Question

Eicosane, \(\mathrm{C}_{20} \mathrm{H}_{42}\), has a higher melting point and a higher boiling point than water, even though water has hydrogen bonds between molecules and eicosane does not. Explain how this can be.

Short answer

Eicosane has a higher melting point and boiling point than water because its larger molecular size and numerous atoms cause the overall strength of the London dispersion forces to be greater than the strong hydrogen bonding present in water. This results in a higher temperature needed to overcome these forces and cause eicosane to melt or boil.

Step-by-step solution

Understand the structure of eicosane

Eicosane is an alkane with the formula C20H42. It is a hydrocarbon consisting of 20 carbon atoms in a long chain with each carbon atom bonded to 2 or 3 hydrogen atoms. The bonding within the molecule is covalent.

Understand the structure of water

Water has the molecular formula H2O. It is a polar molecule due to the difference in electronegativity between oxygen and hydrogen atoms. In addition, water molecules form hydrogen bonds with each other, which are relatively strong intermolecular forces.

Understand the intermolecular forces in eicosane

Eicosane is a non-polar molecule due to the similar electronegativity of carbon and hydrogen atoms. The primary intermolecular forces present in eicosane are London dispersion forces (also known as van der Waals forces). These forces are relatively weak individually, but the large number of carbon and hydrogen atoms in eicosane contributes to a significant overall dispersion force between eicosane molecules.

Understand the intermolecular forces in water

In water, hydrogen bonding is the dominant intermolecular force. Hydrogen bonds are relatively strong and account for many of the unique properties of water, including its high boiling and melting points when compared to other molecules of similar size.

Compare the melting and boiling points

Even though water has hydrogen bonding, which is stronger than the London dispersion forces found in eicosane, eicosane has a higher melting and boiling point. The reason for this is that the overall strength of the intermolecular forces in eicosane, due to its much larger molecular size and number of atoms, is greater than that of water. The greater the intermolecular forces, the higher the temperature required to overcome these forces and cause the substance to melt or boil. In summary, eicosane has a higher melting point and boiling point than water due to the larger size of its molecules and the strength of the London dispersion forces outweighing the strong hydrogen bonding present in water.

Key concepts

Melting and Boiling Points

Understanding the melting and boiling points of substances can seem puzzling, but it's all about the forces at play between molecules. In solids, molecules are tightly packed together and vibrate in place, whereas in liquids, they have enough energy to move around each other. For a substance to transition from solid to liquid (melting) or from liquid to gas (boiling), enough energy must be supplied to overcome the intermolecular forces holding the molecules together.

For example, eicosane, a substance made of long chains of carbon and hydrogen, has a higher melting and boiling point than water despite lacking the relatively strong hydrogen bonds found in water. This can be explained by considering the overall strength and number of intermolecular interactions present in eicosane—particularly, the London dispersion forces—that require more energy to overcome compared to the hydrogen bonds in water.

Hydrogen Bonding

When water molecules get cozy with each other, they experience a special type of intermolecular attraction called hydrogen bonding. This bond occurs when a hydrogen atom from one water molecule is attracted to the electronegative oxygen atom of another water molecule.

Hydrogen bonds are significantly stronger than most other types of intermolecular forces, lending water its unique properties, like high surface tension and the fact that it expands when it freezes. However, these bonds are not always the determining factor for boiling and melting points, as seen in the comparison with eicosane, which relies on a different type of force altogether.

London Dispersion Forces

Even the most seemingly unreactive molecules like eicosane can experience the fleeting attractions known as London dispersion forces. These are temporary, weak intermolecular forces that appear when electron clouds of atoms or molecules momentarily induce dipoles in neighbouring particles. The strength of these forces increases with the size and number of electrons in a molecule.

While individually weak, the collective strength of London dispersion forces in a large molecule such as eicosane can be sufficient to give it higher melting and boiling points than substances with smaller molecules, even those with stronger intermolecular forces like hydrogen bonds.

Molecular Structure

Peeking into the molecular structure can reveal a lot about a substance's physical properties. The long-chain structure of eicosane allows for extensive contact between molecules, magnifying the collective strength of the London dispersion forces across the entire molecule. This is in contrast to the structure of water which, although having strong hydrogen bonds, has a smaller molecular size limiting the extent of intermolecular interactions.

This demonstrates that besides the type of intermolecular force, the molecular structure, including the size and shape of the molecule, plays a critical role in determining melting and boiling points. The larger and more complex the molecule, the more extensive its intermolecular interactions, leading to higher energy requirements for melting and boiling.