Question

A hydrogen bond is a special case of what type of intermolecular force?

Short answer

Hydrogen bonds are a special case of dipole-dipole interactions.

Step-by-step solution

- Identify the Types of Intermolecular Forces

Intermolecular forces are the forces that occur between molecules. The main types include London dispersion forces, dipole-dipole interactions, and hydrogen bonds. London dispersion forces are the weakest, while hydrogen bonds are among the strongest.

- Define a Hydrogen Bond

A hydrogen bond is a strong dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine, which exposes the hydrogen nucleus and creates a strong attraction.

- Relate Hydrogen Bonds to Dipole-Dipole Interactions

Since hydrogen bonds are a type of dipole-dipole interaction, they are considered a special case of dipole-dipole forces. These are stronger because they involve highly electronegative elements and a hydrogen atom, which is very small and can get closer to other atoms.

- Conclusion

Based on the definitions and relationships documented, one can conclude that hydrogen bonds are a special case of dipole-dipole intermolecular forces.

Key concepts

Intermolecular Forces

Intermolecular forces are the attractions that occur between molecules. These forces are crucial for determining the physical properties of substances, such as boiling points, melting points, and solubilities. There are three main types of intermolecular forces, which include:

• London Dispersion Forces: These are the weakest intermolecular forces and are present in all molecules, regardless of polarity.
• Dipole-Dipole Interactions: These forces occur between molecules that have permanent dipoles, which means they have a positive and a negative end.
• Hydrogen Bonds: These are strong dipole-dipole interactions involving hydrogen atoms bonded to highly electronegative atoms such as nitrogen, oxygen, or fluorine.

Understanding these forces helps in explaining why different substances exhibit different physical behaviors.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules. These molecules have regions with slight electrical charges due to differences in electronegativity between the atoms involved. In a polar molecule, one part is slightly negative while the opposite part is slightly positive.

When such molecules approach each other, the positive end of one molecule is attracted to the negative end of another. This creates a net intermolecular force that holds the molecules together.

• For example, in a molecule of hydrogen chloride (HCl), the chlorine atom is more electronegative than the hydrogen atom. This creates a dipole with the hydrogen being slightly positive and the chlorine being slightly negative.
• When two HCl molecules are close together, the positive hydrogen end of one molecule will be attracted to the negative chlorine end of the other.

Although these interactions are stronger than London dispersion forces, they are not as strong as hydrogen bonds.

London Dispersion Forces

London dispersion forces are the weakest type of intermolecular forces and arise due to temporary fluctuations in electron density within molecules. Even in non-polar molecules, electrons are always moving and can occasionally create temporary dipoles.

Here’s how London dispersion forces work:

• An instantaneous dipole can induce a dipole in a neighboring molecule, temporarily attracting them to each other.
• These forces are present in all molecular interactions, but they are especially significant in non-polar molecules where no other forces are at play.

The strength of London dispersion forces increases with the size (molecular weight) and shape of the molecules involved. Larger molecules have more electrons, which can form stronger temporary dipoles. Although they are the weakest intermolecular force, they play a crucial role in the behavior of gases and in the formation of liquids and solids for nonpolar substances.