Colligative Properties of Solutions

Colligative Properties of Solutions

• Colligative properties are properties of solutions that depend on the concentration of solute molecules or ions, but not on the identity of the solute.

Types of Colligative Properties

• Vapor pressure lowering
• Boiling point elevation
• Freezing point depression
• Osmotic pressure

Vapor Pressure Lowering

• Vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phase in a closed container.
• When a non-volatile solute is dissolved in a solvent, the vapor pressure of the solvent decreases.

Boiling Point Elevation

• The boiling point of a liquid is defined as the temperature at which the vapor pressure of that liquid equals the atmospheric pressure.
• Adding a non-volatile solute to a liquid elevates its boiling point.

Freezing Point Depression

• The normal freezing point is the temperature at which a solid and liquid are in equilibrium under 1 atm.
• Adding a solute to a solvent lowers its freezing point.

Osmotic Pressure

• This is the external pressure that must be applied to a solution to prevent it from being diluted by the entry of solvent via osmosis.
• Osmosis is the movement of solvent particles across a semipermeable membrane from a dilute solution into a concentrated solution.
• Osmotic pressure is directly proportional to the concentration of the solution.

Electrolytes vs. Nonelectrolytes

• Electrolytes: Substances that form solutions that can conduct electricity. Their solutions are known as electrolytic solutions. Electrolyte solutions contain ions, atoms, or molecules that have lost or gained electrons..

• Nonelectrolytes: Substances whose solutions do not conduct electricity. Examples include nonpolar gases (e.g., H₂, noble gases), hydrocarbons (e.g., CH₄), and nonpolar organic compounds.

Colligative Properties of Electrolytes and Nonelectrolytes

• Electrolytes: Provide more solutes to the solution via dissociation, leading to significant changes in colligative properties. The effect of electrolytes on colligative properties is high, compared to nonelectrolytes.

• Nonelectrolytes: Provide relatively low solute to their solution since there is no dissociation. The effect of nonelectrolytes on colligative properties is low compared to electrolytes.

Boiling Point Elevation and Freezing Point Depression Calculations

• Boiling Point Elevation: The elevation in the boiling point (∆t_b) is calculated using the formula: ∆t_b = m * k_b * i, where

  • m = molality of the solution
  • k_b = molal boiling point elevation constant of solvent
  • i = the number of particles formed when the compound dissolves (for covalent compounds, i = 1).

• Freezing Point Depression: The freezing point depression (∆t_f) is calculated using the formula: ∆t_f = m * k_f * i, where

  • m = molality of the solution
  • k_f = molal freezing point depression constant of solvent
  • i = the number of particles formed when the compound dissolves (for covalent compounds, i = 1).

Additional Examples

• Salt is added to icy roadways to lower the freezing point of water, preventing ice formation.
• Colligative properties are crucial in various applications, like antifreeze in cars.