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Free Energy and Equilibrium

AP Course

AP Chemistry

Updated for the AP Chemistry framework effective fall 2024, with particle models, quantitative reasoning, labs, quizzes, and practice problems.

Build general chemistry fluency through atomic structure, bonding, reactions, kinetics, equilibrium, acids and bases, thermodynamics, and electrochemistry.

AP Chemistry · Unit 9 · Topic 9.5

Free Energy and Equilibrium

Entropy, free energy, equilibrium, and electrochemical work provide compatible tests of thermodynamic favorability.

Energy profile · Free Energy and EquilibriumThe peak represents the activation barrier; compare reactant and product levels separately from the barrier height.

1. Topic Lens

Free Energy and Equilibrium is studied through thermodynamics and electrochemistry. Connect the system boundary, interacting parts, and measurable evidence before applying a formula.

\[\Delta G^\circ=-nFE^\circ\]
Chemical equation or molecular relation
\[Zn(s)+Cu^{2+}(aq)\rightarrow Zn^{2+}(aq)+Cu(s)\]

2. Why the Formula Works

The relationship is built from definitions and conservation reasoning:

  1. Electrical work for n moles of electrons crossing potential E is nFE.
  2. A spontaneous galvanic cell can deliver work, giving ΔG°=-nFE°.
  3. Combining this with ΔG°=-RT lnK connects cell voltage to equilibrium.
\[E^\circ=\frac{RT}{nF}\ln K\]

3. Detailed Visual Model

Free Energy and EquilibriumTopic-matched fine pixel-art model · use the lesson's equations and labeled diagrams for exact scientific meaning.

4. Worked Example and Lab Link

A cell transfers 2 mol e⁻ at E°=1.10 V. Using F=96485 C/mol, estimate ΔG°.

Answer: ΔG°≈-2.12×10⁵ J/mol, or -212 kJ/mol.

Investigation idea: Construct safe microscale galvanic cells with several metal pairs and compare measured and predicted voltage.

Common trap: Positive E° pairs with negative ΔG° for the reaction as written; it says nothing about how fast the reaction occurs.

Checkpoint · Topic 9.5

Explain how free energy and equilibrium supports or limits this conclusion: ΔG°≈-2.12×10⁵ J/mol, or -212 kJ/mol.

Official curriculum reference: College Board AP Chemistry course page. The explanation and worked example are independently written for this study site.