Dipole-dipole and Vdw forces

I'm doing as chem and I'm confused with how you are supposed to identify a molecule which has dipole-dipole and a molecules which has vdW forces? I know that Vdw forces take place in all molecules but when simple given the names of molecules how do you distinguish which has dipole-dipole and which has Vdw?

A think you're a bit confused. Permanent dipole-dipole forces are one of the theee Van der walls forces no? There's PDD, London dispersion forces and hydrogen bonding. These are all intermolecular forces. Then you have the intramolecular forces which are covalent, ionic and metallic bonding.


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London dispersion forces are VDW forces, but they're the American name and you can't use that name in UK exams.

Google "element electronegativity"

e.g. "Carbon electronegativity"

2.55 for carbon, hydrogen is 2.2

there has to be a significant difference in electronegativity for dipole dipole interactions to occur, (there is a number but im not 100% sure - it might be 2.5 or 1.5?)


CONVERSELY

van der waals forces occur between all molecules,
for example, in methane, at any given point, the carbon atom will be delta +, whilst a surrounding hydrogen is delta -, this small negative charge will attract slightly positive atoms from nearby methane molecules. e.g. a hydrogen from another molecule that is delta -.

@abbigm1

London dispersion forces are VDW forces, but they're the American name and you can't use that name in UK exams.

In every atom and molecule on this earth there will be Van der waal (London dispersion forces).
(Nobody be a smartass about this; the spec says these very words).

Dipole Dipole interaction exists between Dipoles.
A Dipole is simply a difference in Electronegativity, Pretty much if you have a C-X or H-X where X is the Halogen, You will have a Dipole Dipole Interaction, but don't forget you still have Van der waals forces present, it's just that Dipole-Dipole Interaction is the strongest.

I think people are misinterpreting my question.
So is the general rule that dipole dipole interactions occur with halogens because the difference in electronegativity is significant enough?
For example hydrogen sulfide has only Van der waal forces despite a difference in electronegativity, is this because the difference isn't large enough?