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Topic: Determining intermolecular forces from chemical formulas.  (Read 21087 times)

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Offline Eric_m22

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Determining intermolecular forces from chemical formulas.
« on: January 28, 2009, 06:19:35 PM »
Hello, I'm having trouble identifying what intermolecular forces are at play based of a chemical formula. For instance

Based simply on the molecular formula pro-
vided, which of the following compounds is
least likely to be miscible in cyclohexane
(C6H12)?
1. methanol (CH3OH) correct
2. benzene (C6H6)
3. phenol (C6H5OH)
4. Each contains a hydrocarbon unit and
should be equally miscible.
5. naphthalene (C10H8)

I don't understand why benzene or napthalene aren't more miscible in C6H12 since "like dissolves like" and the intermolecular forces are what determine miscibility and these seem to have similar IMF's and the methanol is a polar molecule which violates the "like dissolves like" ??

Also: 1. Rank the following in order of decreasing vapor pressure:
Ar, NH2, TiO2, C6H6, CH3OH, LiBr, CH2Cl2, C2H6.
1. TiO2 > LiBr > NH2 > CH3OH > CH2Cl2 > C6H6 > C2H6 > Ar.
2. Ar > C6H6 > CH2Cl2 > C2H6 > CH3OH > LiBr > NH2 > TiO2.
3. Ar > C2H6 > C6H6 > CH2Cl2 > CH3OH > NH2 > LiBr > TiO2. correct
4. Ar > C2H6 > C6H6 > CH2Cl2 > NH2 > CH3OH > LiBr > TiO2

This is where I'd get the most help from, how do I find out what IMF's exist within a molecular formula? For instance I understand there's an inverse relationship between the strength of the IMF and the vapor pressure i.e. a strong IMF = weak vapor pressure.

I think AR has the weakest IMF since the only IMF is Van Der Waal forces since it's a noble gas which means no dipole no hydrogen bonding and no ion-ion interaction..

However, why isn't CH2Cl2 ahead of C6H6? There's more hydrogen bonds in the C6H6 which constitutes a stronger IMF right?

Actually I'm quite confused as to why the order goes for all of them except for the AR, can someone please explain to me a more quanitative or qualitative way of figuring out relative IMF strengths of molecules? Thanks.


Offline macman104

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Re: Determining intermolecular forces from chemical formulas.
« Reply #1 on: January 28, 2009, 06:41:28 PM »
The first question asks for the least soluble compound.  Which you are correct, methanol is probably the least miscible with cyclohexane.

C6H6 does not have any hydrogen bonding, there are no O-H, N-H, or similar bonds.  Also, look at the structure for CH2Cl2, do you see any other bonding that might occur that would lower it's vapor pressure?

Offline Eric_m22

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Re: Determining intermolecular forces from chemical formulas.
« Reply #2 on: January 28, 2009, 08:03:43 PM »
Oh so hydrogen bonding only occurs when hydrogen bonds to a high electronegative element?

Would the Cl's be able to bond with other hydrogens creating hydrogen bonding to lower the vapor pressure?

Offline macman104

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Re: Determining intermolecular forces from chemical formulas.
« Reply #3 on: January 28, 2009, 08:15:56 PM »
Right, hydrogen has to be bound to an electronegative element, and then ANOTHER electronegative atom will participate in the hydrogen bonding.  So, first you need a hydrogen that is bound to an electronegative atom, which does not exist in CH2Cl2, if you were to look at the lewis structure.

Offline Eric_m22

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Re: Determining intermolecular forces from chemical formulas.
« Reply #4 on: January 28, 2009, 08:25:41 PM »
Oh I see and the methanol CH3OH is slightly polar, and has one hydrogen bond but it's not as bonded as NH2 which has two hydrogen bonds making it have higher IMF and the LiBr is ionic and ionic IMF is always stronger than hydrogen which makes it greater than NH2, but how do I differentiate LiBr from TiO2, is it because TiO2 is more electronegative which gives it a stronger IMF?

Also, I take it I need to draw out the lewis structures of all formula to determine how they would act together in a molar specimen to truly determine their IMF?

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