London dispersion forces

Examples of London dispersion forces in the following topics:

• Dispersion Force

  • These London dispersion forces are often found in the halogens (e.g., F2 and I2), the noble gases (e.g., Ne and Ar), and in other non-polar molecules, such as carbon dioxide and methane.
  • London dispersion forces are part of the van der Waals forces, or weak intermolecular attractions.
  • London dispersion forces allow otherwise non-polar molecules to have attractive forces.
  • There are two kinds of attractive forces shown in this model: Coulomb forces (the attraction between ions) and Van der Waals forces (an additional attractive force between all atoms).
  • Explore the role of size and shape in the strength of London dispersion attractions.

• The Effect of Intermolecular Forces

  • At high pressures and low temperatures, intermolecular forces between gas particles can cause significant deviation from ideal behavior.
  • Intermolecular forces describe the attraction and repulsion between particles.
  • At high pressures, gas particles are forced into close proximity with one another, causing significant intermolecular interactions.
  • When the weight of individual gas molecules becomes significant, London dispersion forces, or instantaneous dipole forces, tend to increase, because as molecular weight increases, the number of electrons within each gas molecule tends to increase as well.
  • At high pressures and low temperatures, these attractive forces can become significant.

• Cycloalkanes

  • Cycloalkanes, like alkanes, are subject to intermolecular forces called London dispersion forces.

• Introduction to Bonding

  • They are related to weaker intermolecular forces, such as dipole-dipole interactions, the London dispersion forces, and hydrogen bonding.
  • The weaker forces will be discussed in a later concept.
  • Chemical bonds are the forces of attraction that tie atoms together.

• Dipole-Dipole Force

  • Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions).
  • These forces are weak compared to the intramolecular forces, such as the covalent or ionic bonds between atoms in a molecule.
  • Instantaneous dipole-induced dipole forces or London dispersion forces: forces caused by correlated movements of the electrons in interacting molecules, which are the weakest of intermolecular forces and are categorized as van der Waals forces.
  • In this video, Paul Andersen describes the intermolecular forces associated with dipoles.
  • An important type of dipole-dipole forces are hydrogen bonds.

• Steric Effects

  • When two neutral atoms (A & B) are close enough to detect each other, they experience an attraction due to dispersion forces.

• Molecular Crystals

  • Molecules held together by van der Waals forces form molecular solids.
  • Recall that a molecule is defined as a discrete aggregate of atoms bound together sufficiently tightly by directed covalent forces to allow it to retain its individuality when the substance is dissolved, melted, or vaporized.
  • Liquids and solids composed of molecules are held together by van der Waals (or intermolecular) forces, and many of their properties reflect this weak binding.
  • Because dispersion forces and the other van der Waals forces increase with the number of atoms, large molecules are generally less volatile, and have higher melting points than smaller ones.
  • There are two kinds of attractive forces shown in this model: Coulomb forces (the attraction between ions) and Van der Waals forces (an additional attractive force between all atoms).

• Properties of Alkenes

  • Alkene isomers that can achieve more regular packing have higher melting and boiling points than molecules with the same molecular formula but weaker dispersion forces.

• Intermolecular Forces and Solutions

  • The strength of the intermolecular forces between solutes and solvents determines the solubility of a given solute in a given solvent.
  • There are two conceptual steps to form a solution, each corresponding to one of the two opposing forces that dictate solubility.
  • If the solute is a solid or liquid, it must first be dispersed — that is, its molecular units must be pulled apart.
  • Many intermolecular forces can contribute to solvation, including hydrogen bonding, dipole-dipole forces, and Van Der Waals forces.
  • Another common example of these forces at work is an ion-dipole interaction, which arises when water solvates ions in solution.

• Solutions and Entropy Changes

  • An increase in entropy occurs when a solution is formed, providing one of the many driving forces for this process.
  • However, when they become spread out and disordered, the thermal energy they carry with them is also dispersed; the availability of this energy as measured by the temperature is also of importance.
  • As time proceeds, these molecules of dye are dispersed more uniformly throughout the solution even without mixing.