The term "solubility" refers to the capacity of a substance to dissolve in a solvent under specified conditions. The solubility of a solute in a solvent depends on several factors and prediction of solubility is not always easy.
The process of dissolution involves forming bonds with the solvent, usually water. All changes occur if the Gibbs Free Energy (see section 4.62) is favourable for the process, dissolution is no exception.
|ΔG = ΔH - TΔS|
The process is usually favourable in terms of entropy change, as it is nearly always positive.
Essentially, it comes down to whether the sum of the bonds formed with the solvent particles are stronger than the sum of the bonds broken between the particles in the solid. Water is a polar molecule and can form strong attachments to ions.
It is also small enough to surround the ions leaning that each ion, both positive and negative, can form at least six bonds to water molecules. This process is called hydration, and the energy released on forming the solvated ion is called hydration enthalpy.
Na+(g) + xH2O
Na+(aq) ΔH = hydration enthalpy of sodium
Cl-(g) + xH2O Cl-(aq) ΔH = hydration enthalpy of chloride ions
Water solvation of sodium ions
water solvation of chloride ions
The overall process of dissolution, in terms of energy, requires firstly that the lattice enthalpy is overcome and then the ions are solvated.
+ Cl-(g) ΔH = lattice enthalpy
Na+(g) + Cl-(g) Na+(aq) + Cl-(aq) ΔH = the sum of the hydration enthalpies of the ions
|overall: NaCl(s) Na+(aq) + Cl-(aq) ΔH = the enthalpy of solution|
Giant covalent structures
Giant covalent substances cannot dissolve in water. However, as the relative molecular mass of giant covalent molecules is enormous, the force of dispersion attraction is large for non-polar solvents.
Graphite has a layer type structure, which allows it to dissolve in some non-polar solvents with high relative molecular mass.
Metals are giant structures that do not dissolve in solvents.
Giant ionic compounds
Ionic substances are well suited to dissolve in water, as the polar water molecules can form 'cages' around the ions, effectively bonding six water molecules to each ion. The oxygen atom in water can coordinate an electron pair from the lone pair of electrons into a vacant orbital on the positive ion. The partially positive hydrogen atoms of water are attracted to the negative ion of the ionic compound.
Water solvated sodium ion
water solvated chloride ion
Whether the ionic substance is soluble, or not, is a complicated issue involving the energy required to break the lattice and the energy gained in forming bonds with the water molecules, the hydration energy. For this reason, it is difficult to predict solubility from first principles.
In general, ionic compounds are soluble and this solubility usually increases with increased temperature.
Simple molecular substances
The solubility of covalent compounds in water required them to be polar, otherwise the water molecules are far more attracted to one another than to the potential solute.
The more polar a compound is, the more likely it is to be soluble in water. Componds which can hydrogen bond can also do so with water increasing the likelihood of solubility. As the relative mass of any non-polar region of a molecule increases, so it becomes more likely that the molecules of the solute will stick together, rather than associate with water.
Factors favouring solubility in water (or another polar solvent)
- Presence of hydrogen bonds
- Presence of dipoles
- Low relative mass
Factors hindering solubility in water
- No dipoles
- Large relative mass
Substances that are themselves non-polar tend to dissolve well in non-polar solvents. This is because they can for attractions well with the non-polar solvent by means of dispersion forces.
There is a simple aphorism that says "like dissolves like", or to put it another way, similar substances dissolve in similar solvents. For example, iodine dissoves easily in tetrachloromethane, as both solute and solvent are non-polar.
However, iodine only dissolves slightly in water, which is a polar solvent. It should be noted that iodine partially reacts with water, giving the impression of solubility.
|I2(s) + H2O(l) HOI(aq) + HI(aq)|
The solubility of different types of structures in polar (eg water) and non-polar solvents are summarised in the following table:
|Giant molecular||insoluble||high relative mass solvent|