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Intermolecular forces

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StilgarMD

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On the GChem Assessment, it was asked "what forces keep NH3 together"

i figured since its interactions are limited to electrons, and the H's hanging off of it, that it would be only hydrogen bonding, but the answer was H-bonding and dipole dipole. is this saying that anywhere hydrogen bonding exists, dipole-dipole interactions exist? is that the case?
 

gtbROX

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Any molecule that has hydrogen bonding will also have dipole dipole and van der waals. Dipole just suggests that there is partial charge separation on the molecule, and hydrogen bonding is a strong form of that. Van der Waals is an intermolecular force found in all compounds.
 

dontbeanegaton

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On the GChem Assessment, it was asked "what forces keep NH3 together"

i figured since its interactions are limited to electrons, and the H's hanging off of it, that it would be only hydrogen bonding, but the answer was H-bonding and dipole dipole. is this saying that anywhere hydrogen bonding exists, dipole-dipole interactions exist? is that the case?

You have to think about the structure of the molecule. NH3 has sp3/tetrahedral electronic shape, but it's actual atomic shape is trigonal pyrimidal. Thus, NH3 is polar. For dipole-dipole interactions you must have a permanently polar molecule!
Hydrogen bonds require a deshielded hydrogen "donor" and an electronegative atom (N, O, F) with a lone pair. So theoretically ammonium (which cannot form dipole-dipole interactions because it is not polar) could act as a hydrogen bond donor.
The fact that NH3 can form dipole-dipole interactions is a result of the fact that it is trigonal pyrimidal and thus polar.
 
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