Solution Properties Notes

Chapter 14: Solution Properties

Mixture Types

• Suspensions:
  • Large particles dispersed in a medium.
  • Particles will settle over time.
• Colloids:
  • Microscopic particles dispersed in a medium.
  • Identified by the Tyndall effect.
  • Examples: Milk, Jello, fog.
• Solutions:
  • Ions/atoms/molecules dispersed individually in a medium.

Solutions

• Homogeneous mixtures.
• Solute: The component present in the minority, dispersed uniformly throughout the solvent.
• Solvent: The component present in the majority.

Student, Beware!

• Dissolution is a physical change; the original solute can be recovered by evaporating the solvent.
• If the substance reacts instead of dissolving, the solute cannot be recovered through evaporation.
• Acids reacting with metals is a chemical reaction, not dissolution.

The Dissolving Process

• Dissolving involves a tug-of-war between:
  • Solute-solvent attractions.
  • Solute-solute attractions.

How a Solution Forms

• The solvent pulls solute particles apart and surrounds (solvates) them.

Ion-Dipole Interactions

• If an ionic salt is soluble in water, ion-dipole interactions are present.
  • Not necessarily stronger than ionic bonds of the salt

Entropy

• Increasing disorder or randomness (entropy) in a system tends to lower the energy of the system.

Types of Solutions

• Saturated:
  • The solvent holds as much solute as possible at a given temperature.
• Unsaturated:
  • The solvent can dissolve more solute.
• Supersaturated:
  • The solvent holds more solute than normally possible at a given temperature.
  • Unstable; crystallization can be stimulated by adding a “seed crystal” or scratching the side of the flask.

Factors Affecting Solubility

• “Like dissolves like.”
  • Polar substances tend to dissolve in polar solvents.
  • Nonpolar substances tend to dissolve in nonpolar solvents.

Examples

• Glucose (with hydrogen bonding) is very soluble in water.
• Cyclohexane (with only dispersion forces) is not very soluble in water.
• Vitamin A is soluble in nonpolar compounds (like fats).
• Vitamin C is soluble in water.

Temperature and Solubility

• Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.
• The opposite is true of gases.
  • Carbonated soft drinks are more “bubbly” when refrigerated.
  • Warm lakes have less O_2 dissolved in them than cool lakes.

Ways of Expressing Concentrations of Solutions

Parts per Million (ppm) and Parts per Billion (ppb)

• Parts per Million (ppm):
  • ppm = \frac{mass \ of \ A \ in \ solution}{total \ mass \ of \ solution} × 10^6
• Parts per Billion (ppb):
  • ppb = \frac{mass \ of \ A \ in \ solution}{total \ mass \ of \ solution} × 10^9

Mole Fraction (X)

• X_A = \frac{moles \ of \ A}{total \ moles \ in \ solution}
• Determine whether the mole fraction of the solvent or solute is needed for the application.

Molarity (M)

• M = \frac{mol \ of \ solute}{L \ of \ solution}
• Volume is temperature-dependent, so molarity can change with temperature.

Molality (m)

• m = \frac{mol \ of \ solute}{kg \ of \ solvent}
• Moles and mass do not change with temperature, so molality is not temperature-dependent.

Colligative Properties

• Depend only on the number of solute particles present, not on the identity of the solute particles.
  • Vapor pressure lowering
  • Boiling point elevation
  • Melting point depression
  • Osmotic pressure

Vapor Pressure

• Solute-solvent intermolecular attractions cause higher concentrations of nonvolatile solutes, making it harder for the solvent to escape to the vapor phase.
• The vapor pressure of a solution is lower than that of the pure solvent.

Boiling Point Elevation and Freezing Point Depression

• Nonvolatile solute-solvent interactions cause solutions to have higher boiling points and lower freezing points than the pure solvent.

Boiling Point Elevation

• \Delta Tb = Kb ∙ m
  • \Delta T_b = change in boiling point
  • K_b = molal boiling point elevation constant (property of the solvent)
  • m = molality of the solution
• \Delta T_b is added to the normal boiling point of the solvent.

Freezing Point Depression

• \Delta Tf = Kf ∙ m
  • \Delta T_f = change in freezing point
  • K_f = molal freezing point depression constant of the solvent
  • m = molality of the solution
• \Delta T_f is subtracted from the normal freezing point of the solvent.

Note: \Delta T does not depend on the solute's identity but only on the number of dissolved particles. Each ion from an ionic compound counts as one particle.

Van’t Hoff Factor

• The van’t Hoff factor (i) is the number of particles a formula unit breaks up into.
  • C6H{12}O6(s) \rightarrow C6H{12}O6(aq) \quad i = 1
  • NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq) \quad i = 2
  • CaCl_2(s) \rightarrow Ca^{2+}(aq) + 2Cl^-(aq) \quad i = 3
• Calcium chloride has triple the effect on boiling point and freezing point as glucose.

Techniques to get stuff to dissolve

Performing Dilutions

• To dilute a solution, take a small volume of the original solution and add distilled water until a new volume is reached.
  • M1V1 = M2V2

Henry’s Law

• To dissolve more gas, increase the pressure of the gas above the liquid.
  • \frac{S1}{P1} = \frac{S2}{P2}
    • S = Solubility