AP Chemistry Unit 9 Study Notes

AP Chemistry 9.4: Free Energy, Reaction Quotients, and Equilibrium

Connect standard and nonstandard free energy with Q and K.

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.

20. Standard vs. Nonstandard Free Energy

  • ΔG° describes a reaction under standard-state conditions.

  • Actual laboratory conditions are often not standard.

    • For nonstandard conditions:
    • ΔG = ΔG° + RT ln Q
    • where:
    • ΔG = free-energy change under current conditions
    • ΔG° = standard free-energy change
    • R = gas constant
    • T = temperature in K
    • Q = reaction quotient
    • Use:
    • R = 8.314 J/(mol·K)
    • when energy is in joules.
21. Using Q to Understand ΔG

  • Remember from Unit 7:

    • Q < K → reaction shifts right
    • Q > K → reaction shifts left
    • Q = K → equilibrium
    • Free energy tells the same story.
Q < K

  • ΔG < 0

    • Forward reaction favored.
Q > K

  • ΔG > 0

    • Reverse reaction favored.
Q = K

  • ΔG = 0

    • System is at equilibrium.
22. ΔG° and the Equilibrium Constant

  • At equilibrium:

    • ΔG = 0
    • and:
    • Q = K
    • Combining the free-energy equations gives:
    • ΔG° = −RT ln K
    • This equation connects thermodynamics directly with equilibrium.
23. Interpreting ΔG° and K

  • If:

    • ΔG° < 0
    • then:
    • K > 1
    • Products are favored.
    • If:
    • ΔG° > 0
    • then:
    • K < 1
    • Reactants are favored.
    • If:
    • ΔG° = 0
    • then:
    • K = 1
    • Neither side is strongly favored under standard-state comparison.
24. Very Important Connection

  • You should recognize these as describing the same basic idea:

Products favored

  • ΔG° < 0

    • K > 1
Reactants favored

  • ΔG° > 0

    • K < 1