| |
Answer:
Dr.
Toshiyuki Osakai, who is actually studying solution chemistry, kindly answered the question above.
The solubility data for salt (electrolyte) in organic solvent is found in “Kagaku Binran (Maruzen)”. The solubility of NaCl in methanol is 1.39 (g in 100g of saturated solution). This value is much smaller than the solubility of NaCl in water.
It is thought that organic solvents generally have low capacity to dissolve electrolytes, since their dielectric constants are usually smaller than that of water. Thus, typical inorganic electrolytes, such as NaCl and KOH, are more likely to have lower solubility to organic solvents than in water.
However, there are some exceptions, such as salt whose anion and cation (either or both) are hydrophobic. Tetrabutyl tetraphenyl borate
(Bu4N+ • Ph4B-), for example, is barely soluble in water but easily soluble in nitrobenzene (> 0.1 mol/L). Although the specific dielectric constant of nitrobenzene is high (ε = 34.8) for an organic solvent, it is still lower than that of water (ε = 78.5). So the phenomenon cannot be explained only by means of dielectric constant. This implies that the “chemistry”
between ion and solvent is important.
The solvation Gibbs energy (ΔG◦tr) is used as the scale to measure the “chemistry” between ion and solvent. This energy is defined as the energy applied to put 1 mol of ion from a vacuum into the solvent. Born proposed the theoretical equation of ΔG◦tr based on electrostatic theory. However, it was revealed later that it was not accurate enough to estimate the interaction energy between ion and solvent using Born’s theory based on the size of ions and dielectric constant of solvent only. So it is necessary to build a new theory which includes the “chemistry” between ion and solvent, but a sufficient theory has not been proposed yet.
It is difficult to explain the solubility of an electrolyte, as
described above. Let me show you one more important factor governing the solubility of an electrolyte. It is the stability of the solid ionic crystal. Let’s think about the process of dissolving an electrolyte into a solvent by dividing the process into two like this:
Ion in crystal →
Free ion in vacuum →
Solvated ion in solvent
The second process is related to ΔG◦tr described above. The energy for the first process corresponds to the crystal lattice energy. This is the energy to cut bonds between cation and anion in a crystal and to take them apart. If the crystal is stable, the crystal lattice energy is high so that the crystal is difficult to dissolve. Remember that AgCl, for example, is barely soluble in water. That is because the crystal lattice energy is so high, even though both
Ag+ and Cl- are very hydrophilic (i.e., -ΔG◦tr is large).
Acknowledgement
Dr.
Toshiyuki Osakai
He is studying fundamental electrochemistry (chemistry related to electrons and ions) and its applications to other fields, such as life
science.
This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp. Please let us know if you find any errors.
|
|
