# II: Principles of Thermodynamics (Entropy and Gibbs Energy)

The name "thermodynamics" is really a silly name. Think about it; the field deals primarily with the condition of equilibrium, that is, no change or a "static" situation. The actual field that describes non-equilibration properties is "kinetics" or "dynamics," hence the more apt term for this field is really "thermostatics." However, since we cannot change a name that has been in use for 150 years, we will stick with it and pretend otherwise.

• 2.1: The Nature of Spontaneous Processes
Chemical and physical processes have a natural tendency to occur in one direction under certain conditions. A spontaneous process occurs without the need for a continual input of energy from some external source, while a nonspontaneous process requires such. Systems undergoing a spontaneous process may or may not experience a gain or loss of energy, but they will experience a change in the way matter and/or energy is distributed within the system.
• 2.2: Entropy and Spontaneity - A Molecular Statistical Interpretation
These forms of motion are ways in which the molecule can store energy. The greater the molecular motion of a system, the greater the number of possible microstates and the higher the entropy. A perfectly ordered system with only a single microstate available to it would have an entropy of zero. The only system that meets this criterion is a perfect crystal at a temperature of absolute zero (0 K), in which each component atom, molecule, or ion is fixed in place within a perfect crystal lattice.
• 2.3: Entropy and Heat - Experimental Basis of the Second Law of Thermodynamics
A reversible process is one for which all intermediate states between extremes are equilibrium states; it can change direction at any time. In contrast, an irreversible process occurs in one direction only. The change in entropy of the system or the surroundings is the quantity of heat transferred divided by the temperature. The second law of thermodynamics states that in a reversible process, the entropy of the universe is constant.
• 2.4: Entropy Changes and Spontaneity
All spontaneous changes cause an increase in the entropy of the universe. This is the Second Law of Thermodynamics.
• 2.5: Entropy Changes in Reversible Processes
Changes in internal energy, that are not accompanied by a temperature change, might reflect changes in the entropy of the system. Changes in internal energy, that are not accompanied by a temperature change, might reflect changes in the entropy of the system.
• 2.6: Quantum States, Microstates, and Energy Spreading
Entropy (S) is a state function whose value increases with an increase in the number of available microstates.For a given system, the greater the number of microstates, the higher the entropy. During a spontaneous process, the entropy of the universe increases.
• 2.7: The Third Law of Thermodynamics
As the absolute temperature of a substance approaches zero, so does its entropy. This principle is the basis of the Third law of thermodynamics, which states that the entropy of a perfectly-ordered solid at 0 K is zero.
• 2.8: Gibbs Energy
One of the major goals of chemical thermodynamics is to establish criteria for predicting whether a particular reaction or process will occur spontaneously. We have developed one such criterion, the change in entropy of the universe. This is not particularly useful and a criterion of spontaneity that is based solely on the state functions of a system would be much more convenient and is provided by a new state function: the Gibbs free energy.
• 2.E: Principles of Thermodynamics (Exercises)
These are homework exercises to accompany the Textmap created for "Principles of Modern Chemistry" by Oxtoby et al.