Chapter 23: Entropy and Gibbs Free Energy

The chapter provides a comprehensive introduction to the principles of Entropy and Gibbs free energy for predicting chemical reaction feasibility. Entropy, symbolized as S, is defined as the measure of the dispersal or number of arrangements of particles and their energy in a given system. The Second Law of Thermodynamics establishes that all spontaneous changes—those that tend to continue naturally once initiated—occur in the direction of increased total disorder, meaning the total entropy change (Delta S total) must be positive. Physical state and molecular complexity are critical factors in determining entropy: gases generally possess significantly higher standard molar entropy values (S standard) than liquids, which, in turn, have higher values than solids. Entropy increases when a substance undergoes a phase change from solid to liquid, or liquid to vapor, due to the large increase in particle disorder, and also increases when temperature rises. For chemical reactions, the change in the number of gaseous molecules is the most significant factor for estimating the change in system entropy (Delta S system). Calculations for the system entropy change follow the rule that Delta S standard system equals the sum of standard entropies of products minus the sum of standard entropies of reactants. While exothermic reactions increase the entropy of the surroundings, enhancing feasibility, chemists often use Gibbs free energy (G) to simplify the prediction of spontaneity by focusing only on the system. The Gibbs equation defines the standard free energy change as Delta G standard equals Delta H standard reaction minus T Delta S standard system. For a reaction to be feasible (spontaneous), its Delta G standard value must be negative. Temperature (T) plays a crucial role in feasibility, particularly in endothermic reactions, where increasing the temperature can make the negative T Delta S system term large enough to overcome a positive Delta H standard term, resulting in a negative Delta G standard. Gibbs free energy change of reaction can also be calculated using standard Gibbs free energies of formation (Delta G standard reaction equals the sum of Delta G standard formation products minus the sum of Delta G standard formation reactants). Finally, the feasibility of an electrochemical reaction can be predicted using the standard cell potential (E standard cell) via the relationship Delta G standard equals minus n F E standard cell, where n is the number of transferred electrons and F is the Faraday constant.