Sciencemadness Discussion Board

2 chemistry problems

LHcheM - 26-12-2011 at 23:44

These are simple chemistry problems, but I can't find any explanation web, so someone please help me!

1. Why the liquid range of aluminium is so large compared to Mg and Na?
2. Why some substances shows negative solubility coefficient with temperature? (ie more insoluble in hot water, soluble in cold water eg Nd2(SO4)3 )

Thanks!!!

neptunium - 29-12-2011 at 10:17

right underneath Al we find Gallium on the periodic table, and again Ga has one of the longest liquid phase of all the elements.
i`ve red something about the vapor preassure of this metal ( and Al) they keep a high viscosity when liquid i have a few ideas as of why but i m gonna let the forum go on for a while

neptunium - 3-1-2012 at 15:05

either nobody is interested or no one has a clue so far....

AJKOER - 3-1-2012 at 17:28

Quote: Originally posted by LHcheM  
These are simple chemistry problems, but I can't find any explanation web, so someone please help me!

2. Why some substances shows negative solubility coefficient with temperature? (ie more insoluble in hot water, soluble in cold water eg Nd2(SO4)3 )


One possible explanation is that upon reaction with water at different temperatures, there could be more significant progress in a hydrolysis/ionization reaction, or the formation/dissolution of complexes or bonds. In any of these cases, we are not looking at the solubility of truly the same 'substance' either from a chemical or physical perspective. Hence, there could be observed a seeming reversal in solubility, which is actually the solubility of the new compound.

[Edited on 4-1-2012 by AJKOER]

[Edited on 4-1-2012 by AJKOER]

Ozone - 3-1-2012 at 18:21

We see inverse solubility of calcium salts (especially Ca(OH)2 in the sugar industry) every day, a bit of information is given here:

http://books.google.com/books?id=5px8GzscIRsC&pg=PA104&a...

For salts like these, there is a negative entropy of solution (see also "hydrophobic effect"): Hopkins, H.P. and Wulff, C.A. (1965). The Solution Thermochemistry of Polyvalent Electrolytes. I. Calcium Hydroxide. J. Phys. Chem. 69 (1), pp. 6-8.

DH(sol): -4290 cal/mol
DS(sol): -38.2 cal/mol.K

Because DG = DH - TDS, DG(298K) = +7094 cal/mol. (30 kJ/mol).

This means that solution is disfavored (both enthalpic and entropic factors), and becomes even less favorable as T increases. For example, NaCl, which is spontaneously soluble in water, DG is negative.

This is (at least in some cases) because "the entropy of a system is lowered when the solution forms because solvent molecules form cagelike structures around the solute molecules." (http://books.google.com/books?id=4R6hb1OIMRUC&pg=PA327&a...).

My stab at it, anyway,

O3

Attachment: Hopkins and Wulff 1965 CaOH2.pdf (278kB)
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Attachment: Entropy and Gibbs Energy_ch15_student.pdf (198kB)
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Attachment: Feakins et al 1989 Thermo of solutions.pdf (1.1MB)
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