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Understanding Free Energy in Dissolution

May 5, 2025

Lecture Notes: AP Chemistry - Free Energy of Dissolution

Instructor: Jeremy Kug

Overview

  • Topic: Free Energy of Dissolution (Unit 9, Topic 6)
  • Focus: Understanding thermodynamic favorability in dissolution processes through Gibbs Free Energy, Enthalpy (螖H), and Entropy (螖S).

Key Concepts

Gibbs Free Energy (螖G)

  • 螖G < 0: Thermodynamically favored process.
  • 螖G > 0: Not thermodynamically favored.

Thermodynamic Driving Forces

  • Enthalpy (螖H):
    • Positive 螖H: Endothermic process, generally not favorable.
    • Negative 螖H: Exothermic process, generally favorable.
  • Entropy (螖S):
    • Positive 螖S: Increase in disorder, favorable.
    • Negative 螖S: Decrease in disorder, not favorable.

Dissolution Examples

Potassium Chloride (KCl)

  • 螖G = -5.3 kJ/mol
    • Negative 螖G indicates solubility and thermodynamic favorability.
  • 螖H = +17.1 kJ/mol (Endothermic)
  • 螖S = +76.4 J/mol路K
    • High increase in entropy counteracts endothermic 螖H.
    • Conclusion: KCl is soluble.

Silver Chromate (Ag2CrO4)

  • 螖G = Positive
    • Positive 螖G indicates low solubility, not thermodynamically favored.
  • 螖H = +61.7 kJ/mol (Endothermic)
  • 螖S = -22 J/mol路K
    • Both 螖H and 螖S are not favorable.
    • Conclusion: Ag2CrO4 is insoluble.

Calcium Chloride (CaCl2)

  • 螖H = -81.4 kJ/mol (Exothermic)
    • Exothermic 螖H makes it favorable despite a decrease in entropy.
  • 螖S = -45.1 J/mol路K
  • Conclusion: CaCl2 is very soluble due to exothermic nature.

Problem Solving

Barium Sulfate (BaSO4)

  • 螖G = +56.8 kJ/mol
    • Indicates non-solubility in water under standard conditions.

Iron(III) Chloride (FeCl3)

  • Soluble in water implies 螖G is negative.
  • If 螖S is negative, process must be exothermic to be thermodynamically favored.

Additional Resources

  • Ultimate Review Packet: Comprehensive review tool available at ultimatereviewpacket.com
  • Practice problems, unit summaries, and guides to score a 5 on the AP exam.

Final Remarks

  • The lecture emphasizes understanding of the principles behind thermodynamic favorability using Gibbs Free Energy.
  • The instructor encourages students to use additional resources for further preparation.
  • Note: The concepts discussed are crucial for understanding solubility and dissolution in chemistry, particularly for AP exam preparation. Regular practice and review are advised.

Full transcript