Dispersion forces

Definition

Dispersion forces, also known as London dispersion forces, are a type of intermolecular force that arise from temporary shifts in electron density within atoms and molecules. These forces are the weakest among all intermolecular interactions but play a crucial role in the physical properties of substances, especially in nonpolar compounds. Despite their weak nature, dispersion forces can accumulate in larger molecules, significantly influencing boiling points, melting points, and solubility.

5 Must Know Facts For Your Next Test

1. Dispersion forces occur due to temporary fluctuations in electron distribution within molecules, creating temporary dipoles.
2. These forces increase with the size and number of electrons in a molecule; larger molecules tend to have stronger dispersion forces.
3. Dispersion forces are significant in nonpolar molecules where other types of intermolecular forces (like dipole-dipole) are absent.
4. The presence of dispersion forces can lead to unexpected behaviors in physical properties, such as higher boiling points for larger nonpolar molecules compared to smaller ones.
5. While dispersion forces are weak individually, they can become quite strong when many molecules are involved, leading to notable effects in bulk substances.

Review Questions

• How do dispersion forces compare to other types of intermolecular forces in terms of strength and significance?
  • Dispersion forces are the weakest type of intermolecular force compared to dipole-dipole interactions and hydrogen bonds. While they are individually weaker than these other forces, their significance grows in larger molecules where many dispersion interactions can sum up. This accumulation means that even nonpolar substances can exhibit notable physical properties due to dispersion forces alone.
• What factors influence the strength of dispersion forces in molecules, and how do these factors affect physical properties?
  • The strength of dispersion forces is influenced primarily by the size of the molecule and the number of electrons it contains. Larger molecules with more electrons tend to have greater dispersion forces due to increased polarizability. This enhanced polarizability leads to higher boiling and melting points, as the energy required to overcome these forces is greater in larger molecules than in smaller ones.
• Evaluate the role of dispersion forces in determining the solubility of nonpolar substances in solvents. What implications does this have for biological systems?
  • Dispersion forces play a key role in the solubility of nonpolar substances in nonpolar solvents. Because both solute and solvent experience similar intermolecular interactions, they can mix well. In biological systems, understanding these interactions is crucial for processes such as membrane formation and drug delivery. For instance, the solubility of lipids (which are largely nonpolar) in cell membranes affects how nutrients and drugs are absorbed and utilized by cells.

Related terms

dipole-dipole interactions:

A type of intermolecular force that occurs between polar molecules, where the positive end of one molecule is attracted to the negative end of another.

hydrogen bonding: A strong type of dipole-dipole interaction that occurs specifically when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine.

intermolecular forces: Forces that exist between molecules, affecting their physical properties such as boiling and melting points, viscosity, and solubility.