Question

Without referring to Table 6 , predict which compound in each of the following pairs has the higher heat of fusion \((\mathrm{cal} / \mathrm{mol})\) (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) \(\mathrm{H}_{2}\mathrm{O}\); (b) \(\mathrm{H}_{2}\mathrm{Se}\).

Step-by-step solution

Understanding Heat of Fusion

The heat of fusion is the amount of energy required to convert a solid into a liquid at its melting point. It is largely influenced by the strength of intermolecular forces present in the substance.

Analyze the Intermolecular Forces in (a)

In \(\mathrm{H}_{2}\mathrm{O}\), strong hydrogen bonding exists due to the high electronegativity of oxygen. In contrast, \(\mathrm{H}_{2}\mathrm{~S}\) has weaker dipole-dipole interactions because sulfur is less electronegative than oxygen. Thus, \(\mathrm{H}_{2}\mathrm{O}\) has stronger intermolecular forces and a higher heat of fusion.

Analyze the Intermolecular Forces in (b)

Both \(\mathrm{H}_{2}\mathrm{~S}\) and \(\mathrm{H}_{2}\mathrm{Se}\) primarily exhibit dipole-dipole interactions. However, selenium has a larger atomic size compared to sulfur, leading to more polarizable electron clouds and stronger London dispersion forces, which can increase heat of fusion slightly. Therefore, \(\mathrm{H}_{2}\mathrm{Se}\) is expected to have a slightly higher heat of fusion than \(\mathrm{H}_{2}\mathrm{~S}\).

Key concepts

Intermolecular Forces

Intermolecular forces are the forces that act between molecules, holding them together and influencing various physical properties like boiling points, melting points, and the heat of fusion. These forces vary in strength and include hydrogen bonding, dipole-dipole interactions, and dispersion forces (often London dispersion forces).

Among these, dispersion forces are generally the weakest, arising due to fluctuations in electron distribution within molecules, leading to temporary dipoles. Dipole-dipole interactions occur between polar molecules due to the permanent dipoles these substances possess.

Stronger intermolecular forces mean more energy is needed to disrupt these interactions during melting, leading to a higher heat of fusion. For example, in the provided exercise, water ( ext{H}_2 ext{O} ) has a higher heat of fusion than hydrogen sulfide ( ext{H}_2 ext{S} ) due to the stronger hydrogen bonds present in water, compared to the dipole-dipole interactions in hydrogen sulfide.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction. It occurs specifically when hydrogen is bonded to highly electronegative elements like oxygen, nitrogen, or fluorine. The intense electron-withdrawing ability of these elements leaves hydrogen slightly positive. This positive charge makes hydrogen atoms attract to the electronegative atoms in neighboring molecules, forming a hydrogen bond.

This bonding significantly affects properties like boiling and melting points. For example, in the exercise example, water has hydrogen bonds due to the oxygen-hydrogen bond. This makes water's heat of fusion considerably higher than that of hydrogen sulfide, where sulfur, a less electronegative element, does not allow for hydrogen bonding.

• Hydrogen bonds contribute to the unique properties of water, like its high surface tension and boiling point.
• These bonds can impart significant strength and stability, often necessitating more energy input to melt hydrogen-bonded materials.

Understanding hydrogen bonding is key in explaining why some substances require more heat to melt, as illustrated by ext{H}_2 ext{O} 's higher heat of fusion compared to ext{H}_2 ext{S} .

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that have permanent net dipoles due to their polar nature. These dipoles arise from an uneven distribution of electron density, often because of a difference in electronegativity between the atoms in a molecule. This causes the molecules to align themselves such that the positive pole of one molecule is near the negative pole of another.

Such interactions are observable in hydrogen sulfide ( ext{H}_2 ext{S} ) and selenium hydride ( ext{H}_2 ext{Se} ) where both have permanent electric dipoles.

• These interactions are stronger than London dispersion forces but weaker than hydrogen bonds.
• In the provided exercise, these forces are responsible for the properties of both hydrogen sulfide and selenium hydride, influencing their heats of fusion.
• The example shows that despite both having dipole-dipole interactions, selenium's larger atomic size enhances London dispersion forces, giving ext{H}_2 ext{Se} a slightly higher heat of fusion than ext{H}_2 ext{S} .

Understanding dipole-dipole interactions provides insight into why some molecules bond more tightly than others, impacting their melting points and related thermal properties.