Solubility of Ionic & Molecular Compounds (College Board AP® Chemistry): Revision Note
Solubility of Ionic & Molecular Compounds
The solubility of two substances depends on their intermolecular interactions
Ions and molecules can exhibit can exhibit the following interactions:
Relative strength comparison between the intermolecular interactions
Interaction | Relative strength |
|---|---|
Ion-dipole | Strongest |
Hydrogen bonding |
|
Dipole-dipole |
|
London dispersion forces | Weakest |
A solution is made from two components a solvent and a solute
In a solution system, intermolecular interactions exist in three possible scenarios:
Solvent–solute, i.e. solvent particles with solute particles
Solvent–solvent, i.e. solvent particles with themselves
Solute–solute, i.e. solute particles with themselves
When does a solution form?
Substances with similar intermolecular interactions tend to be miscible or soluble in one another, because the solute–solvent attractions can replace the solute–solute and solvent–solvent attractions
Two liquids that mix completely in all proportions are miscible; a solid that dissolves in a liquid is soluble
E.g. ethanol and water molecules hydrogen bond to each other, so ethanol and water are miscible
When the solvent-solvent interaction and the solute-solute interaction are not equal, the strength of the solvent-solute interaction must be considered
The three interactions between the particles of solute and solvent
Solvent-solvent, solvent-solute and solute-solute interactions must be considered for solubility
A solution is formed when the solvent-solute interactions are stronger than the solvent-solvent interactions
E.g. Salt and water present ion-dipole interactions. Water molecules present hydrogen bonding between themselves
ion-dipole > hydrogen bonding
Therefore, they are miscible in one another
A solution is unlikely to form when the solute–solvent attractions are much weaker than the solvent–solvent attractions they would have to replace
E.g. the weak attractions between hexane and water molecules cannot replace the hydrogen bonds between water molecules
Therefore, they are not miscible in one another
The rules of solution formation are summarized in the table below
Rules of Solution Formation by comparing the intermolecular interactions strengths
Comparison of the strength between intermolecular interactions | Solution formed? |
|---|---|
Solvent-solute = Solvent-solvent | Yes |
Solvent-solute > Solvent-solvent | Yes |
Solvent-solute < Solvent-solvent | May or may not form |
Examiner Tips and Tricks
Explain solubility using the attractions between solute and solvent particles. Describing only the forces in the pure solute or the pure solvent does not earn credit
When a molecular solute such as ethanol dissolves, its molecules separate: no covalent bonds break and no ions form
Worked Example
Predict if a solution will be formed when chloroform is mixed with an acetone solvent.
Answer:
Step 1 (Optional): Draw the molecular structure of the solvent molecules and the solute molecules. This step is not applicable to ions since they exist as lattice structures
Step 2: Identify the solvent-solvent intermolecular interactions
Acetone is the solvent. Since oxygen is more electronegative than carbon, a dipole moment will be formed
Therefore, acetone molecules attract each other through dipole–dipole forces in addition to London dispersion forces
Step 3: Identify the solute-solute intermolecular interactions
Chloroform is the solute. Since it is a non-symmetrical molecule and chlorine is more electronegative than carbon, dipole moments will be formed
Therefore, chloroform molecules attract each other through dipole–dipole forces in addition to London dispersion forces
Step 4: Identify the solute–solvent attractions and compare them with the solute–solute and solvent–solvent attractions they must replace
Acetone and chloroform molecules attract each other through dipole–dipole and London dispersion forces that are similar in strength to the attractions in each pure liquid, so acetone and chloroform are miscible
Worked Example
Predict if a solution will be formed when hexane is mixed with water as solvent.
Answer:
Step 1 (Optional): Draw the molecular structure of the solvent molecules and the solute molecules. This step is not applicable to ions since they exist as lattice structures
Step 2: Identify the solvent-solvent intermolecular interactions
Water is the solvent. Since hydrogen is bonded to a highly electronegative atom such as oxygen, a strong dipole moment will be formed
Therefore, water molecules attract each other through hydrogen bonding (and London dispersion forces)
Step 3: Identify the solute-solute intermolecular interactions
Hexane is the solute. Since hexane is a symmetrical molecule, all its dipole moments are canceled out
Therefore, the only attractions between hexane molecules are London dispersion forces
Step 4: Identify the solute–solvent attractions and compare them with the solute–solute and solvent–solvent attractions they must replace
The solvent-solvent interaction is not the same as solute-solute interaction
The solvent-solute interaction must be considered
As shown in the image above, water and hexane attract each other through dipole–induced dipole forces (the water dipole induces a temporary dipole in hexane) and London dispersion forces
The attractions between hexane and water molecules are much weaker than the hydrogen bonds between water molecules that they would have to replace
So water molecules stay hydrogen-bonded to each other, hexane and water are not miscible, and they form two layers
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