Question

Without referring to Table \(6,\) predict which compound in each of the following pairs has the higher melting point. (a) \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{H}_{2} \mathrm{~S}\) (b) \(\mathrm{H}_{2} \mathrm{~S}\) or \(\mathrm{H}_{2} \mathrm{Se}\)

Short answer

(a) \(\mathrm{H}_2\mathrm{O}\) has a higher melting point; (b) \(\mathrm{H}_2\mathrm{Se}\) has a higher melting point.

Step-by-step solution

Identify Intermolecular Forces

When predicting melting points, it's critical to recognize the types of intermolecular forces present in each compound. \(\mathrm{H}_2\mathrm{O}\) (water) experiences hydrogen bonding due to the high electronegativity of oxygen, which attracts the hydrogen atoms of adjacent molecules. In contrast, \(\mathrm{H}_2\mathrm{S}\) (hydrogen sulfide) mainly exhibits dipole-dipole interactions and London dispersion forces due to the lower electronegativity of sulfur compared to oxygen.

Decide the stronger Intermolecular Force in Pair (a)

Hydrogen bonding, present in \(\mathrm{H}_2\mathrm{O}\), is generally stronger than the polar forces and dispersion forces in \(\mathrm{H}_2\mathrm{S}\). Stronger intermolecular forces usually result in a higher melting point. Thus, \(\mathrm{H}_2\mathrm{O}\) has a higher melting point than \(\mathrm{H}_2\mathrm{S}\).

Compare Atomic Mass and Size for Pair (b)

For \(\mathrm{H}_2\mathrm{S}\) versus \(\mathrm{H}_2\mathrm{Se}\), both have similar types of intermolecular forces (mainly dispersion forces and some dipole-dipole interactions). In this situation, you compare their molar masses. \(\mathrm{Se}\) is heavier than \(\mathrm{S}\), increasing the strength of dispersion forces in \(\mathrm{H}_2\mathrm{Se}\), which usually results in a higher melting point than \(\mathrm{H}_2\mathrm{S}\).

Key concepts

Melting Point comparison

The melting point of a substance is a key physical property that tells us at what temperature a solid turns into a liquid. Whether a compound has a high or low melting point can often be predicted by looking at the types of intermolecular forces at play. Substances with stronger intermolecular forces typically have higher melting points.

For example, comparing the melting points of different compounds means we should always consider the types of bonds and forces that hold the molecules together:

• Molecules with hydrogen bonding, such as water (\(\mathrm{H}_2\mathrm{O}\)), usually have higher melting points compared to those that exhibit only dispersion or dipole-dipole interactions, like hydrogen sulfide (\(\mathrm{H}_2\mathrm{S}\)).
• When molecules have similar intermolecular forces, differences in melting points might stem from the size of the molecules. Larger molecules usually mean stronger dispersion forces, which can increase the melting point.

Understanding these elements helps us predict why some substances remain solids or become liquids at various temperatures.

Hydrogen Bonding

Hydrogen bonding is one of the most significant types of intermolecular forces and is crucial in determining the properties of many compounds, especially water (\(\mathrm{H}_2\mathrm{O}\)). It occurs when hydrogen (\(\mathrm{H}\)) is bonded to highly electronegative atoms like oxygen (\(\mathrm{O}\)), nitrogen (\(\mathrm{N}\)), or fluorine (\(\mathrm{F}\)). This results in a special type of attraction between molecules.

Characteristics of hydrogen bonding include:

• Its surprisingly strong nature compared to other dipole-dipole interactions.
• It significantly increases the melting and boiling points of compounds, as seen in water's high melting point compared to hydrogen sulfide (\(\mathrm{H}_2\mathrm{S}\)).
• Hydrogen bonds affect the physical state of compounds at room temperature, influencing whether substances are gases, liquids, or solids.

Understanding hydrogen bonding helps explain why water has such unique properties, playing an essential role in the environment and chemistry.

Dipole-Dipole Interactions

Dipole-dipole interactions are one type of intermolecular force that occurs between polar molecules. These interactions happen when the positive end of one molecule is attracted to the negative end of another. These interactions are stronger than dispersion forces but weaker than hydrogen bonds.

Key points about dipole-dipole interactions include:

• They contribute to the higher melting and boiling points of polar compounds compared to non-polar ones, though less so than hydrogen bonding.
• In hydrogen sulfide (\(\mathrm{H}_2\mathrm{S}\)), these interactions are present, but the effect is less pronounced compared to hydrogen bonding in water.
• As the molecular weight of the substances increases, so does the overall strength of dipole-dipole interactions due to stronger dispersion forces.

Recognizing the role of dipole-dipole interactions helps explain the behavior of molecules and the trends in their melting and boiling points.