Colligative Properties

Colligative Properties

Introduction

• Colligative properties are phenomena you've likely encountered without realizing it.
• Examples include making ice cream, dealing with snow/ice on roads, and using car radiators.
• The main idea is that a solute affects the solvent's behavior; adding "stuff" to water changes its boiling and freezing points.

Freezing Point Depression

• Adding a solute to a solvent lowers the freezing point.
• Ions in the solution attract water molecules, hindering their interactions with each other.
• This makes it more difficult for water molecules to freeze into a consistent structure.

Boiling Point Elevation

• Antifreeze/coolant is added to car radiators to prevent overheating.
• The solute in the liquid stops water molecules from vaporizing, thus raising the boiling point.

Definitions

• Colligative Properties: Properties that depend on the concentration of solute particles rather than the solute's identity.
• Freezing Point Depression: The decrease in the freezing point of a solvent upon the addition of a solute.
• Boiling Point Elevation: The increase in the boiling point of a solvent upon the addition of a solute.

Particle Models and Separation

• Ionic compounds separate into individual ions in solution (electrolytes).
• Molecular (covalent) compounds separate into individual molecules (no charges).
• Examples:
  • Sodium Bromide (NaBr):
    • $NaBr \rightarrow Na^+ + Br^-$
    • One NaBr unit yields two particles (Na+ and Br-).
  • Calcium Chloride (CaCl2):
    • $CaCl_2 \rightarrow Ca^{2+} + 2Cl^-$
    • One CaCl2 unit yields three particles (one Ca2+ and two Cl-).
  • Sugar (C12H22O11):
    • $C<em>{12}H</em>{22}O<em>{11} (aq) \rightarrow C</em>{12}H<em>{22}O</em>{11} (aq)$
    • One sugar molecule remains one particle in solution.

Effect of Number of Particles

• Colligative properties depend on the number of particles in solution.
• More particles result in a greater effect on boiling and freezing points.
  • Least Effect: Sugar (1 particle)
  • Medium Effect: Sodium Bromide (2 particles)
  • Most Effect: Calcium Chloride (3 particles)

Molarity and Colligative Properties

• The effect on colligative properties can be related to molarity.
• Consider the number of particles formed and the concentration (moles) of the solution.
• Examples:
  • 2 M Lithium Nitrate (LiNO3):
    • $LiNO<em>3 \rightarrow Li^+ + NO</em>3^-$
    • 2 particles x 2 moles = 4
  • 4 M Ribose (C5H10O5):
    • Covalent molecule, does not split.
    • 1 particle x 4 moles = 4
  • 0. 5 M Aluminum Acetate (Al(C2H3O2)3):
    • $Al(C<em>2H</em>3O<em>2)</em>3 \rightarrow Al^{3+} + 3C<em>2H</em>3O_2^-$
    • 4 particles x 0.5 moles = 2
• Lithium nitrate and ribose have the greatest effect (same number of particles x moles).
• Aluminum acetate has a smaller effect due to a smaller number of particles.