Van der Waals force

(noun)

Attractive forces between molecules (or between parts of the same molecule). These include interactions between partial charges (hydrogen bonds and dipole-dipole interactions), and weaker London dispersion forces.

Related Terms

  • intermolecular force
  • molecular solid
  • doping

Examples of Van der Waals force in the following topics:

  • Dispersion Force

    • 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).
    • How does changing the Van der Waals attraction or charging the atoms affect the melting and boiling point of the substance?
  • Introduction to Intermolecular Forces

    • All atoms and molecules have a weak attraction for one another, known as van der Waals attraction.
    • 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.
  • Hydrogen Bonding and Van der Waals Forces

    • Hydrogen bonds and van der Waals interactions are two types of weak bonds that are necessary to the basic building blocks of life.
    • Two weak bonds that occur frequently are hydrogen bonds and van der Waals interactions.
    • Like hydrogen bonds, van der Waals interactions are weak attractions or interactions between molecules.
    • Van der Waals attractions can occur between any two or more molecules and are dependent on slight fluctuations of the electron densities, which are not always symmetrical around an atom.
    • Explore how Van der Waals attractions and temperature affect intermolecular interactions.
  • Molecular Crystals

    • Molecules held together by van der Waals forces form molecular solids.
    • 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).
    • How does changing the Van der Waals attraction or charging the atoms affect the melting and boiling point of the substance?
  • Friction: Static

    • When surface area is below the micrometer range, Van der Waals' forces, electrostatic interactions and hydrogen bonding can cause two materials to adhere to one another.
    • Like kinetic friction, the force of static friction is given by a coefficient multiplied by the normal force.
    • As with all frictional forces, the force of friction can never exceed the force applied.
    • Thus the force of static friction will vary between 0 and $_sF_n$ depending on the strength of the applied force.
    • Any force larger than that overcomes the force of static friction and causes sliding to occur.
  • The Effect of Intermolecular Forces

    • Intermolecular forces describe the attraction and repulsion between particles.
    • To correct for intermolecular forces between gas particles, J.D. van der Waals introduced a new term into the Ideal Gas Equation in 1873.
    • By adding the term n2a/V2 to pressure, van der Waals corrected for the slight reduction in pressure due to the interaction between gas particles:
    • In the term above, a is a constant specific to each gas and V is the volume. van der Waals also corrected the volume term by subtracting out the excluded volume of the gas.
    • The full van der Waals equation of state is written as:
  • Van der Waals Equation

    • 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.
    • Isotherm (plots of pressure versus volume at constant temperature) can be produced using the van der Waals model.
    • Notice that the van der Waals equation becomes the Ideal Gas Law as these two correction terms approach zero.
    • The van der Waals model offers a reasonable approximation for real gases at moderately high pressures.
    • Distinguish the van der Waals equation from the Ideal Gas Law.
  • The Effect of the Finite Volume

    • At low temperatures where the contribution of intermolecular forces becomes significant
    • The van der Waals equation modifies the ideal gas law to correct for this excluded volume, and is written as follows:
    • Demonstrate an understanding of the van der Waals equation for non-ideal gases.
  • Real Gases

    • The model also fails for most heavy gases (including many refrigerants) and for gases with strong intermolecular forces (such as water vapor).
    • a is an empirically determined factor that corrects for the intermolecular forces between gas particles; it is specific for each gas
    • It is almost always more accurate than the van der Waals equation and frequently more accurate than some equations with more than two parameters.
    • Note that a and b here are defined differently than in the van der Waals equation.
    • However, these systems are used less frequently than are the van der Waals and Redlich-Kwong models.
  • Covalent Bonds and Other Bonds and Interactions

    • Two types of weak bonds that frequently occur are hydrogen bonds and van der Waals interactions.
    • Individual hydrogen bonds are weak and easily broken; however, they occur in very large numbers in water and in organic polymers, and the additive force can be very strong.
    • Like hydrogen bonds, van der Waals interactions are weak interactions between molecules.
    • Van der Waals attractions can occur between any two or more molecules and are dependent on slight fluctuations of the electron densities, which can lead to slight temporary dipoles around a molecule.
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