Factors Affecting Solubility

Factors that can affect solubility:

• Properties of solute
• Properties of solvent
• Temperature
• Pressure (Gases)

Gases

• Gases are gases because the attractive forces are typically weak - involve primarily London dispersion forces
• Consequently, attractive forces (basis of solubility) between gas molecules and solvent are also primarily London dispersion forces
• London dispersion forces increase with increasing size and mass of molecules involved

• Kr (83.8 amu) more soluble in water than Ar (39.9 amu), which is more soluble than O₂ (32 amu), which is more soluble than N₂(28 amu)

Some gases appear to be a lot more soluble than what their mass would predict

• Cl₂(70.8 amu) is about 50 times more soluble than Kr (83.8 amu)
• Cl₂ dissolved in H₂O can undergo a chemical reaction to produce hypochlorous acid:

• This chemical reaction with H₂O removes Cl₂(aq) from solution and allows more Cl₂(g) to dissolve

Polar solutes tend to dissolve readily in polar solvents

• Interactions between polar solutes are typically dipole-dipole (or Hydrogen-bonds)
• Interactions between molecules of a polar solvent are also dipole-dipole (or Hydrogen-bonds)
• Thus, the energies associated with disrupting solute-solute interactions and solvent-solvent interactions are approximately equivalent
• Entropic forces can subsequently drive the dissolution process
• Polar liquids tend to dissolve readily in polar solvents

• Ethanol contains a hydroxyl (OH) functional group that is similar in structure to water. Attractive forces between ethanol molecules include Hydrogen bonds, like the attractive forces between water molecules. Ethanol is miscible in H₂O
• Octane (gasoline) molecules contain only C-H and C-C bonds and are essentially non-polar molecules. Attractive forces between octane molecules include primarily London dispersion forces. Octane is not miscible in H₂O
• Ethanol is an alcohol (contains an OH functional group). Octanol is also an alcohol (it contains a single OH functional group). However, it contains a string of 8 carbon groups compared to the two in ethanol. The carbon groups cannot participate in Hydrogen bonding (only dispersion forces). The single OH group in octanol is not enough to provide solubility for octanol, and octanol is essentially immiscible in H₂O.

The observation that if similar attractive forces exits between solute-solute molecules and solvent-solvent molecules results in miscible solutions (i.e. the ability of the solvent to dissolve the solute), has led to the following generalization:

• If we increase the pressure of a gas (at constant T), the physical interpretation is that more gas molecules are striking the surface of the container in a given amount of time (Kinetic Molecular Theory)
• A gas in contact with a solution is "dissolved" when gas molecules strike the surface of the solution (and are surrounded and dispersed by the solvent).
• Thus, increasing the pressure (at constant T) results in more collisions of the gas molecules, per unit time, with the surface of the solvent. This results in greater solubility.

• Solvated gas molecules with enough kinetic energy can escape from the surface of a liquid (requires de-solvation of solvent molecules)
• Kinetic energy increases with increasing temperature
• Thus, increasing the temperature reduces the solubility of gas molecules in a solvent

• Insolubility of solid solutes is related to the inability of solvent molecules to overcome the attractive forces between solute molecules
• Increases temperature results in increased kinetic energy of the solvent molecules, as well as the solute molecules
• The increased kinetic energy of the solute molecules favors separation of solute molecules. Increased kinetic energy of the solvent molecules allows them to separate the solute molecules easier
• Thus, increasing temperature increases the solubility of solid solutes