Chapter 20: Entropy, Free Energy, & Reaction Direction

The Second Law of Thermodynamics is then introduced, establishing that in any spontaneous process, the entropy (disorder) of the universe increases. Students learn to calculate entropy changes (ΔS) for systems, surroundings, and the universe, with worked examples connecting entropy to molecular motion, freedom of particle movement, and energy dispersal. The chapter introduces Gibbs free energy (ΔG) as the central criterion for spontaneity, combining enthalpy (ΔH) and entropy (ΔS) into the relationship ΔG = ΔH – TΔS. Students learn how to calculate ΔG under standard conditions, interpret its sign, and connect it to equilibrium. The relationship between ΔG° and the equilibrium constant (K) is explored, showing how thermodynamics predicts both spontaneity and the extent of reaction. The chapter also emphasizes the role of temperature in shifting spontaneity, explaining how endothermic reactions can become spontaneous at high temperatures if entropy increases sufficiently. Practical applications include coupling reactions in biological systems, particularly the role of ATP hydrolysis in driving nonspontaneous processes. By the end of the chapter, students understand how energy transformations and entropy changes govern chemical reactivity, equilibrium, and the feasibility of both laboratory and biological reactions.