# Interesting question

1. Feb 9, 2006

### bomba923

Hmm...well, it may seem rather immature

Using any quantity of (any) halogen and SPONCH atoms, draw a molecule of the molecular solid possessing the highest

$$\frac{\text{Melting point}}{\text{Molar mass}}$$

ratio.

(Note: Diamond is not a molecular solid; it is a network solid)

What molecular solid (composed of any quantity of halogen & SPONCH atoms) might have the highest melting point (in °Celsius) to molar mass (in grams) ratio?

(Only halogen and SPONCH atoms are allowed.)

Last edited: Feb 10, 2006
2. Feb 10, 2006

### Gokul43201

Staff Emeritus
What is water (ice) ?

3. Feb 10, 2006

### bomba923

Ice (water) is a molecular solid, as the water molecules are held together NOT by covalent or ionic bonds....but rather by such intermolecular foces as H-bonds, dipole-dipole attractions, and general dispersion forces.

Acquired from http://chemmovies.unl.edu/Chemistry/DoChem/DoChem037.html [Broken],
By the way, the melting point (in °Celsius) to molar mass (in grams) ratio for water is zero (0 °C/g).

Back to the question,

What molecular solid (composed of any quantity of halogen & SPONCH atoms) might have the highest melting point (in °Celsius) to molar mass (in grams) ratio?

(Only halogen and SPONCH atoms are allowed for the molecules!)

Last edited by a moderator: May 2, 2017
4. Feb 10, 2006

### Gokul43201

Staff Emeritus
I didn't think you'd mentioned the units in your previous post. Your particular choice of units makes the nature of the ratio somewhat arbitrary and meaningless (or at least, less interesting from a scientific point of view).

In units of K-mol/gm, there's at least some meaning to the ratio (it would be strongly correlated to the molar enthalpy of fusion, I'd imagine; and would say something about the strength of intermolecular bonding in the solid state). And in these units, water has a very large ratio of about 15.

5. Feb 11, 2006

### bomba923

Less interesting?
I never said that the ratio should be meaningful /
Yes, 15 on that scale. But I chose Celsius to better emphasize the proportional difference in ratios for different molecular solids, and to also exclude the water molecule (a trivial answer).

For example, take glucose and scyllo-inositol, both of which have equivalent molar masses (180.g/mol).

For glucose, the melting point to molar mass ratio in units K*mol/g is
$$\frac{{432\,{\text{K}}}}{{180.\,\tfrac{{\text{g}}}{{{\text{mol}}}}}} = 2.35\frac{{{\text{K}} \cdot {\text{mol}}}}{{\text{g}}}$$

Or, with units °C*mol/g, the ratio is
$$\frac{{150.^\circ {\text{C}}}}{{180.\,\tfrac{{\text{g}}}{{{\text{mol}}}}}} \approx 0.833\frac{{^\circ {\text{C}} \cdot {\text{mol}}}}{{\text{g}}}$$

And scyllo-inositol, in units K*mol/g:
$$\frac{{623{\text{K}}}}{{180.\,\tfrac{{\text{g}}}{{{\text{mol}}}}}} \approx 3.46\frac{{{\text{K}} \cdot {\text{mol}}}}{{\text{g}}}$$

And with units °C*mol/g,
$$\frac{{350.^\circ {\text{C}}}}{{180.\,\tfrac{{\text{g}}}{{{\text{mol}}}}}} \approx 1.94\frac{{^\circ {\text{C}} \cdot {\text{mol}}}}{{\text{g}}}$$

In units °C*mol/g, the scyllo-inositol ratio is 233% that of the glucose ratio. In units K*mol/g, the scyllo-inositol ratio is only 47% greater than that of glucose (147% of the glucose ratio).

The purpose of using Celsius, as opposed to Kelvin, was to
1) eliminate the water molecule (trivial answer) and
2) more strongly emphasize the proportional differences in the ratios for various molecular solids.
------------------------------------------------------------
If you wish, I suppose K*mol/g can be used, and will produce a more meaningful result, in terms of units; in that case, I'd revise the original question-->

|->Not considering ice (water), what molecular solid (composed of any quantity of halogen & SPONCH atoms) might have the highest melting point (in Kelvin) to molar mass (in grams/mol) ratio?

(Only halogen and SPONCH atoms are allowed for the molecules!)

Last edited: Feb 11, 2006
6. Feb 11, 2006

### GCT

are you supposing that molecules with hydrogen bonding with the optimal intermolecular arragments (with some degree of symmetry, no intramolecular interactions, high density) would have the high mp/mw ratios?

Are you suggesting a correlation with thermodynamica/physical parameters (such as density), systematic molecular modeling (e.g. based on group theory), or a naieve hypothesis (you've been convinced through some simple though experiments)?

7. Feb 11, 2006

### bomba923

Well, I would expect the molecules to have good H-bonding and dipole-dipole interactions.

But no, I'm not suggesting or supposing anything (no experiments have been done, nor any data collected...nothing of the such); my question is just one of general curiousity~

Not considering ice (water), what molecular solid (composed of any quantity of halogen & SPONCH atoms) might have the highest melting point (in Kelvin) to molar mass (in grams/mol) ratio?

(Only halogen and SPONCH atoms are allowed for the molecules!)

8. Feb 11, 2006

### Gokul43201

Staff Emeritus
bomba :

1. The stronger emphasis that you want (by asking for the temerature in C) will happen only if the solid melts closer to 273K than to 0K. For instance in centrigrade units, O2 is less than 7% better than N2, but in Kelvin units N2 is nearly 50% better.

2. For the above reason (and others), I think perhaps, you want to restrict the quest to solids that freeze above 273K. But I'd still recommend keeping Kelvin units (centigrade is a somewhat arbitrary unit of temperature and how no real thermodynamic meaning).

3. Working with K-mol/g (and not imposing the condition in point 2 above) : HF has a ratio of 9.5 (I know this is different from the direction in which you are looking, but what the heck !)

Last edited: Feb 11, 2006
9. Feb 12, 2006

### bomba923

Good point, as I originally intended the molecular solid to melt above 273K

That was the original intent!

I see...so in that case, I'll revise the question as such:

What molecular solid (composed of any quantity of halogen & SPONCH atoms) might have the highest melting point (which must be greater than 500K) to molar mass ratio (in units K*mol/g)?

(Only halogen and SPONCH atoms are allowed for the molecules!)

Last edited: Feb 13, 2006