This document summarizes Chapter 20 from the textbook "Prentice-Hall General Chemistry" by Petrucci, Harwood, and Herring. The chapter discusses spontaneous change, entropy, free energy, and their relationships to determining the spontaneity of chemical reactions. It defines key concepts such as entropy, the Boltzmann equation, and Gibbs free energy. It also explains how these quantities can be used to evaluate spontaneity based on the second law of thermodynamics and establish equilibrium using the equilibrium constant Keq. Examples are provided for calculating entropy changes during phase transitions as well as determining temperature dependence of free energy and equilibrium. The chapter concludes by discussing how coupled reactions can be used to drive nonspontaneous processes.