Intermolecular Forces question

1. Apr 13, 2012

1. The problem statement, all variables and given/known data

The question asks to list all the intermolecular forces operating in each of the two liquids.

$H_2O$
$CH_3OH$

2. Relevant equations

N/A

3. The attempt at a solution

I'm mostly getting confused about the London Dispersion Force with water, and London Dispersion Force and Dipole-Dipole with Methanol.

I know that both of these molecules have a Hydrogen Bond, but I can't seem to pinpoint the other forces. I've googled it and seen several people state that water has all three, and methanol has only LDF and Dipole-Dipole. Drawing the lewis dot diagram of these I just don't see how water can have a LDF, and how methanol can have either an LDF or a Dipole-Dipole. Is it because they are asking for it in a liquid state, and they can orient themselves more freely?

Any help is appreciated, or let me know if perhaps I have a concept wrong.

Mac

*EDIT* Just to add to this, $CH_3F$ would appear to me to have an obvious Hydrogen bond, as you have extreme polar opposites from the Electronegative Fluorine on one side of the molecule, and Hydrogen on the other side. However, it seems from what I've found online that this molecule does not have a hydrogen bond. I think I am definitely missing something.

*2nd EDIT* Okay, as for above I see that the Fluorine is not bonded directly with the Hydrogen, so there can not be a Hydrogen Bond. That does not apply to the Methanol though.

Last edited: Apr 13, 2012
2. Apr 13, 2012

babita

hey hydrogen bond is the force which binds H atom of one molecule with an electronegative atom of another......so H bonding is possible in CH3F,H2O and CH3OH

3. Apr 13, 2012

babita

i mean to say i don't think that H has to necessarily bonded with an electronegative atom for hydrogen bonding to occur

4. Apr 13, 2012

babita

And i read somewhere that besides dipole-dipole interaction, all polar molecules can also interact by London forces. And the cumulative effect is that the total of inter molecular forces increase. Though i don't know how :P
Considering above all 3 forces operate in both the liquids.(i think so, i might be wrong)

5. Apr 13, 2012

Staff: Mentor

What kind of hydrogens take part in hydrogen bonding? Every hydrogen, or only some?

6. Apr 13, 2012

I think I am starting to see this. It almost seems that all polar molecules and non-polar molecules do have some dispersion force. Hopefully Borek can verify this.

That's an interesting question, and I'm not sure. I would guess only the Hydrogen atom and not the Ion, as I don't see any mention of hydrogen bonding in an ionic compound.

If you look at the attachment you'll see a flow chart on how my book instructs us to go about it. This chart only shows one force for each molecule though, and I know that there can be more than one.

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7. Apr 13, 2012

Staff: Mentor

Dig deeper then, as you are missing an important piece of information. Hydrogens are definitely not equal.

8. Apr 13, 2012

Attempting to use my brain a bit I suppose it has to be the Hydrogen Proton, or H+. Is that correct? Or at least it becomes the Hydrogen Proton after interacting with either N, O, F, or Cl.

I am seeing so much conflicting information online that I think I just need to ask. Does $CH_3OH$ have a Hydrogen Bond, a Dipole-Dipole, and a London Dispersion Force? Because from what I can see I believe it does. At this point I think I can safely say that Oxygen has all three forces.

9. Apr 14, 2012

Staff: Mentor

It is about the bond and the atom hydrogen is bonded to. Don't waste time on guessing, look in your book, or google for hydrogen bond.

10. Apr 14, 2012