Solubility of Ionic & Molecular Compounds (College Board AP® Chemistry)
Study Guide
Written by: Martín
Reviewed by: Stewart Hird
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 exists 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?
A solution is formed when solvent-solvent interactions and the solute-solute interactions are the same
When this happens, the substances are said to be miscible
E.g. Ethanol and water present hydrogen bonding interactions. Therefore, they are miscible in one another
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 will not be formed when the solvent-solute interactions are weaker than the solvent-solvent interactions
E.g. Hexane and water present London Dispersion Forces. Water molecules present hydrogen bonding themselves
London-Dispersion Forces < hydrogen bonding
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 |
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 our solvent. Since oxygen is more electronegative than carbon, a dipole moment will be formed
Therefore, the solvent-solvent intermolecular interaction is dipole-dipole
Step 3: Identify the solute-solute intermolecular interactions
Chloroform is our solute. Since it is a non-symmetrical molecule and chlorine is more electronegative than carbon, dipole moments will be formed
Therefore, the solute-solute intermolecular interaction is dipole-dipole
Step 4: If the solute-solute interaction and the solvent-solvent interactions are the same, they are miscible. Otherwise, the table should be used and the solvent-solute interactions must be considered
Since acetone presents dipole-dipole interactions and chloroform presents dipole-dipole interactions, both solute and solvent will be miscible in one another
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 our solvent. Since hydrogen is bonded to a highly electronegative atom such as oxygen, a strong dipole moment will be formed
Therefore, the solvent-solvent intermolecular interaction is hydrogen bonding
Step 3: Identify the solute-solute intermolecular interactions
Hexane is our solute. Since hexane is a symmetrical molecule, all its dipole moments are canceled out
Therefore, the solute-solute intermolecular interaction are London dispersion forces
Step 4: If the solute-solute interaction and the solvent-solvent interactions are the same, they are miscible. Otherwise, the table should be used and the solvent-solute interactions must be considered
The solvent-solvent interaction is not the same as solute-solute interaction
The solvent-solute interaction must be considered
As shown in the image below, hexane and water present London dispersion forces interactions because oxygen induces momentaneous dipoles in hexane
Using the rules of solution, if solvent-solute < solvent-solvent, a solution will not be formed
Since London-dispersion forces < hydrogen bonding in terms of relative strength, hexane and water are not miscible, and a solution will not be formed
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