Expert: Dr. Henry Boyter, Jr. - 11/29/2004

Question
Dear Expert!

I'm trying to produce formulas which are able to give me a (more or less) correct estimation of the boiling and melting point of a substance.

I started with the melting point and tried to construct the formula using the approach Tmelt = X - 273,15 where X is a measurement for the quantity of intermolecular forces.
First I wanted to find out the effect of the molecular weight on the melting point changing my formula to Tmelt = X + a · mw - 273,15 where X is the effect of other intermolecular forces, a is a factor of proportionality and mw is the molecular weight.
So I looked at substances that haven't many intermolecular forces (or X is about zero) - the noble gases.

There my problems started. I was hoping for a linear dependency therefore a linear graph. But I got a curve. Realizing that there are Van der Waals interactions observed in noble gases, which is why they condense into liquids, I wondered if there is any effect of the molecular weight on the melting point. Would a molecule with no intermolecular forces have a boiling and melting point or would it always be a gas? Or reworded: Is the molecular weight alone an independent effect or is it contained in the effect of the Van der Waals force?

Whatever it was I was observing that bent the graph into a curve I found out that for the smaller noble gases a linear approximation was sufficient because not only the coefficient of determination was quite good (R2 = 0,9922) but also the absolute zero the approximation delivered (Tmelt = 1,27 · mw - 272,97) was about correct.

Next I looked at other molecules with few intermolecular forces: H2, N2, O2, F2, Cl2, Br2, J2
But they didn't show a very good correspondence to molecular weight.
And when I combined the bimolecules with the noble gases the correspondence to molecular weight got even worse.
Here my attempt failed.


Now my questions:
Is the effect of molecular weight and Van der Waals force equivalent?
Can I describe the effects of ionic interactions, hydrogen bonds, dipole-dipole interactions and Van-der-Waals forces simply with the difference in electronegativity and the resulting strength of the charge?
Is it at all possible to describe the boiling and melting point as sum of independent effects such as absolute zero, molecular weight, electronegativity or presence of H-bonds?
Why did I fail?


Christopher Brimstone

Answer
First, I'm not an expert in this area.  Here are some websites that may help.

www.che.wisc.edu/~eamastny/jdp_group_Sept04.pdf

nr.stic.gov.tw/ejournal/ProceedingA/v23n5/622-629.pdf

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1...