Question

It is known that the heat of vaporization of water is 5 times as great as the heat of fusion. Explain this fact.

Short answer

In conclusion, the heat of vaporization of water is 5 times as great as the heat of fusion because it requires a significantly larger amount of energy to break almost all the hydrogen bonds and transition the water molecules from a liquid to a gaseous state, compared to the energy required to break only a few hydrogen bonds and transition water molecules from a solid to a liquid state. The unique molecular structure and hydrogen bonding in water play a crucial role in the differences observed in the energy requirements for these phase transitions.

Step-by-step solution

Understand the molecular structure of water

Water is a polar molecule with a unique hydrogen bonding structure. The molecule has one oxygen atom bonded to two hydrogen atoms, forming a bent shape with a positive and a negative side. In the solid (ice) and liquid states of water, these polar water molecules are attracted to each other through hydrogen bonds, which create an ordered, lattice-like structure in ice and a less-ordered structure in the liquid state.

Analyze the phase transition from solid to liquid

When ice melts, the heat of fusion is the energy required to break the hydrogen bonds between the water molecules in the solid lattice structure without changing their temperature. This process leads to an increase in the disorder of these molecules as the solid-ordered structure becomes a more disordered liquid structure. During the phase transition from solid to liquid, only a few of the hydrogen bonds need to be broken, allowing the molecules to move more freely.

Analyze the phase transition from liquid to gas

On the other hand, when water boils, the heat of vaporization is the energy required to transform the liquid state into the gaseous state, also called steam. In this case, a much larger amount of energy is required to break almost all the hydrogen bonds between the water molecules completely. This process causes the water molecules to become much more disordered and widely spaced, as they transition from the liquid state to the gaseous state.

Compare the energy required for both phase transitions

Comparing both phase transitions, it becomes clear that the phase transition from liquid to gas requires more energy than the transition from solid to liquid. This is because in the former, nearly all hydrogen bonds need to be broken, causing a significantly greater increase in disorder as well as separation between the molecules. On the other hand, only a few hydrogen bonds need to be broken during the phase transition from solid to liquid, which leads to comparatively lesser increase in disorder and distance between the molecules.

Conclusion

In conclusion, the heat of vaporization of water is 5 times as great as the heat of fusion because it requires a substantially larger amount of energy to break almost all the hydrogen bonds and transition the water molecules from a liquid to a gaseous state, compared to the energy required to break only a few hydrogen bonds and transition water molecules from a solid to a liquid state. The unique molecular structure and hydrogen bonding in water play a crucial role in the differences observed in the energy requirements for these phase transitions.

Key concepts

Molecular Structure of Water

Water, a substance essential to life on Earth, has a simple but unique molecular design that significantly influences its physical properties. Each water molecule consists of one oxygen atom and two hydrogen atoms, with the oxygen at the vertex of a V-shape and the hydrogens at the tips. This geometry leads to an asymmetric distribution of charge, making water a polar molecule endowed with positive and negative ends.

The significance of water's bent structure lies in its impact on cohesive forces, particularly at play when water transitions between different states of matter, such as freezing and boiling. These transformations are deeply connected to the strength of interactions between individual water molecules, which are dictated by its molecular architecture.

Hydrogen Bonding

Hydrogen bonds, although weaker than covalent or ionic bonds, are powerful enough to profoundly affect water's properties. Formed due to the polarity of water, these bonds occur when the hydrogen part of one water molecule is attracted to the oxygen part of another water molecule. Picture each water molecule as a tiny magnet, with its hydrogen atoms seeking to align with the oxygen atoms of its neighbors.

In ice, this attraction results in a well-ordered, hexagonal lattice where molecules are locked in place. As a consequence of these extensive hydrogen bonds, water has an unusually high melting point compared to other molecules of similar size. When it comes to vaporizing liquid water, severing these pervasive bonds demands a significant amount of energy, far more than merely overcoming the weaker intermolecular forces in most other liquids.

Phase Transitions

Phase transitions—such as melting, freezing, boiling, and condensing—are physical processes involving a state change in matter. When it comes to water, these transitions are particularly intriguing due to the substance's extensive hydrogen bonding. During melting, the orderly crystal structure of ice is disrupted, allowing molecules to move freely as the substance becomes liquid. Conversely, when water freezes, the molecules slow down and organize into an ice lattice held together by hydrogen bonds.

During boiling, water molecules must obtain enough energy to break free from the collective hold of these bonds, transitioning into the spaciousness of the gaseous state. Each of these transitions requires a specific amount of energy, reflecting the strength of hydrogen bonds at different states: heavy-duty bonds in the solid state, a loose network in the liquid state, and almost no bonds in the gas phase.

Energy Requirements in Phase Changes

The amount of energy required to induce phase changes in water is a direct consequence of its molecular structure and hydrogen bonding. The heat of fusion, the energy needed to convert ice to liquid water, largely involves loosening the rigid lattice of hydrogen bonds found in ice. As the bonds are only partially disrupted, this transformation demands less energy compared to the heat of vaporization.

The heat of vaporization, the energy needed to transform water to steam, is immense because it entails breaking nearly all hydrogen bonds, allowing water molecules to scatter and fill a greater volume as a gas. The difference in energy requirements—five times more for vaporization than fusion—highlights the resilient nature of hydrogen bonds in water, requiring significant energy to transition from a structured liquid to an unordered gas.