AP Chemistry Unit 9 Review: Thermodynamics and Electrochemistry
Every Unit 9 topic, what's actually tested, and what the exam explicitly excludes -- with diagrams for entropy, favorability, and galvanic cells.
Unit 9 closes out the course by asking two connected questions: does a process happen on its own (entropy and Gibbs free energy), and can chemistry generate or consume electrical energy (electrochemistry). It's 7–9% of the multiple-choice section across 11 topics and 10–13 class periods. Here's everything in it, topic by topic, with diagrams and full worked calculations, verified against the current CED.
Unit 9 at a Glance
Unit 9 (Thermodynamics and Electrochemistry) pulls together reasoning from across the whole course. See the full AP Chemistry units breakdown for how it fits alongside the other 8 units, and the Course and Exam Description for the full framework.
Naming note: this unit was renamed in the Fall 2024 CED update. If you see "Applications of Thermodynamics" in an older resource, that's the same unit under its previous name.
Introduction to Entropy
Entropy increases when matter or energy becomes more dispersed. A solid melting, a liquid boiling, a gas expanding into a larger volume, or a reaction that produces more moles of gas than it consumes -- all increase entropy. Raising temperature also increases entropy, because the Maxwell-Boltzmann distribution of particle kinetic energies broadens.
Absolute Entropy and Entropy Change
The standard entropy change of a reaction is calculated the same way as enthalpy of reaction: ΔS°reaction = ΣS°products − ΣS°reactants, using tabulated standard molar entropies.
Gibbs Free Energy and Thermodynamic Favorability
ΔG° combines enthalpy and entropy into a single favorability test: ΔG° = ΔH° − TΔS°. When ΔG° < 0, the process is thermodynamically favored (the term the CED uses in place of "spontaneous," to avoid implying the process happens suddenly or without cause). The signs of ΔH° and ΔS° determine whether temperature matters at all:
| ΔH° | ΔS° | Favored at: |
|---|---|---|
| < 0 | > 0 | all temperatures |
| > 0 | < 0 | no temperature |
| > 0 | > 0 | high temperature |
| < 0 | < 0 | low temperature |
Worked example: A reaction has ΔH° = +50.0 kJ/mol and ΔS° = +200. J/(mol·K). Since both are positive, it's favored only above some threshold temperature -- find it by setting ΔG° = 0: 0 = 50,000 J/mol − T(200 J/(mol·K)), so T = 50,000/200 = 250 K. Above 250 K, this reaction is thermodynamically favored; below it, it isn't.
Thermodynamic and Kinetic Control
A process can be thermodynamically favored and still not happen at any noticeable rate -- that's a process under kinetic control, usually because of a high activation energy. Thermodynamic favorability tells you nothing about how fast a reaction goes; that's kinetics, not thermodynamics (see Unit 5).
Free Energy and Equilibrium
ΔG° and the equilibrium constant K are directly linked: ΔG° = −RT ln K, or equivalently K = e−ΔG°/RT. A negative ΔG° means K > 1 (products favored at equilibrium); a positive ΔG° means K < 1 (reactants favored).
Worked example: What's K for a reaction with ΔG° = −10.0 kJ/mol at 298 K? K = e−(−10,000)/(8.314 × 298) = e4.03 = 56.4. A negative ΔG° correctly predicts K > 1.
Free Energy of Dissolution
Dissolving a solid involves three competing energy factors: breaking the solid's intermolecular/ionic attractions, reorganizing the solvent around the dissolved particles, and the new solute-solvent interactions themselves. You can estimate the sign and rough size of each piece, but predicting the overall ΔG° of dissolution is genuinely hard because these three factors partially cancel.
Coupled Reactions
An external energy source -- electricity charging an electrolytic cell, or light driving photosynthesis -- can force a thermodynamically unfavorable process to occur. Alternatively, two reactions sharing a common intermediate (like ATP converting to ADP in biology) can be "coupled" so the favorable reaction drags the unfavorable one along, producing a combined ΔG° < 0.
Galvanic (Voltaic) and Electrolytic Cells
A galvanic (voltaic) cell harnesses a thermodynamically favored redox reaction to generate electrical energy; an electrolytic cell uses an external power source to force a thermodynamically unfavorable redox reaction to occur. In both, oxidation happens at the anode, reduction happens at the cathode -- that rule never flips, regardless of cell type.
Not tested: labeling an electrode as positive or negative is explicitly excluded. Know which electrode is the anode (oxidation) and which is the cathode (reduction) -- that's what the exam actually asks for.
Cell Potential and Free Energy
A cell's standard potential connects directly to ΔG°: ΔG° = −nFE°, where n is moles of electrons transferred and F is Faraday's constant. A positive E° means a thermodynamically favored (negative ΔG°) reaction; a negative E° means an unfavored one.
Worked example: A Zn/Cu galvanic cell has E°cell = +1.10 V, transferring n = 2 mol electrons. ΔG° = −nFE° = −(2)(96,485 C/mol)(1.10 V) = −212,000 J/mol = −212 kJ/mol. The negative value confirms the reaction is thermodynamically favored, consistent with the positive cell potential.
