Dispersion Force - The intermolecular force

In summary, the dispersion force is the weakest of the intermolecular forces and was discovered by Fritz W. London. It is similar to the gravity force in that it is a universal and permanent force between any two masses, but it is different because it is an electric force.
  • #1
srmeier
100
0
(I would like to put an idea of mine to rest) About one year ago I took a college chemistry course in which I learned about intermolecular forces. The weakest of these intermolecular forces, I learned, was the dispersion force (also called London force). This force was discovered by Fritz W. London. Here I would like to quote my text:

"Dispersion forces are the result of fluctuations in the electron distribution within molecules or atoms. Since all atoms and molecules have electrons, they all exhibit dispersion forces... An instantaneous dipole on anyone atom induces instantaneous dipoles on neighboring atoms, which then attract one another..."

-by CHEMISTRY: A Molecular Approach, Nivaldo J. Tro (pg.466)

This (at the very least) seems a lot like gravity. So, why can't we explain gravity as just another aspect of the electromagnetic force which manifests itself through millions of intermolecular electron fluctuations?

Thanks,
 
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  • #2
This is not gravity, it is due to the dipole, and is an electric force.
 
  • #3
MikeyW said:
This is not gravity, it is due to the dipole, and is an electric force.

I understand that they are classified as different things. I just feel as though if dispersion forces can account for atoms attracting one another then why do we need gravity?
 
  • #4
Dispersion forces are instantaneous, and they form dipole moments, not permanent dipoles. A simple way of putting this is that, depending on the location of the electron, the dipole is either turned on or off like a switch.

Gravity, on the other hand, is universal and permanent. It is a constant force between any two masses regardless of electrical charge (or lack thereof). We need gravity in addition to the electrical intermolecular forces simply because they are two different forces.

You might be interested in looking into the "Theory of Everything" for more information on the unification of forces. This is one of the biggest and most important problems in physics today.
 
  • #5
pzona said:
Dispersion forces are instantaneous, and they form dipole moments, not permanent dipoles. A simple way of putting this is that, depending on the location of the electron, the dipole is either turned on or off like a switch.

Gravity, on the other hand, is universal and permanent. It is a constant force between any two masses regardless of electrical charge (or lack thereof). We need gravity in addition to the electrical intermolecular forces simply because they are two different forces.

You might be interested in looking into the "Theory of Everything" for more information on the unification of forces. This is one of the biggest and most important problems in physics today.

I read that book and liked it a lot (although i didn't understand most of it), Thanks!
 
  • #6
Keep reading! It'll start to make more sense the more you study it, I promise.
 

1. What is a dispersion force?

A dispersion force, also known as London dispersion force, is an intermolecular force that occurs between nonpolar molecules. It is caused by temporary dipoles induced by the constant motion of electrons within the molecule.

2. How do dispersion forces affect the properties of a substance?

Dispersion forces are responsible for the attractive forces between nonpolar molecules. These forces increase as the size of the molecule increases, leading to higher boiling points and melting points. They also contribute to the surface tension and viscosity of liquids.

3. Can dispersion forces occur between polar molecules?

Yes, although dispersion forces are more significant in nonpolar molecules, they can also occur between polar molecules. The strength of the dispersion force in polar molecules is typically weaker compared to nonpolar molecules due to the presence of other intermolecular forces such as dipole-dipole interactions.

4. How does temperature affect dispersion forces?

As temperature increases, the motion of molecules also increases, leading to a higher likelihood of induced dipoles and thus stronger dispersion forces. This is why substances with stronger dispersion forces tend to have higher boiling points and melting points.

5. How do dispersion forces play a role in solubility?

Dispersion forces play a significant role in the solubility of nonpolar substances in nonpolar solvents. This is because the dispersion forces between the solute and solvent molecules are similar in strength, allowing for a homogenous mixture to form. However, they do not play a significant role in the solubility of polar substances in polar solvents.

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