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🔋 Unit 9 · Applications of Thermodynamics 🏠 Unit Hub 🗂 Flashcards 🗺 Cheat Sheet ⭐ Essentials 🎨 Visual Review 📝 MC Practice ✍️ SAQ Practice

AP Chemistry Unit 9 Essentials

The must-know terms and big ideas for Unit 9: Applications of Thermodynamics. Every vocabulary word and concept you need to master.

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Big Idea 1
Spontaneity is governed by Gibbs free energy, combining enthalpy and entropy
Whether a process happens on its own (is thermodynamically favorable) depends on both how much energy is released or absorbed (ΔH, from Unit 6) and how much disorder increases or decreases (ΔS). Gibbs free energy combines both into a single equation, ΔG = ΔH − TΔS, and a negative ΔG means the process is favorable at that temperature. Temperature itself can flip the outcome: a process with unfavorable ΔH but favorable ΔS (or vice versa) can become favorable or unfavorable depending on how hot or cold the system is.
Entropy Gibbs Free Energy Thermodynamic Favorability
Big Idea 2
ΔG unifies thermodynamics, equilibrium, and electrochemistry
Free energy isn't just a stand-alone quantity — it's the bridge between three major topics in this course. ΔG° = −RT ln K connects favorability to the size of an equilibrium constant (Unit 7/8), while ΔG = −nFE connects it to the voltage of an electrochemical cell. A reaction with a very negative ΔG° has a large K (strongly favors products) AND would generate a large cell potential if set up as a galvanic cell — they're three different lenses on the exact same underlying chemistry.
Free Energy & K Galvanic Cells Cell Potential
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Entropy (S)
A measure of the number of accessible microstates, or disorder, in a system; entropy tends to increase for a system left to itself (second law of thermodynamics).
Entropy
Second law of thermodynamics
The total entropy of the universe (system + surroundings) increases for any spontaneous process.
Entropy
Absolute entropy (S°)
The entropy of a substance in its standard state; generally increases from solid to liquid to gas, and with increasing molecular complexity.
Entropy
Entropy change (ΔS)
The change in entropy for a process; positive when moles of gas increase, a solid/liquid becomes a gas, or molecules become more disordered.
Entropy
Microstates
The number of possible arrangements of particles and energy in a system; more accessible microstates corresponds to higher entropy.
Entropy
Gibbs free energy (G)
A thermodynamic quantity that combines enthalpy and entropy to predict whether a process is thermodynamically favorable at a given temperature.
Gibbs Free Energy
ΔG = ΔH − TΔS
The equation for Gibbs free energy change; a negative ΔG indicates a thermodynamically favorable (spontaneous) process at temperature T.
Gibbs Free Energy
Thermodynamic favorability
Describes a process with ΔG < 0, meaning it can occur without continuous outside energy input — also called spontaneity.
Gibbs Free Energy
Thermodynamic vs. kinetic control
The distinction between whether a reaction is favorable (ΔG < 0, a thermodynamic property) and whether it actually proceeds quickly (a kinetic property, governed by activation energy).
Kinetics vs. Thermo
ΔG° = −RT ln K
The equation relating the standard free energy change of a reaction to its equilibrium constant K; a more negative ΔG° corresponds to a larger K.
Free Energy & K
Coupled reactions
Pairing a thermodynamically unfavorable reaction with a favorable one so that their combined ΔG is negative overall, driving the unfavorable reaction forward.
Free Energy & K
Galvanic (voltaic) cell
An electrochemical cell that uses a spontaneous redox reaction to generate electrical current.
Electrochemistry
Electrolytic cell
An electrochemical cell that uses an external power source to drive a nonspontaneous redox reaction.
Electrochemistry
Anode / cathode
The anode is the electrode where oxidation occurs; the cathode is the electrode where reduction occurs (in both galvanic and electrolytic cells).
Electrochemistry
Salt bridge
A connection between the two half-cells of a galvanic cell that allows ion flow to maintain charge balance without mixing the solutions.
Electrochemistry
Standard reduction potential (E°)
A measure of a half-reaction's tendency to be reduced (gain electrons), measured relative to the standard hydrogen electrode.
Cell Potential
Cell potential (E°cell)
The voltage produced by a galvanic cell, calculated as E°cathode − E°anode; a positive value indicates a spontaneous (favorable) redox reaction.
Cell Potential
ΔG = −nFE
The equation relating Gibbs free energy change to cell potential, where n is moles of electrons transferred and F is Faraday's constant.
Cell Potential
Faraday's constant (F)
The charge of one mole of electrons, equal to 96,485 coulombs per mole; used to connect electrical and chemical quantities.
Cell Potential
Nernst equation
An equation used to calculate cell potential under nonstandard conditions (concentrations other than 1 M), accounting for how Q affects E.
Cell Potential