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Difference between van der Waals forces and dispersion forces

by soopo
Tags: difference, dispersion, forces, waals
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soopo
#1
Dec10-08, 12:41 PM
P: 226
What is the main difference between van der Waals forces and dispersion forces?

This statement confuses me:
"van der Waals forces are nearly the same thing as dispersion forces."

Right now, I have an idea that van der Waals forces includes these attractions:
1. permanent dipole - permanet dipole force such as H-bonding
2. permanent dipole - induced dipole forces
3. instantaneous induced dipole - induced dipole dispersion forces

I have observed that van der Waals forces are sometimes used as an synonym for intermolecular forces. I am not sure this completely correct.

In my opinion, dispersion forces are then again a subgroup of van der Waals forces which arise from the interactive forces between temporary multipoles in molecules without permanent dipoles.

Please, let me know the main difference between van der Waals forces and dispersion forces?
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#2
Dec10-08, 06:51 PM
P: 282
Van Der Walls forces account for both weak dipole dipole interactions in slightly polar molecules and for even weaker interactions between nonpolar molecules. London Forces are specifically limited to weaker interactions between nonpolar molecules. One is more general than the other (at least that is my understanding anyways), but the two terms can be used (scratch that..."are" used) interchangeably to mean weak interactions between non-polar molecules.
BioPhysicDino
#3
Aug23-09, 12:53 PM
P: 2
You are correct that van der Walls is the more general term, and dispersion forces is a particular case of van der Walls force.

van der Walls is the term used for any kind of inter- or intra-molecular force other than covalent and ionic bonds, so yes it includes all interactions between charges and dipoles, and dipoles and dipoles, whether those dipoles are permanent, induced, or transient. Also, any place where I just said dipole you can also include multipoles.

Dispersion force, also called London force, is the case where one or both of the dipoles (or multipoles) are transient (that is arise entirely from continual fluctuations in electron density). [As an aside point, Fritz London himself called them Dispersion Forces for the following reason: The ability of transient dipoles to attract one another depends strongly on the frequency of the fluctuations in electron density - that is two transiently induced dipoles will attract each other only if their frequencies match (are equal or multiples of one another). Quantities that vary with freqency are said to exhibit dispersion].

The confusion here comes from the fact that, while all agree on the basic definition of Dispersion forces, there is a lack of consistency among various textbooks on the use of the term van der Waals force - specifically in how general to make the term. Most textbooks include the definitition I gave above. However some textbooks vary in either direction: Some will use van der Waals force to describe ALL inter-moleculecular forces of any type, including ionic. Others go to the other extreme to include only induced dipoles and dispersion forces - leaving out permanent dipoles. I even saw one textbook that defined van der Waals force as identical to dispersion force (i.e. transient dipole interactions only).

The most correct use of van der Waals force, in my opinion, is EITHER the definition I originally gave above (i.e. all intra- and inter-molecular forces other than covalent and ionic), OR EQUALLY CORRECT is include even ionic forces when they are inter-molecular or otherwise not the classical ionic bonds found in salts and crystalline solids.

Hope this helps. --Dino.

soopo
#4
Aug23-09, 08:47 PM
P: 226
Difference between van der Waals forces and dispersion forces

Quote Quote by BioPhysicDino View Post
You are correct that van der Walls is the more general term, and dispersion forces is a particular case of van der Walls force.

van der Walls is the term used for any kind of inter- or intra-molecular force other than covalent and ionic bonds, so yes it includes all interactions between charges and dipoles, and dipoles and dipoles, whether those dipoles are permanent, induced, or transient. Also, any place where I just said dipole you can also include multipoles.

Dispersion force, also called London force, is the case where one or both of the dipoles (or multipoles) are transient (that is arise entirely from continual fluctuations in electron density). [As an aside point, Fritz London himself called them Dispersion Forces for the following reason: The ability of transient dipoles to attract one another depends strongly on the frequency of the fluctuations in electron density - that is two transiently induced dipoles will attract each other only if their frequencies match (are equal or multiples of one another). Quantities that vary with freqency are said to exhibit dispersion].

The confusion here comes from the fact that, while all agree on the basic definition of Dispersion forces, there is a lack of consistency among various textbooks on the use of the term van der Waals force - specifically in how general to make the term. Most textbooks include the definitition I gave above. However some textbooks vary in either direction: Some will use van der Waals force to describe ALL inter-moleculecular forces of any type, including ionic. Others go to the other extreme to include only induced dipoles and dispersion forces - leaving out permanent dipoles. I even saw one textbook that defined van der Waals force as identical to dispersion force (i.e. transient dipole interactions only).

The most correct use of van der Waals force, in my opinion, is EITHER the definition I originally gave above (i.e. all intra- and inter-molecular forces other than covalent and ionic), OR EQUALLY CORRECT is include even ionic forces when they are inter-molecular or otherwise not the classical ionic bonds found in salts and crystalline solids.

Hope this helps. --Dino.

I completely agree with you.
Thank you for your answer!


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