Sciencemadness Discussion Board

Melting Point & Lattice energy

guy - 12-8-2006 at 20:50

I was taught that as lattice energy increases, melting point increases. Lattice energy goes down when going down a group.
So what's the deal with this?

Compound Melting Point (°C)
MgCl2 ...........................712
CaCl2 ...........................772
SrCl2 ............................868
BaCl2 ............................963

Group I chlorides follow the "expected" trend quite nicely for chlorides except for LiCl. It should have the highest melting point but its almost as low as CsCl!

Is there some factor that is missing here?

12AX7 - 13-8-2006 at 08:37

Yes. Chemistry is an art, not a science. :P

With the alkaline earths, it's probably like the page about decomposition temperature of the carbonates -- the intense charge of a small magnesium ion tends to stress the anion, whereas the fatter Ba(2+) has a more diffuse field.

Tim

turd - 13-8-2006 at 10:28

Quote:
I was taught that as lattice energy increases, melting point increases. Lattice energy goes down when going down a group.

Common sense should tell you that how well the ions fit in the lattice will have a big impact on the stability of the crystal phase.
Quote:
Compound Melting Point (°C)
MgCl2 ...........................712
CaCl2 ...........................772
SrCl2 ............................868
BaCl2 ............................963

These do not even have the same crystal structure! Comparing their m.p. is useless.

guy - 13-8-2006 at 12:13

turd can you elaborate? I understand now about crystal structure. So what makes a crystal stronger?

turd - 14-8-2006 at 10:16

I'm not exactly sure what you mean with stronger, but if you consider that melting/solidifying is a chemical reaction
A(s) <---> A(l)
then it is no surprise that it is driven by the (minimisation of the) free energy G=H-TS. In the solid phase the entropy S is lower (more ordered system) as is the enthalpy H (bonding energy). In other words, once you reach a temperature where the entropy gain in the liquid phase multiplied by T surpasses the energy lost by weaker bonding, the substance melts.

For purely ionic compounds you can model them by a force field approach (balls and springs), for realistic compounds you have to compute their electronic structure with methods like DFT, which are approximations too and have the disadvantage of simulating T=0K. For T>0K, you would have to consider things like the Boltzmann distribution and phonons, which is out of the question at the moment, AFAIK.

But I'm not a physical chemist, so take with a grain of salt.

Engager - 24-8-2006 at 02:12

Quote:
Originally posted by guy
I was taught that as lattice energy increases, melting point increases. Lattice energy goes down when going down a group.
So what's the deal with this?

Compound Melting Point (œC)
MgCl2 ...........................712
CaCl2 ...........................772
SrCl2 ............................868
BaCl2 ............................963

Group I chlorides follow the "expected" trend quite nicely for chlorides except for LiCl. It should have the highest melting point but its almost as low as CsCl!

Is there some factor that is missing here?


Melting point is not only based on lattice energy, it also depends from molecular mass of compound. Heavier molecules require more thermal energy to reach speeds that reqired to escape from crystall cage. The general rule is - higher molecular weight - higher melting and boiling points are.

not_important - 24-8-2006 at 02:58

But for the alkali metals, excluding lithium, the melting and boiling points of the halides and chalcogens all go down as the atomic number goes up. Within those halides the melting and boiling points go down as the atomic number goes up.

I think that with Ca-Sr-Ba, Sr halides have the higher melting points, but I'm too lazy to look it up.

turd - 24-8-2006 at 10:37

Quote:
Melting point is not only based on lattice energy, it also depends from molecular mass of compound. Heavier molecules require more thermal energy to reach speeds that reqired to escape from crystall cage. The general rule is - higher molecular weight - higher melting and boiling points are.

First of all this thread is not about organic molecules, but inorganic solids - a different kettle of fish. And then the notion of molecules having to reach a certain speed to leave the crystal is ridiculous. It's probably got more to do with the fact that in crystals of big molecules you have more contribution of covalent bonds vs. VdW-interaction.

Stop spreading misinformation.

12AX7 - 24-8-2006 at 11:28

No kiddin'. Ever notice that stearic acid melts *higher* than its triglyceride? Unsaturated fatty acids (and their glycerides) have even lower melting points (hence are mostly oils rather than fats).

Tim

guy - 24-8-2006 at 11:28

Quote:
Originally posted by turd
Quote:
Melting point is not only based on lattice energy, it also depends from molecular mass of compound. Heavier molecules require more thermal energy to reach speeds that reqired to escape from crystall cage. The general rule is - higher molecular weight - higher melting and boiling points are.

First of all this thread is not about organic molecules, but inorganic solids - a different kettle of fish. And then the notion of molecules having to reach a certain speed to leave the crystal is ridiculous. It's probably got more to do with the fact that in crystals of big molecules you have more contribution of covalent bonds vs. VdW-interaction.

Stop spreading misinformation.


Wouldn't more intermolecular interaction show in the Lattice energy?
I think the matter is entropy, so what factors will affect entropy?

[Edited on 8/24/2006 by guy]