Entropy, Gibbs free energy, and thermodynamic favorability โ and how they connect to equilibrium and electrochemistry through galvanic and electrolytic cells.
Entropy (S) as a measure of disorder or the number of accessible microstates in a system.
Topic 9.2
Absolute Entropy & Entropy Change
Predicting the sign of ฮS for a process based on changes in moles of gas, temperature, or physical state.
Topic 9.3
Gibbs Free Energy & Thermodynamic Favorability
ฮG = ฮH โ TฮS, and how the signs of ฮH, ฮS, and temperature together determine whether a process is thermodynamically favorable.
Topic 9.4
Thermodynamic & Kinetic Control
The distinction between a reaction being thermodynamically favorable (ฮG < 0) and actually occurring quickly (kinetics).
Topic 9.5
Free Energy & Equilibrium
The relationship ฮGยฐ = โRT ln K, connecting thermodynamic favorability to the size of the equilibrium constant.
Topic 9.6
Coupled Reactions
How an unfavorable reaction can be driven forward by pairing it with a more favorable one, summing their ฮG values.
Topic 9.7
Galvanic (Voltaic) Cells
Spontaneous redox reactions that generate electrical current, with separate oxidation (anode) and reduction (cathode) half-cells.
Topic 9.8
Electrolytic Cells
Using an external power source to drive a nonspontaneous redox reaction, reversing the natural direction of electron flow.
Topic 9.9
Cell Potential & Free Energy
The relationship ฮG = โnFE, connecting a cell's voltage directly to the free energy change of its redox reaction.
Topic 9.10
Cell Potential Under Nonstandard Conditions
Using the Nernst equation to calculate cell potential when concentrations differ from standard 1 M conditions.
About Unit 9
Unit 9 brings together every major thread of the course โ thermodynamics, equilibrium, and redox chemistry โ under a single unifying quantity: Gibbs free energy (ฮG). You'll learn to predict the sign of entropy change (ฮS) for a process, then combine it with enthalpy in ฮG = ฮH โ TฮS to determine whether a reaction is thermodynamically favorable at a given temperature. From there, the equation ฮGยฐ = โRT ln K connects favorability directly to the size of an equilibrium constant, while ฮG = โnFE connects it to the voltage of a galvanic or electrolytic cell.
This unit is roughly 7โ9% of the AP Chem exam and takes about 12โ14 class periods โ shorter than most units, but conceptually dense, since it ties Units 6 (thermochemistry), 7 (equilibrium), and electrochemistry together into one coherent framework.
The College Board ties Unit 9 to two core Big Ideas:
Big Idea ENE
Energy โ spontaneity is governed by Gibbs free energy, combining enthalpy and entropy