Predicting Solubility of Simple
Molecular Compounds
For the sake of simplicity, I will copy the title as given in the Chemical Bonding notes. However, here
I will attempt to give a more simplistic but comprehensive explanation on how polarity of the
solvent or solute affects the solubility.
Firstly, solubility is not as simple as how much of a substance, for example salt, can one mix into
water until some of the salt cannot ‘disappear’ no matter how much you stir it.
If a substance is soluble in a certain solvent, the substance must interact with the solvent to a
certain extent through intermolecular interactions.
An example is the dissolution of NaCl, when table salt is dissolved in water, the end result (the salt
solution) is like the following:
1. We can see that the sodium ions and chloride ions do interact with water through (in this
case) electrostatic forces of attraction between the charge on the sodium/chloride ion and
the partial charge of the water molecule.
2. We also can see that for NaCl to dissolve in water, the sodium ions and chloride ions must be
separated from the ionic crystal lattice in order for it to interact with water molecules as
shown above.
These 2 points are the same for the dissolution of all compounds in any solvent.
From these 2 points, we can then draw the necessary conclusion to determine the effects of how
polarity of the solvent or solute affects the solubility.
The most important conclusion is that the interactions between the solvent and solute must be on
an approximately equal scale as the interactions between solvent molecules as well as between
solute molecules.
In other words,
𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑠𝑜𝑙𝑣𝑒𝑛𝑡 − 𝑠𝑜𝑙𝑢𝑡𝑒 𝑖𝑛𝑡𝑒𝑟𝑎𝑐𝑡𝑖𝑜𝑛𝑠
≈ 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑠𝑜𝑙𝑣𝑒𝑛𝑡 − 𝑠𝑜𝑙𝑣𝑒𝑛𝑡 𝑖𝑛𝑡𝑒𝑟𝑎𝑐𝑡𝑖𝑜𝑛𝑠
≈ 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒 − 𝑠𝑜𝑙𝑢𝑡𝑒 𝑖𝑛𝑡𝑒𝑟𝑎𝑐𝑡𝑖𝑜𝑛𝑠
1. Ionic Solutes (NaCl)
a. Polar solvent (water)
The ion-dipole interactions between water and the ion (electrostatic attractions) are of
similar strength to the ion-ion interactions in the solute (electrostatic attractions). Therefore,
the water molecules are able to abstract individual ions from the ionic lattice and thus
dissolve the ionic compound.
Here, a more scientific explanation would be the discussion of Gibbs free energy, entropy and
enthalpy; but that is for a later time.
b. Non-polar solvent (hexane)
The dispersion forces between hexane and the ion are much weaker strength than the ionion interactions in the solute (electrostatic attractions). Therefore, the ionic interactions
cannot be overcome by the dispersion forces. Therefore, the ionic lattice does not break up
and thus does not dissolve.
2. Polar solutes (ethanol)
Similar discussion to the one on ionic solutes, except the electrostatic attractions change to
hydrogen bonding or permanent-dipole-permanent-dipole.
3. Non-polar solutes (alkane)
a. Polar solvent (water)
Between water molecules, there are strong hydrogen bonds. These bonds are much stronger
than the weak dispersion forces between the non-polar solute and the solvent molecules
(water molecules). Therefore, the attraction between the non-polar solute and the water
molecules are not strong to attract the water molecule to interact with the solute as
compared to other water molecules. This is like the reverse of the case in ionic and polar
solutes in non-polar solvent.
b. Non-polar solvent
Do it yourself.
Thanks
Yuan Han