CHEM 106: 1/31/25

Solubility Trends in Alcohols

  • Solubility in Water vs. Hexane

    • Alcohols are more soluble in water at lower temperatures.

    • Solubility decreases in warmer temperatures for alcohols in water, but increases for hydrocarbons (like hexane).

    • Ethanol vs. Propanol vs. Hexane: Ethanol remains soluble because of its hydroxyl (–OH) group, while propanol and hexane show different solubility trends.

  • Hydroxyl Group Position

    • The number in front of the alcohol’s name indicates the position of the hydroxyl group.

    • A hydroxyl group at the end (1-position) can form hydrogen bonds more easily than one farther away (2-position or 3-position), influencing solubility.

The Solution Process

  • Steps in Solution Formation

    1. Separation of Solute - Energy is absorbed (positive enthalpy change, ΔH > 0).

    2. Separation of Solvent - Energy is absorbed (ΔH > 0).

    3. Mixing Solute and Solvent - Energy is released (ΔH < 0).

  • Energy changes during dissolution differ based on solutes (e.g., sodium chlorate, ammonium nitrate).

  • Endothermic vs. Exothermic Solutions

    • Ammonium Nitrate: Dissolves endothermically (positive ΔH), yet forms a solution due to favorable entropy changes.

    • Calcium Chloride: Dissolves with a large negative ΔH, providing instant heat in hot packs.

Entropy and Spontaneity

  • Entropy (S): Measure of disorder within a system. Higher disorder corresponds to higher entropy.

  • Second Law of Thermodynamics: Processes tend to increase the universe's entropy (ΔS > 0).

  • Phase Relationship

    • Solid < Liquid < Gas concerning entropy.

    • Phase transitions like melting and vaporization lead to positive entropy changes. Conversely, freezing and condensation lead to negative changes.

Impact of Temperature on Solubility

  • General Trend: Solubility of salts usually increases with temperature, but exceptions exist (e.g., cesium sulfate).

  • Gases and Temperature: Contrarily, the solubility of gases typically decreases as temperature increases due to increased kinetic energy that allows gas molecules to escape the solution.

Henry's Law and Gas Solubility

  • Henry's Law: The solubility of a gas in liquid is directly proportional to its partial pressure above the solution. Formula:

    • Solubility = Henry's law constant × Partial pressure (in atm).

  • Example Calculation: For nitrogen gas at 25°C and 1 atm, solubility = (Henry's law constant) × (mole fraction in mixture). Denote the constant for nitrogen in water and calculate.

Concentration Units in Solutions

  • Mole Fraction (x): Number of moles of solute divided by the total number of moles in solution (solute + solvent).

  • Molality (m): Moles of solute per kilogram of solvent.

  • Molarity (M): Moles of solute per liter of solution; used for calculating osmotic pressure.

Colligative Properties Overview

  • Colligative Properties: Properties that depend on the number of solute particles rather than their nature. Crucial examples include vapor pressure lowering, boiling point elevation, and freezing point depression.

  • Review sections that delve into the application of these concepts and calculations.