Cell Potential Under Nonstandard Conditions
Under nonstandard concentrations, cell potential shifts according to the Nernst equation: E = E° − (RT/nF) ln Q. The farther the reaction is from equilibrium, the larger the magnitude of E; as the system approaches equilibrium (Q approaches K), E shrinks toward zero. Algorithmic Nernst-equation plug-and-chug isn't the point on the AP Exam -- you're expected to reason qualitatively about which direction E shifts as Q changes.
Important distinction: Le Chatelier's principle does not apply to a running electrochemical cell, because the cell is not at equilibrium while current flows. Don't reach for Le Chatelier language when a question describes a working galvanic or electrolytic cell.
Electrolysis and Faraday's Law
Faraday's law connects charge, current, and time (I = q/t) to the stoichiometry of an electrolysis reaction -- letting you calculate moles of electrons transferred, and from there, mass deposited or removed at an electrode.
Worked example: A 2.00 A current runs for 965 seconds through a solution of Cu²⁺. How many grams of Cu are deposited? Charge: q = It = (2.00 A)(965 s) = 1930 C. Moles of electrons: 1930 C / 96,485 C/mol = 0.0200 mol e⁻. Since Cu²⁺ + 2e⁻ → Cu requires 2 mol electrons per mol Cu: 0.0200 mol e⁻ × (1 mol Cu / 2 mol e⁻) = 0.0100 mol Cu. Mass = 0.0100 mol × 63.5 g/mol = 0.635 g Cu.
Common Mistakes in Unit 9
- Assuming a thermodynamically favored reaction happens quickly. Favorability (ΔG° < 0) says nothing about rate -- a favored reaction can still be kinetically stalled by a high activation energy.
- Mixing units between ΔS° and ΔH°/ΔG°. Entropy is reported in J/(mol·K); enthalpy and free energy are in kJ/mol. Convert before combining them in ΔG° = ΔH° − TΔS°.
- Applying Le Chatelier's principle to a running electrochemical cell. The cell isn't at equilibrium while it operates -- Le Chatelier reasoning doesn't apply until current stops flowing.
- Forgetting the n value in Faraday's law and ΔG° = −nFE°. Both calculations require the correct moles of electrons transferred per the balanced half-reactions, not just 1.
- Trying to label an electrode "positive" or "negative." That labeling is explicitly excluded -- focus on identifying anode/cathode and the oxidation/reduction happening at each.
How Unit 9 Connects to the Rest of the Course
- The ΔG°–K relationship here is the same equilibrium-constant framework built in Unit 7 and applied to acids and bases in Unit 8.
- The half-reaction balancing from Unit 4's redox reactions is exactly what you use to construct a galvanic cell here.
- Thermodynamic vs. kinetic control directly revisits the activation-energy reasoning from Unit 5.
Related Resources
- AP Chemistry Units (All 9)
- AP Chemistry Unit 1 Review
- AP Chemistry Unit 2 Review
- AP Chemistry Unit 3 Review
- AP Chemistry Unit 4 Review
- AP Chemistry Unit 5 Review
- AP Chemistry Unit 8 Review
- AP Chemistry Course and Exam Description
- AP Chemistry FRQ Calculator
Frequently Asked Questions
What topics are in AP Chemistry Unit 9?
Introduction to Entropy, Absolute Entropy and Entropy Change, Gibbs Free Energy and Thermodynamic Favorability, Thermodynamic and Kinetic Control, Free Energy and Equilibrium, Free Energy of Dissolution, Coupled Reactions, Galvanic (Voltaic) and Electrolytic Cells, Cell Potential and Free Energy, Cell Potential Under Nonstandard Conditions, and Electrolysis and Faraday's Law -- 11 topics in total.
How much is Unit 9 worth on the AP Chemistry exam?
Seven to nine percent of the multiple-choice section across roughly 10 to 13 class periods -- the final unit of the course.
Was AP Chemistry Unit 9 renamed?
Yes. The current CED (Effective Fall 2024) renamed it "Thermodynamics and Electrochemistry" -- previously "Applications of Thermodynamics." Many older study guides and search results still use the outdated name.
Does AP Chemistry test which electrode is positive or negative?
No. Labeling an electrode as positive or negative is explicitly excluded. You're responsible for identifying the anode (oxidation) and cathode (reduction), not assigning a plus or minus sign to either one.
Does Le Chatelier's principle apply to a running electrochemical cell?
No. An operating electrochemical cell is not at equilibrium, so Le Chatelier's principle doesn't apply to it. Cell potential is a measure of how far the reaction is from equilibrium, not a system you can "shift" with a stress.
Sourced from College Board's official AP Chemistry Course and Exam Description, Effective Fall 2024. This page describes the document's real content and current exclusion statements; it is not a copy of it and is not affiliated with or endorsed by College Board.