London Dispersion versus Wan der Waals

[MedGrl@2022]

Should I be able to differentiate between Van der Waals and London Dispersion for MCAT? They seem to be very similar, right? How would you differentiate between the two?

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[sdm33]

I read that dipole dipole, H- bonding and London dispersion are all types of van der waals forces, however for the MCAT I think Van der waals is the same as London Dispersion, I don't think that both going to show as answers at the same time, if they do let me know..

 

[Temperature101]

I read that dipole dipole, H- bonding and London dispersion are all types of van der waals forces, however for the MCAT I think Van der waals is the same as London Dispersion, I don't think that both going to show as answers at the same time, if they do let me know..

Are sure that H-bonding is a type of Van der Waals force? I was under the impression that all the Van der Waals forces are weak forces.

 

[Marika11]

Are sure that H-bonding is a type of Van der Waals force? I was under the impression that all the Van der Waals forces are weak forces.

H-bonding is not a Van der Waals force. However, both dipole/dipole and london dispersion forces are considered Van der Waals forces.

 

[sdm33]

Yes it is little tricky, you see H bonding can be classified as both inter-molecular and intra-molecular bonds , H bond is not that strong in comparison to ionic or covalent bonds, however H Bonding is stronger than Dipole Dipole and London dispersion forces.
For Example, Boiling water involves the breaking of intermolecular bonds but leaves intramolecular bonds intact.

 

[MD Odyssey]

Yes it is little tricky, you see H bonding can be classified as both inter-molecular and intra-molecular bonds , H bond is not that strong in comparison to ionic or covalent bonds, however H Bonding is stronger than Dipole Dipole and London dispersion forces.

This is not entirely accurate and unfortunately, the MCAT does test this pretty heavily, so it's worth your time to get it right. Hydrogen bonds are not stronger than dipole-dipole interactions. They are dipole-dipole interactions. As dipole-dipole interactions go, they're not necessarily the strongest either. What makes hydrogen bonds special is that you can make lots of them. An individual hydrogen bond is not especially strong, but when your solvent is capable of accepting and donating hydrogen bonds, the interactions become extremely favorable.

This is where the MCAT gets a little annoying. The definition of van der Waal's forces is not entirely agreed upon. Some use van der Waal's forces to refer to all of the intermolecular attractive forces except for covalent, ionic, or hydrogen bonds. Others use the term to refer to induced-dipole attraction, which is what seems to be how the MCAT treats it. Technically speaking, London dispersion forces refer strictly to induced-dipole interactions, which are a subset of van der Waal's forces.

This distinction doesn't really get stressed on the MCAT anyway. It's far more important that you understand what leads to all of the intermolecular interactions and are able to rank them. The MCAT often seems to test these topics in the context of solubility, which tends to be governed by the energetics (i.e., enthalpies) of the interactions, although not always. A good example of this is the fact that non-polar substances do not dissolve in water. Most people attribute this to the strength of hydrogen bonding, but that turns out to not be the case. The reason non-polar substances do not dissolve in water has nothing to do with the energetics of hydrogen bonding.

 

[sdm33]

Mr Odyssey can you explain how Hydrogen bonds are not stronger than dipole-dipole interactions. They are dipole-dipole interactions ??? Are Republicans and Democrats The Same Thing?