Question #fb753

1 Answer
Sep 18, 2017

Because of a very negative energy balance.


Most of solubilisation processes of solids are energetically unfavourable, that is the solute-solvent interactions are weaker than solute-solute interactions and solvent-solvent intermolecular interactions.

So, when a solubilisation process occurs, this is because the entropy, or "randomness" of the system increases enough to drive the energetically unfavourable transformation.

In fact a solution is always a more chaotic, or disordered system than are crystal an solvent separately.

Then, if we explain why the solubilisation of a ionic solid in a organic - unpolar - solvent uses up too much energy, we can understand why the process does not occur usually.

Because water molecules are very polar, they interact strongly with positive and negative ions with ion-dipole interactions, unlike the organic solvent molecules, which are unpolar or weakly polar.

Thus, the energetic gain in water molecule-ion interaction is the term, or subprocess, which makes the water solubilisation energetically "less unfavourable".

Using an organic solvent instead of water, laking this positive contribution, solubilisation entails only energetically unfavourable subprocesses: a) separation of the positive and negative ions of the crystal, the major negative contribution; b) separation of solvent molecules.