Kavita a
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What is the difference between Dipole-Dipole forces and Van Der Waals forces?

Just when I think I understand them, I keep getting the question wrong.
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k4l397
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Okay well firstly one is temporary and one is permanent, van der waals are temporary and the other one is permanent. Remember to mention permanent otherwise they may not give you the mark.

So a permanent dipole-dipole force is when you have one atom that is more electronegative than the other across a bond so the electrons are drawn closer to that atom. As a result a dipole forms (charge difference across the covalent bond). This is permanent and as a result permanent dipole-dipole intermolecular forces formed between that molecule and others with dipoles are known as permanent dipole-dipole forces.

Van Der Waals forces are only temporary and are induced. As a very basic example, if you had a hydrogen atom alone (I know it's usually in a two but it's just an example), when the electron in it's outer shell is on one side of the atom, that side has a slight negative charge and as a result the other side has a slight positive charge. This allows it to form Van Der Waal forces with other atoms, but they are only temporary. The negative side of the Hydrogen atom can repel electrons on the atom next to it, this induces a new van der waal force as it means the atom next to it now has a charge difference across it so can form van der waal forces.

Hope I explained that okay, if I didn't let me know and I can try and explain it better
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Kavita a
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(Original post by k4l397)
Okay well firstly one is temporary and one is permanent, van der waals are temporary and the other one is permanent. Remember to mention permanent otherwise they may not give you the mark.

So a permanent dipole-dipole force is when you have one atom that is more electronegative than the other across a bond so the electrons are drawn closer to that atom. As a result a dipole forms (charge difference across the covalent bond). This is permanent and as a result permanent dipole-dipole intermolecular forces formed between that molecule and others with dipoles are known as permanent dipole-dipole forces.

Van Der Waals forces are only temporary and are induced. As a very basic example, if you had a hydrogen atom alone (I know it's usually in a two but it's just an example), when the electron in it's outer shell is on one side of the atom, that side has a slight negative charge and as a result the other side has a slight positive charge. This allows it to form Van Der Waal forces with other atoms, but they are only temporary. The negative side of the Hydrogen atom can repel electrons on the atom next to it, this induces a new van der waal force as it means the atom next to it now has a charge difference across it so can form van der waal forces.

Hope I explained that okay, if I didn't let me know and I can try and explain it better
I think I've got it. So, the dipole - dipole forces are formed only when the difference in electronegativy is high?
How do you know when the difference is enough to form a dipole-dipole?

Am I even making sense?!
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k4l397
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(Original post by Kavita a)
I think I've got it. So, the dipole - dipole forces are formed only when the difference in electronegativy is high?
How do you know when the difference is enough to form a dipole-dipole?

Am I even making sense?!
A dipole is formed when there is a difference in electro-negativity between two atoms in a covalent bond, usually you'll be able to tell how electronegative an atom is by it's position on the periodic table. Atoms in the top right are very electronegative but as you move towards the bottom left they aren't so electronegative. This results in one end of the covalent molecule being slightly positive and the other slightly negative. It should be obvious in the question if a molecule is polar (slightly positive on one side, slightly negative on the other). A polar molecule has a dipole. I'm not too sure if the difference has to be over a certain amount but it will be obvious.

Water for example:
Name:  covalent-bond-oxygen-hydrogen.gif
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Size:  8.3 KB

Oxygen is very electronegative, Hydrogen isn't as electronegative, so the electrons are pulled closer to the oxygen. The oxygen becomes slightly negative and as a result the Hydrogen becomes slight positive. This means water molecules are polar.

It's the polarity that forms the dipole-dipole intermolecular forces, they are called permanent dipole-dipole forces because in order for a molecule to be permanently polar they must have a permanent charge difference across them (one end is more positive the other more negative). They then form bond very much like a magnet, the positive end is attracted to the negative end of other polar molecules forming this intermolecular bond.


