abbigm1
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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?
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SANTR
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(Original post by abbigm1)
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?
If you're given the names then I'd suggest you draw them out.
Permament dipole-dipole interacters occurs in molecules that have polar bonds i.e. there's a difference in electronegativity between two bonding atoms e.g. C-Cl
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KyleH123
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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
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RossB1702
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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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KyleH123
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(Original post by RossB1702)
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.
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RossB1702
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(Original post by KyleH123)
London dispersion forces are VDW forces, but they're the American name and you can't use that name in UK exams.
Alright thanks for clearing that up.
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KyleH123
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there we go
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abbigm1
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(Original post by KyleH123)
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
So is there a certain electronegativity that an element must have when bonded to hydrogen for it to have dipole dipole forces? It's hard to explain what I mean but what I'm trying to say is how different must the electronegativites be in order for the forces to be dipole dipole as aqa does not exactly specifiy a value.
And im assuming the difference in electronegativity between carbon and hydrogen is not significant enough to induce dipole dipole forces?
Thanks for the help
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username943595
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(Original post by KyleH123)
London dispersion forces are VDW forces, but they're the American name and you can't use that name in UK exams.
in the new spec for Edexcel, they accept London. In fact, they prefer you use London
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Bulletzone
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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.
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hassantsr
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yh permanent dipole dipole forces with molecules that have polar bonds so a difference in electronegativity
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abbigm1
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(Original post by Bulletzone)
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?
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Bulletzone
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(Original post by abbigm1)
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?
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