding, as the partially positive hydrogen of one water molecule can be strongly attracted to the partially negative oxygen of another water molecule. In substances with less electronegative elements, such as which do not form significant polar covalent bonds, this kind of strong attraction does not occur, resulting in weaker intermolecular interactions and, therefore, lower boiling points.