Problem 34 - Chemistry The Molecular Science

Question

The fusion enthalpy for \(\mathrm{H}_{2} \mathrm{O}\) is about 2.5 times larger than the fusion enthalpy for \(\mathrm{H}_{2} \mathrm{~S}\). What does this say about the relative strengths of the forces between the molecules in these two solids? Explain.

Step-by-Step Solution

Answer

Water has stronger intermolecular forces (hydrogen bonds) than hydrogen sulfide.

Step 1: Understanding Fusion Enthalpy

Fusion enthalpy, also known as the heat of fusion, is the amount of energy required to convert a substance from a solid to a liquid at its melting point. This value reflects the strength of the forces that hold the molecules together in the solid state. The higher the fusion enthalpy, the stronger the intermolecular forces that need to be overcome for melting to occur.

Step 2: Compare Fusion Enthalpies of H2O and H2S

The problem states that the fusion enthalpy for water ( H2O) is about 2.5 times larger than that for hydrogen sulfide ( H2S). This implies that the intermolecular forces within solid water are significantly stronger than those within solid hydrogen sulfide.

Step 3: Analyze Intermolecular Forces

Water molecules engage in hydrogen bonding, which is a particularly strong type of dipole-dipole interaction due to the high electronegativity of oxygen. In contrast, hydrogen sulfide, while also capable of dipole-dipole interactions, does not engage in hydrogen bonding to the same extent because sulfur is less electronegative than oxygen.

Step 4: Conclusion on Molecular Forces

Since water can form stronger hydrogen bonds compared to the weaker forces in hydrogen sulfide, it requires more energy to overcome these interactions during melting. Therefore, the higher fusion enthalpy of water reflects stronger intermolecular forces compared to hydrogen sulfide.

Key Concepts

Fusion Enthalpy

Fusion enthalpy is a key concept in understanding how substances transition from a solid to a liquid. When a solid melts, energy is required to break the bonds holding the solid particles together. This energy is known as the fusion enthalpy or heat of fusion. It's a measure of the strength of the forces between molecules in a solid. The higher the fusion enthalpy, the more energy is needed. This means stronger intermolecular forces between molecules. To imagine this, think about chocolate and ice. Melting chocolate requires less energy than melting ice. Why? Because the bonds in chocolate are weaker. When we compare substances like two solids— water (H₂O) and hydrogen sulfide (H₂S)— and their fusion enthalpies, we see that water has a higher fusion enthalpy. This means it takes more energy to break the bonds between water molecules than between hydrogen sulfide molecules. Here, the implication is straightforward: stronger molecular forces in water mean higher fusion enthalpy.

Hydrogen Bonding

Hydrogen bonding is a special type of molecular interaction that plays a critical role in the unique properties of water. These bonds occur when hydrogen atoms are attracted to electronegative atoms, like oxygen or nitrogen. In water, oxygen is highly electronegative. It attracts the hydrogen atoms from neighboring water molecules, forming strong hydrogen bonds. These bonds significantly influence the properties of water, such as its high boiling point and high fusion enthalpy. Water's ability to form these strong hydrogen bonds accounts for the extra energy needed to overcome them during melting, compared to substances like hydrogen sulfide, which forms weaker dipole-dipole interactions. With hydrogen bonds, water gains its extraordinary capacity to store energy (latent heat), contributing to its higher fusion enthalpy in comparison to hydrogen sulfide.

Molecular Interaction

Understanding molecular interactions is key to explaining why different substances require varying amounts of energy to undergo phase changes. These interactions are forces that occur between molecules, and they determine how molecules stick together in different states of matter. There are several types of molecular interactions:

• Hydrogen bonding: A strong form of interaction seen in water.
• Dipole-dipole interaction: Occurs between polar molecules like hydrogen sulfide but is generally weaker than hydrogen bonding.
• Van der Waals forces: Weak attractions that exist even in nonpolar molecules.

In the case of water and hydrogen sulfide, the differences in molecular interaction help explain their differing fusion enthalpies. Water's stronger hydrogen bonds contrast with the less potent dipole-dipole interactions in hydrogen sulfide. This means that the molecular interactions in water act more strongly to keep the molecules together as a solid, requiring more energy to transform into a liquid. This is why water's fusion enthalpy is much higher compared to hydrogen sulfide.