AP Chemistry Unit 9 Study Notes

AP Chemistry 9.5: Free Energy of Dissolution

Explain how enthalpy and entropy work together when substances dissolve.

Aligned to Thermodynamics and Electrochemistry from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.

Study these notes

Start with each main idea, then follow the indented explanations and worked examples. Try the next calculation before reading its answer.

Organized from the provided Unit 9 study document. Further study: Khan Academy.

17. Free Energy of Dissolution

  • Dissolving a substance involves competition among several interactions.

    • Conceptually, dissolution can involve:
    • separating solute particles
    • separating some solvent particles
    • forming attractions between solute and solvent

  • Breaking existing attractions requires energy.

  • Forming new attractions releases energy.

  • Entropy also changes because dissolved particles can become more dispersed throughout the solution.

  • Whether dissolution is thermodynamically favored depends on the combined effect of:

    • ΔH and ΔS
    • through:
    • ΔG = ΔH − TΔS
18. Endothermic Dissolution Can Still Be Favored

  • A common mistake is:

    • If dissolving absorbs heat, it cannot happen naturally.
    • That is false.
    • Suppose:
    • ΔH > 0
    • but:
    • ΔS > 0
    • If the entropy term is large enough:
    • TΔS > ΔH
    • then:
    • ΔG < 0
    • and dissolution can still be thermodynamically favored.