Entropy, Gibbs free energy, and thermodynamic favorability — and how they connect to equilibrium and electrochemistry through galvanic and electrolytic cells.
Six ways to master Unit 9 — pick whichever fits how you like to study.
Eleven topics from the College Board CED, in order.
Unit 9 brings together every major thread of the course — thermodynamics, equilibrium, and redox chemistry — under a single unifying quantity: Gibbs free energy (ΔG). You'll learn to predict the sign of entropy change (ΔS) for a process, then combine it with enthalpy in ΔG = ΔH − TΔS to determine whether a reaction is thermodynamically favorable at a given temperature. From there, the equation ΔG° = −RT ln K connects favorability directly to the size of an equilibrium constant, while ΔG = −nFE connects it to the voltage of a galvanic or electrolytic cell.
This unit is roughly 7–9% of the AP Chem exam and takes about 10–13 class periods — shorter than most units, but conceptually dense, since it ties Units 6 (thermochemistry), 7 (equilibrium), and electrochemistry together into one coherent framework.
Two core themes run through this unit: