London dispersion forces

Definition

London dispersion forces are weak intermolecular forces that arise from the temporary fluctuations in electron distribution within molecules, leading to the formation of instantaneous dipoles. These forces are a type of Van der Waals force and are present in all molecules, regardless of whether they are polar or nonpolar. They play a significant role in the physical properties of substances, particularly in gases and nonpolar liquids.

5 Must Know Facts For Your Next Test

1. London dispersion forces increase with larger molecular size and mass, as larger molecules have more electrons that can fluctuate.
2. These forces are generally the weakest among intermolecular attractions, but they become significant in nonpolar substances where other types of interactions are absent.
3. The strength of London dispersion forces can vary widely depending on molecular shape; elongated molecules may experience stronger interactions than compact ones.
4. London dispersion forces are crucial for explaining why nonpolar substances can condense into liquids or solids at low temperatures.
5. In noble gases, London dispersion forces are responsible for their liquefaction under low temperatures and high pressures.

Review Questions

• How do London dispersion forces differ from other types of intermolecular forces?
  • London dispersion forces differ from other intermolecular forces primarily in their origin; they result from temporary fluctuations in electron distribution within molecules, creating instantaneous dipoles. Unlike dipole-dipole interactions that occur between polar molecules or hydrogen bonds which involve specific highly electronegative atoms, London dispersion forces are present in all molecules. Their strength is influenced by molecular size and shape, making them unique among intermolecular forces.
• Discuss the significance of London dispersion forces in the behavior of nonpolar substances.
  • London dispersion forces are essential for understanding the behavior of nonpolar substances because they provide the only intermolecular attraction available in these materials. In the absence of stronger interactions like dipole-dipole or hydrogen bonding, these weak attractions allow nonpolar substances to condense into liquids and solids under appropriate conditions. This phenomenon helps explain why gases such as noble gases can liquefy at low temperatures despite lacking permanent dipoles.
• Evaluate how increasing molecular size impacts London dispersion forces and its implications for physical properties like boiling points.
  • As molecular size increases, London dispersion forces become stronger due to a greater number of electrons and a larger electron cloud that can undergo fluctuations. This increase in attraction results in higher boiling points for larger molecules compared to their smaller counterparts. For instance, comparing alkanes, larger ones have significantly higher boiling points because the increased London dispersion forces overcome the kinetic energy that keeps them in a gaseous state at room temperature.

Related terms

Intermolecular forces: Forces that act between molecules, influencing the physical properties of substances such as boiling point and solubility.

Dipole-dipole interactions: Attractive forces between polar molecules, where the positive end of one molecule is attracted to the negative end of another.

Van der Waals forces: A collective term for weak intermolecular forces, including London dispersion forces, dipole-dipole interactions, and hydrogen bonding.