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Intermolecular Forces

by lovelylila
Tags: forces, intermolecular
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lovelylila
#1
Mar29-08, 08:02 PM
P: 17
I am really confused in my advanced chemistry class. We are currently doing intermolecular and intramolecular forces. For example, a book problem that has me frustrated is :

"What is the strongest interaction between hexane and iodine?"

What's the difference between "dipole-dipole" and "instantaneous dipole-dipole"? In general, how do you determine what intermolecular forces are at work when two molecules interact (ie, can a non-polar molecule still have dipoles?) Any help would be very much appreciated, because I am totally lost!
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LtStorm
#2
Mar29-08, 10:51 PM
P: 76
Off the top of my head, London Dispersion Forces/van der Waals Forces would be the strongest interaction between hexane and iodine. LDF works due to transient dipoles in a non-polar molecule (which hexane most certainly is), and iodine being presumably in its diatomic state being similarly non-polar.

There wouldn't be dipole-dipole interaction because neither of them have a permanent dipole due to their non-polar nature.
lovelylila
#3
Mar30-08, 07:32 AM
P: 17
Thank you very much!

Hootenanny
#4
Mar30-08, 08:03 AM
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Intermolecular Forces

In simple terms one can imagine the electrons of a molecule as a 'cloud of electrons' that is that the electrons are not bound to a particular atom, rather they form a collective 'cloud' shared between all the atoms in the molecule.

Now, molecules with a permanent dipole arise as a result of this electron cloud being asymmetrically distributed. For the simple case, consider the covalent molecule hydrogen chloride (HCl). Chlorine has a higher electro-negativity than hydrogen, which means it attracts the electron cloud more than hydrogen does. Therefore, in our simple picture this means that the electron cloud is pulled closer the the chlorine atom. This will result in more electrons spending more time closer to the chlorine atom, which means that the chlorine end of the atom will obtain a 'partial' negative charge and the hydrogen end of the molecule will gain a 'partial' positive charge. In contrast, now consider carbon dioxide (CO2). Again in this case oxygen has a higher electro-negativity than carbon and so the electron cloud will be 'pulled' towards the oxygen atoms. However, since the molecule is linear (i.e. the molecule consists of and oxygen molecule followed by a carbon atom followed by a further oxygen atom in a straight line, O=C=O), the electron cloud is 'pulled' equally in both directions and so the molecule has no overall net dipole.

For the case of instantaneous dipoles, consider once again the electron cloud. One can visualise the electron cloud literally as a cloud of a number of electrons moving randomly in all directions. Now imagine one could freeze the position of the electrons at any instant in time (analogous to taking a picture). If one could do this, one would observe that more often than not the electrons will not be evenly distributed, at any given time there will be more electrons on one side of a molecule than another. Therefore, one side of the molecule will have a 'partial' negative charge (where there are more electrons) and the other side will have a 'partial' positive charge (where there are less electrons). Obviously, the position of these partial positive/negative charges will be changing all the time. This is an instantaneous dipole.

I would like to point out that although I have studied chemistry in the past I am by no means what one would formally call a chemist and the picture I have painted is a rather crude visualisation, but hopefully you will get something out of it.

Edit: After typing that up I've just read your last post . Non of the three options you state are either dipoles nor hydrogen bonds. To answer your question consider how each bond is formed and decide whether it is possible in the case in question.
lovelylila
#5
Mar30-08, 08:10 AM
P: 17
Thanks for that amazing explanation! Yea, the only possibility I can think of is covalent, because they definitely wouldn't form a hydrogen or an ionic--but do these two substances even bond covalently?
Hootenanny
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Mar30-08, 08:23 AM
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Quote Quote by lovelylila View Post
Thanks for that amazing explanation! Yea, the only possibility I can think of is covalent, because they definitely wouldn't form a hydrogen or an ionic--but do these two substances even bond covalently?
From what I remember Iodine is strongly nucleophilic, which means it should be able attach to the carbon in methyl group of the hexane.

Chemists, feel free to correct me.


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