Solutions & Raoult's Law

Solutions (Molecular Dispersions)

• Solutions consist of a solute dispersed within a solvent.
• Molecular dispersion particle size is less than 1 mμ.
• Colloidal dispersion particle size ranges from 1mμ to 0.5μ
• Coarse dispersion particle size is greater than 0.5μ.

Solubility of Liquid in Liquid

• Miscibility is the term, rather than solubility, used when mixing gases or liquids.
• Ideal solutions exhibit no change in properties, while non-ideal solutions do.
• Raoult's Law states the partial pressure of a solvent in an ideal solution is equal to the vapor pressure of the pure solvent multiplied by the mole fraction of the component.

Deviation of Raoult's Law

• Negative deviation occurs when adhesion forces are greater than cohesion forces, and the actual vapor pressure is less than expected, producing a curve with a minimum. Example: chloroform mixed with acetone.
• Positive deviation occurs when adhesion forces are less than cohesion forces, and the actual vapor pressure is more than expected, producing a curve with a maximum. Example: benzene added to ethyl alcohol.

Liquid-Liquid Systems

• Complete miscibility occurs when two liquids mix in all proportions, forming one layer. Examples include water, alcohol, glycerin, alcohol and acetone, benzene and carbon tetrachloride.
• Partial miscibility occurs when liquids do not mix in all proportions, resulting in two layers. Examples include water and phenol, triethylamine and water, and nicotine and water.

Two Component Systems

• Critical Solution Temperature (CST) is the maximum temperature at which two phases can co-exist.
• Upper Consolute Temperature (UCST): completely miscible with heat e.g. phenol-water system at 66.8 °C.
• Lower Consolute Temperature (LCST): completely miscible with cooling e.g. triethylamine-water system with paraldehyde enemas at 18.5 °C.
• Closed-phase diagram: miscible with cooling/heating e.g. Nicotine-water system.
• Ethyl ether and water show partial miscibility over the entire temperature range, which has no critical solution temperature.

Three Component Systems (Triangular Diagram)

• Examples include methyl salicylate-isopropanol-water and peppermint oil-water-polyethylene glycol.

Dissolution of Solids in a Liquid (Fick's Law of Diffusion)

• Noyes-Whitney equation: dm/dt = k A(Cs - C) / h.
• Increased particle size reduces the surface area (A), which decreases the rate of solution.
• Increased stirring decreases the thickness of the stagnant layer and increases the rate of solution.
• Increased viscosity of the liquid decreases the rate of solution.

Solubility Curves

• Indicate the effect of temperature on the solubility of a given substance.
• KNO3: solubility increases with temperature (ΔH = +ve).
• Calcium acetate: solubility decreases with increasing temperature (ΔH = -ve).
• NaCl: solubility does not change with temperature (ΔH = zero).
• Na2S2O4.10H2O: undergoes endothermic transition to anhydrous form at 32.55°C.

Application of Solubility: Distribution of Solutes between Immiscible Liquids

• Nernst Distribution Law defines the oil-water partition coefficient (CA/CB = K).
• CA: concentration in liquid A.
• CB: concentration in liquid B.
• K: partition coefficient, indicates lipophilic or hydrophobic nature.
• Distribution Law applications include drug release from dosage forms.
• Distribution Law applications include formulations of solubilized systems.
• Distribution Law applications include passage of drugs through membranes and preservative action of oil-water systems.

Types of Solutions: Examples

• Gas in gas: Air
• Liquid in gas: Water in oxygen
• Solid in gas: Iodine vapor in air
• Gas in liquid: Carbonated water
• Liquid in liquid: Alcohol in water
• Solid in liquid: Aq. NaCl solution
• Gas in solid: Hydrogen in palladium
• Liquid in solid: Mineral oil in paraffin
• Solid in solid: Gold-silver mixture

Important Note

• Supersaturated solutions do not crystallize on cooling.

Solubility Questions and Answers

• A solution containing the maximum amount of solute that a solvent can dissolve at a specific temperature is a saturated solution.
• A solution containing a dissolved solute in a concentration below that necessary for complete saturation is an unsaturated solution.
• Supersaturated solutions can be achieved by seeding crystals, heating, or adding more solutes.
• When two components forming a solution are either both gases or liquids, this described as miscibility.
• If 1g of sodium chloride requires 20mL of distilled water for its solubility, the solubility is soluble
• If 3g of sodium acetate requires 900 mL of distilled water for its solubility, the solubility is sparingly soluble.
• In the solubility of mineral oil in paraffin, the solute is liquid and the solvent is solid.
• In the solubility of a gold-silver mixture, the solute is solid and the solvent is solid.
• Carbon tetrachloride is an example of a non-polar solvent.
• Propylene glycol is an example of a semi-polar solvent.
• Metronidazole solubility increases by complex formation, using a cosolvent, and increasing the particle size.
• The aqueous solubility of salicylic acid and its sodium salt is an example of the molecular structure of the solute.
• Potassium nitrate solubility increases by complex formation, using a cosolvent, increasing the particle size, and heating the solution.
• Sodium sulfate solubility increases by complex formation, using a cosolvent, increasing the particle size, and cooling the solution.
• Citric acid monohydrate solubility is less than anhydrous citric acid.
• Atropine solubility increases by complex formation, using a cosolvent, increasing the particle size, and decreasing the pH.
• HgI2 solubility increases by complex formation, using a cosolvent, and increasing the particle size.

True or False Statements

• Fick's law describes the relationship between the rate of diffusion and the concentration difference (True).
• Negative deviation from Raoult's law occurs when adhesion forces are smaller than cohesion forces (False).
• Solubility is a spontaneous interaction of two or more substances to form a homogeneous molecular dispersion (True).
• Positive deviation leads to a decrease in solubility due to the association of molecules to form dimers or polymers (True).
• Negative deviation leads to an increase in solubility and is associated with hydrogen bonding between polar components (True).
• Carbonated water is an example of dissolving gas in solid (False).
• Nonpolar solvents cannot reduce the attraction between ions of strong and weak electrolytes (True).