You are making sense don't worry I'm more worried I'm not :P
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Kavita a
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(Original post by k4l397)
A dipole is formed when there is a difference in electro-negativity between two atoms in a covalent bond, usually you'll be able to tell how electronegative an atom is by it's position on the periodic table. Atoms in the top right are very electronegative but as you move towards the bottom left they aren't so electronegative. This results in one end of the covalent molecule being slightly positive and the other slightly negative. It should be obvious in the question if a molecule is polar (slightly positive on one side, slightly negative on the other). A polar molecule has a dipole. I'm not too sure if the difference has to be over a certain amount but it will be obvious.

Water for example:
Name:  covalent-bond-oxygen-hydrogen.gif
Views: 229
Size:  8.3 KB

Oxygen is very electronegative, Hydrogen isn't as electronegative, so the electrons are pulled closer to the oxygen. The oxygen becomes slightly negative and as a result the Hydrogen becomes slight positive. This means water molecules are polar.

It's the polarity that forms the dipole-dipole intermolecular forces, they are called permanent dipole-dipole forces because in order for a molecule to be permanently polar they must have a permanent charge difference across them (one end is more positive the other more negative). They then form bond very much like a magnet, the positive end is attracted to the negative end of other polar molecules forming this intermolecular bond.


You are making sense don't worry I'm more worried I'm not :P

Alright! I've got it. So, based on the electronegativty of two elements, if the electronegativity difference between the two is great, then they will form a dipole-dipole intermolecular force (which is actually the polarity of the molecule?) and a van der waals force is a temporary polarity? Or should I just think of it as something that ISN'T a Dipole Dipole?

I know how hydrogen bonds are formed, thank goodness, it's just.....difficult.

But no! You're making total sense, I just think I'm loosing my mind,
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blueynuey1
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which imf force is stronger temporary dipole-dipole interactions or permanent dipole-dipole interactions.

i've looked at a lot of sources and and they all say different things. the edexcel AS chemisrtry by Ann fullick and my teacher both say that permanent dipole are stronger than instantaneous whereas the edexcel AS chemistry by george facer and chemguide website say that instantaneous dipoles are stronger than permanent dipoles. So which do i believe ...

here is link to the chemguide's view on this: http://www.chemguide.co.uk/atoms/bon...ngths.html#top

can someone explain it to me preferably using the edexcel markscheme as i can't find it anywhere thanks!
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k4l397
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(Original post by blueynuey1)
which imf force is stronger temporary dipole-dipole interactions or permanent dipole-dipole interactions.

i've looked at a lot of sources and and they all say different things. the edexcel AS chemisrtry by Ann fullick and my teacher both say that permanent dipole are stronger than instantaneous whereas the edexcel AS chemistry by george facer and chemguide website say that instantaneous dipoles are stronger than permanent dipoles. So which do i believe ...

here is link to the chemguide's view on this: http://www.chemguide.co.uk/atoms/bon...ngths.html#top

can someone explain it to me preferably using the edexcel markscheme as i can't find it anywhere thanks!
The permanent one is the stronger one because that force is always present where as the van der waals (temporary ones) are always being created and destroyed. What the other two might be suggesting is at the instant the van der waal is created, it is stronger but because it is constantly destroyed and created it has a much smaller effect overall. If you are ever asked to write about it you would always say van der waals are much weaker.

Sorry for not responding on this again before, I think I just forgot.

Edit: Also when you think about it, elements like oxygen and halogens ect that just have van der waals' have much lower melting points than things like water and other covalent molecules with permanent dipole dipole interactions so permanent dipole dipole forces must be stronger. (I know water isn't a great example because it has hydrogen bonds too but I couldn't think of any others :P ).
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blueynuey1
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(Original post by k4l397)
The permanent one is the stronger one because that force is always present where as the van der waals (temporary ones) are always being created and destroyed. What the other two might be suggesting is at the instant the van der waal is created, it is stronger but because it is constantly destroyed and created it has a much smaller effect overall. If you are ever asked to write about it you would always say van der waals are much weaker.

Sorry for not responding on this again before, I think I just forgot.

Edit: Also when you think about it, elements like oxygen and halogens ect that just have van der waals' have much lower melting points than things like water and other covalent molecules with permanent dipole dipole interactions so permanent dipole dipole forces must be stronger. (I know water isn't a great example because it has hydrogen bonds too but I couldn't think of any others :P ).
Lool thanks so much for responding really cleared this up!!
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