London Dispersion Forces

Definition

London dispersion forces are a type of intermolecular force that arises from the temporary, induced dipoles that can form in otherwise nonpolar molecules. These forces are the weakest of the intermolecular forces, but they play a crucial role in the properties of alkanes and other nonpolar compounds.

5 Must Know Facts For Your Next Test

1. London dispersion forces are the weakest of the intermolecular forces, but they are still important in determining the physical properties of nonpolar molecules.
2. The strength of London dispersion forces is directly proportional to the size and polarizability of the molecules involved.
3. Larger molecules with more electrons have greater polarizability and, therefore, stronger London dispersion forces.
4. London dispersion forces are responsible for the relatively low melting and boiling points of alkanes compared to other compounds with similar molar masses.
5. The increase in London dispersion forces with increasing molecular size and complexity is what accounts for the trend of increasing boiling points within a homologous series of alkanes.

Review Questions

• Explain how the size and polarizability of molecules affect the strength of London dispersion forces.
  • The strength of London dispersion forces is directly proportional to the size and polarizability of the molecules involved. Larger molecules with more electrons have greater polarizability, meaning they can more easily form temporary dipoles in response to the electric fields of neighboring molecules. This increased polarizability results in stronger attractive forces between the induced dipoles, leading to higher boiling and melting points for larger, more polarizable molecules.
• Describe the role of London dispersion forces in determining the physical properties of alkanes.
  • London dispersion forces play a crucial role in determining the physical properties of alkanes, such as their relatively low melting and boiling points compared to other compounds with similar molar masses. As the size of the alkane molecule increases, the London dispersion forces between the molecules also increase, leading to higher boiling points. This trend of increasing boiling points within a homologous series of alkanes is a direct result of the strengthening of London dispersion forces as the molecules become larger and more polarizable.
• Analyze how the weakness of London dispersion forces compared to other intermolecular forces affects the behavior of nonpolar molecules.
  • The weakness of London dispersion forces compared to other intermolecular forces, such as hydrogen bonding and ionic interactions, significantly affects the behavior of nonpolar molecules like alkanes. Due to the relatively weak London dispersion forces, alkanes have low melting and boiling points, low viscosities, and high compressibilities. These properties are crucial for the many applications of alkanes, such as in fuels, solvents, and as building blocks for more complex organic compounds. The understanding of London dispersion forces and their impact on the physical properties of nonpolar molecules is essential for predicting and explaining the behavior of these important chemical species.

Related terms

Intermolecular Forces:

Intermolecular forces are the attractive or repulsive forces between molecules that determine the physical properties of a substance, such as boiling point, melting point, and viscosity.

Induced Dipole: An induced dipole is a temporary separation of charge within a molecule or atom caused by the presence of an external electric field, such as the electric field created by a neighboring molecule.

Nonpolar Molecules: Nonpolar molecules are molecules in which the electrons are evenly distributed, resulting in no permanent dipole moment.