Question

Is it possible for the dispersion forces in a particular substance to be stronger than the hydrogen bonding forces in another substance? Explain your answer.

Short answer

Yes, it is possible for the dispersion forces in a particular substance to be stronger than the hydrogen bonding forces in another substance. Dispersion forces arise due to temporary fluctuations in electron density, while hydrogen bonding forces occur between molecules with a hydrogen atom bonded to a highly electronegative atom. An example of this is iodine (I₂) and water (H₂O). Iodine has strong dispersion forces due to its large molecular size, while water has hydrogen bonding forces. The higher melting point of iodine (113.7°C) compared to water (0°C) demonstrates that its dispersion forces can be stronger than the hydrogen bonding forces in water.

Step-by-step solution

Define dispersion forces

Dispersion forces, also known as London dispersion forces or van der Waals forces, are weak intermolecular forces between nonpolar molecules. They arise due to temporary fluctuations in electron density around the molecules, which leads to the creation of instantaneous dipoles. Dispersion forces are generally weaker than other types of intermolecular forces, but they can still play a significant role in certain substances, particularly those with large molecular sizes or multiple electron density fluctuations.

Define hydrogen bonding forces

Hydrogen bonding forces are a type of intermolecular force that occurs between molecules with a hydrogen atom bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine. This bond creates a strong dipole moment, where the electronegative atom attracts the electron density away from the hydrogen atom, resulting in a partial negative charge on the electronegative atom and a partial positive charge on the hydrogen atom. This partial charge difference leads to an attraction between the molecules, known as hydrogen bonding. Hydrogen bonding is generally stronger than dispersion forces, but weaker than covalent or ionic bonds.

Provide an example where dispersion forces are stronger than hydrogen bonding forces

Yes, it is possible for the dispersion forces in a particular substance to be stronger than the hydrogen bonding forces in another substance. For example, let's consider iodine (I₂) and water (H₂O). Iodine molecules are nonpolar and experience relatively strong dispersion forces due to their large molecular size and heavy mass. These forces cause the I₂ molecules to form solid crystals at room temperature. On the other hand, water molecules form hydrogen bonding forces with each other due to the polar nature of the molecule, with the oxygen being more electronegative than the hydrogen atoms. These hydrogen bonding forces are responsible for the relatively high boiling point of water compared to other small molecules. However, if we compare the melting points of iodine (113.7°C) and water (0°C), it is clear that the dispersion forces in iodine are strong enough to hold the molecules together in a solid state at a much higher temperature than the hydrogen bonding forces in water. This example demonstrates that it is possible for dispersion forces in a particular substance to be stronger than hydrogen bonding forces in another substance.