Dispersion forces

Definition

Dispersion forces, also known as London dispersion forces, are weak intermolecular forces that arise from the temporary fluctuations in electron density within molecules, leading to the formation of temporary dipoles. These forces are present in all molecules, regardless of their polarity, and become significant in larger atoms or molecules where there are more electrons available to create these temporary dipoles. Understanding dispersion forces is crucial for explaining the behavior of real gases and their interactions under different conditions.

5 Must Know Facts For Your Next Test

1. Dispersion forces are generally the weakest of all intermolecular forces but can still have a significant impact on the physical properties of substances, especially in larger molecules.
2. The strength of dispersion forces increases with molecular size and surface area because larger molecules have more electrons and a greater chance of creating temporary dipoles.
3. While dispersion forces are present in all substances, they are particularly important in nonpolar molecules where no other type of intermolecular force is present.
4. These forces play a crucial role in the condensation of gases into liquids, as they help to hold gas molecules together when they come close to each other.
5. In statistical mechanics, dispersion forces can affect the behavior of real gases by contributing to deviations from ideal gas behavior, especially under conditions of high pressure and low temperature.

Review Questions

• How do dispersion forces contribute to the physical properties of nonpolar substances?
  • Dispersion forces significantly influence the physical properties of nonpolar substances by providing a source of attraction between molecules. Since nonpolar substances lack permanent dipoles, these weak intermolecular forces are essential for holding the molecules together. The strength of these dispersion forces depends on the size and shape of the molecules; larger nonpolar molecules exhibit stronger dispersion forces due to a larger electron cloud that can create temporary dipoles.
• Evaluate how increasing molecular size impacts dispersion forces and its implications for real gases.
  • As molecular size increases, the strength of dispersion forces also increases due to greater polarizability and more electrons available to form temporary dipoles. This enhancement leads to higher boiling points for larger molecules compared to smaller ones. In the context of real gases, this means that larger gas molecules will deviate more significantly from ideal gas behavior at higher pressures and lower temperatures, where these dispersion interactions become more relevant in promoting attractive interactions.
• Synthesize the relationship between dispersion forces and statistical mechanics in explaining real gas behavior under various conditions.
  • Dispersion forces play a critical role in statistical mechanics when analyzing real gas behavior because they account for intermolecular attractions that deviate from ideal gas assumptions. In conditions such as high pressure and low temperature, where gas particles are closer together, dispersion forces become significant and lead to observable deviations from the ideal gas law. By understanding these interactions, statistical mechanics provides insights into how these weak attractions can influence macroscopic properties like compressibility and phase transitions in real gases.

Related terms

van der Waals forces:

A general term for the weak attractions between molecules that includes dispersion forces, dipole-dipole interactions, and hydrogen bonds.

polarizability:

The ability of a molecule's electron cloud to be distorted by an external electric field or by the presence of nearby charged particles, which influences the strength of dispersion forces.

intermolecular forces: Forces that mediate attraction between molecules, affecting physical properties like boiling points and solubility.