# 13: Spontaneous Processes and Thermodynamic Equilibrium

A spontaneous process is the time-evolution of a system in which it releases free energy and moves to a lower, more thermodynamically stable energy state.

• 13.1: The Nature of Spontaneous Processes
In this section, consider the differences between two types of changes in a system: Those that occur spontaneously and those that occur by force. In doing so, we’ll gain an understanding as to why some systems are naturally inclined to change in one direction under certain conditions and how relatively quickly or slowly that natural change proceeds. We’ll also gain insight into how the spontaneity of a process affects the distribution of energy and matter within the system.
• 13.2: 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.
• 13.3: 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 2nd law states that in a reversible process, the entropy of the universe is constant and in an irreversible process, the entropy of the universe increases.
• 13.4: 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.
• 13.5: Entropy Changes and Spontaneity
In the quest to identify a property that may reliably predict the spontaneity of a process, we have identified a very promising candidate: entropy. Processes that involve an increase in entropy of the system (ΔS > 0) are very often spontaneous; however, examples to the contrary are plentiful. By expanding consideration of entropy changes to include the surroundings, we may reach a significant conclusion regarding the relation between this property and spontaneity.
• 13.6: The Third Law of Thermodynamics
This system may be described by a single microstate, as its purity, perfect crystallinity and complete lack of motion (at least classically, quantum mechanics argues for constant motion) means there is but one possible location for each identical atom or molecule comprising the crystal (Ω=1). According to the Boltzmann equation, the entropy of this system is zero.
• 13.7: The Gibbs Free 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.
• 13.8: Carnot Cycle, Efficiency, and Entropy
The Carnot cycle has the greatest efficiency possible of an engine (although other cycles have the same efficiency) based on the assumption of the absence of incidental wasteful processes such as friction, and the assumption of no conduction of heat between different parts of the engine at different temperatures.
• 13.E: Spontaneous Processes (Exercises)