intermolecular forces

Examples of intermolecular forces in the following topics:

• 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.
  • The contribution of intermolecular forces creates deviations from ideal behavior at high pressures and low temperatures, and when the gas particles' weight becomes significant.
  • At high pressures, gas particles are forced into close proximity with one another, causing significant intermolecular interactions.
  • To correct for intermolecular forces between gas particles, J.D. van der Waals introduced a new term into the Ideal Gas Equation in 1873.

• Introduction to Intermolecular Forces

  • Indeed, many of the physical characteristics of compounds that are used to identify them (e.g. boiling points, melting points and solubilities) are due to intermolecular interactions.
  • This attractive force has its origin in the electrostatic attraction of the electrons of one molecule or atom for the nuclei of another.
  • If there were no van der Waals forces, all matter would exist in a gaseous state, and life as we know it would not be possible.
  • It should be noted that there are also smaller repulsive forces between molecules that increase rapidly at very small intermolecular distances.

• Dispersion Force

  • These intermolecular forces are also sometimes called "induced dipole-induced dipole" or "momentary dipole" forces.
  • London dispersion forces are part of the van der Waals forces, or weak intermolecular attractions.
  • Van der Waals forces help explain how nitrogen can be liquefied.
  • 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).

• 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.
  • Remember that the interactions between X and Y, represented above as X-Y, are classified as intermolecular forces, which are not covalent (bonding) interactions.
  • 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.

• Substances that Exist as Gases

  • Substances that exist in the gas phase exhibit negligible intermolecular forces.
  • These attractive forces are known as intermolecular forces.
  • An ideal gas is assumed to experience no intermolecular forces whatsoever, due to the fact that the particles of an ideal gas are moving so quickly, and are so far apart from one another, that they do not interact at all.
  • This is because under these conditions, intermolecular forces will be minimized.
  • Under standard conditions (1 atm, 273 K), a substance which exists as a gas is called a pure gas and (disregarding any substance-specific intermolecular forces or particle volume that could alter this value) has a volume of 22.4 L per mole.

• Dipole-Dipole Force

  • Dipole-dipole interactions are intermolecular attractions that result from two permanent dipoles interacting.
  • Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions).
  • 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.
  • Dipole–dipole interactions are a type of intermolecular attraction—attractions between two molecules.
  • In this video, Paul Andersen describes the intermolecular forces associated with dipoles.

• Ion-Dipole Force

  • The ion-dipole force is an intermolecular attraction between an ion and a polar molecule.
  • However, ion-dipole forces involve ions instead of solely polar molecules.
  • Ion-dipole forces are stronger than dipole interactions because the charge of any ion is much greater than the charge of a dipole; the strength of the ion-dipole force is proportionate to ion charge.
  • Ion-dipole forces are generated between polar water molecules and a sodium ion.
  • These intermolecular ion-dipole forces are much weaker than covalent or ionic bonds.

• Boiling & Melting Points

  • A clear conclusion to be drawn from this fact is that intermolecular attractive forces vary considerably, and that the boiling point of a compound is a measure of the strength of these forces.
  • The following table illustrates some of the factors that influence the strength of intermolecular attractions.
  • The attractive forces between the latter group are generally greater.
  • The distance between molecules in a crystal lattice is small and regular, with intermolecular forces serving to constrain the motion of the molecules more severely than in the liquid state.
  • Molecular size is important, but shape is also critical, since individual molecules need to fit together cooperatively for the attractive lattice forces to be large.

• Surface Tension and Capillary Action

  • Attractive forces between molecules of the same type are called cohesive forces.
  • Attractive forces between molecules of different types are called adhesive forces.
  • Such forces cause liquid drops to cling to window panes, for example.
  • It occurs because of intermolecular attractive forces between the liquid and solid surrounding surfaces.
  • With some pairs of materials, such as mercury and glass (see ), the intermolecular forces within the liquid exceed those between the solid and the liquid, so a convex meniscus forms, and capillary action works in reverse.

• Van der Waals Equation

  • The van der Waals equation modifies the Ideal Gas Law to correct for the excluded volume of gas particles and intermolecular attractions.
  • The gas particles are affected by the intermolecular forces acting on them, which leads to inelastic collisions between them. 
  • Derived by Johannes Diderik van der Waals in 1873, the van der Waals equation modifies the Ideal Gas Law; it predicts the properties of real gases by describing particles of non-zero volume governed by pairwise attractive forces.
  • The term involving the constant a corrects for intermolecular attraction.
  • Attractive forces between molecules decrease the pressure of a real gas, slowing the molecules and reducing collisions with the walls.