Question

Look up and compare the normal boiling points and normal melting points of \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{H}_{2} \mathrm{~S}\). Based on these physical properties, which substance has stronger intermolecular forces? What kinds of intermolecular forces exist for each molecule?

Short answer

The normal boiling points and normal melting points of water (H₂O) and hydrogen sulfide (H₂S) are as follows: - H₂O: Normal boiling point: 100°C, Normal melting point: 0°C - H₂S: Normal boiling point: -60.7°C, Normal melting point: -82°C Comparing these values, we can conclude that H₂O has stronger intermolecular forces due to its higher boiling and melting points. Intermolecular forces for water include strong dipole-dipole interactions and hydrogen bonding, while hydrogen sulfide has weaker dipole-dipole interactions and does not exhibit hydrogen bonding.

Step-by-step solution

Look up the normal boiling points and normal melting points of H₂O and H₂S

We can find the normal boiling points and normal melting points of water (H₂O) and hydrogen sulfide (H₂S) in a chemistry reference book or online. For water (H₂O): - Normal boiling point: 100°C (212°F) - Normal melting point: 0°C (32°F) For hydrogen sulfide (H₂S): - Normal boiling point: -60.7°C (-77.3°F) - Normal melting point: -82°C (-115.6°F)

Identify the substance with stronger intermolecular forces

Comparing the normal boiling and melting points, we see that H₂O has much higher boiling and melting points than H₂S. This indicates that H₂O has stronger intermolecular forces, as more energy is required to break these forces and cause a phase transition (from solid to liquid or from liquid to gas).

Identify the intermolecular forces in H₂O (water)

There are several types of intermolecular forces, including London dispersion forces, dipole-dipole interactions, and hydrogen bonding. Water contains highly polar O-H bonds due to the difference in electronegativity between oxygen and hydrogen. The oxygen atom carries a partial negative charge, while the hydrogen atoms carry partial positive charges. This results in strong dipole-dipole interactions. In addition, water molecules can form hydrogen bonds (a particularly strong kind of dipole-dipole interaction) due to the presence of highly polar O-H bonds.

Identify the intermolecular forces in H₂S (hydrogen sulfide)

Hydrogen sulfide, like water, has polar S-H bonds due to the difference in electronegativity between sulfur and hydrogen. However, the S-H bond is less polar than the O-H bond in water, leading to weaker dipole-dipole interactions. Although hydrogen sulfide also contains polar S-H bonds, it does not exhibit hydrogen bonding because the sulfur atom is not as electronegative as the oxygen atom (hydrogen bonding typically occurs with more electronegative elements like oxygen, nitrogen, and fluorine). In summary, water (H₂O) has stronger intermolecular forces, mainly due to its ability to form hydrogen bonds, while hydrogen sulfide (H₂S) has weaker intermolecular forces due to its less polar bonds and the absence of hydrogen bonding.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that is significantly stronger than other types of dipole-dipole interactions. It occurs in molecules where hydrogen is directly bonded to a highly electronegative element, such as oxygen, nitrogen, or fluorine. This bond results in a large difference in electronegativity between the two atoms.
In water (H₂O), hydrogen bonding plays a crucial role. Each water molecule has two hydrogen atoms attached to one oxygen atom. Because oxygen is highly electronegative, it pulls the shared electrons towards itself, creating a partial negative charge on the oxygen and partial positive charges on the hydrogens.

• Hydrogen bonds form between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another.
• This results in a network of hydrogen bonds that creates a strong molecular structure.
• The strength of hydrogen bonds is what gives water its unique properties, like high boiling and melting points.

Hydrogen sulfide (H₂S), on the other hand, does not form hydrogen bonds because sulfur is less electronegative than oxygen. This means the S-H bonds in H₂S are less polar, leading to weaker interactions between molecules.

Boiling Points

Boiling points are an important indicator of the strength of intermolecular forces within a substance. They represent the temperature at which a liquid changes into a gas. The stronger the intermolecular forces, the more energy or heat is required to separate the molecules, resulting in a higher boiling point.
Water (H₂O) has a normal boiling point of 100°C (212°F). Its high boiling point compared to many other small molecules can be attributed to its strong hydrogen bonding. This means a lot of energy is needed to break these bonds and turn liquid water into vapor.

• The presence of multiple hydrogen bonds in water results in significant cohesion among molecules.
• This cohesion leads to a higher amount of energy needed to convert water from liquid to vapor.

In contrast, hydrogen sulfide (H₂S) has a boiling point of -60.7°C (-77.3°F). Since H₂S lacks hydrogen bonding, its intermolecular forces are weaker, requiring less energy to transition into a gaseous state.

Melting Points

Melting points provide insight into the strength of forces holding a solid together. It is the temperature at which a solid becomes a liquid. Similar to boiling points, stronger intermolecular forces result in higher melting points because more energy is needed to overcome these forces for the phase change.
Water (H₂O) has a normal melting point of 0°C (32°F). This high melting point for a molecule of its size is also due to the strong hydrogen bonds that hold the water molecules together in a solid state.

• These bonds must be broken for ice to melt into liquid water.
• The unique structure of ice, which is less dense than liquid water, is also a result of hydrogen bonding.

Hydrogen sulfide (H₂S), with a melting point of -82°C (-115.6°F), showcases its weaker intermolecular forces, as less energy is required for H₂S to transition from a solid to a liquid.
This difference highlights why H₂O is a liquid at room temperature, while H₂S is a gas.