Chapter 16 Summary

16.1  Spontaneity

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.

16.2  Entropy

Entropy (S) is a state function that can be related to the number of microstates for a system (the number of ways the system can be arranged) and to the ratio of reversible heat to kelvin temperature. It may be interpreted as a measure of the dispersal or distribution of matter and/or energy in a system, and it is often described as representing the “disorder” of the system.

For a given substance, entropy depends on phase with S_solid < S_liquid < S_gas. For different substances in the same physical state at a given temperature, entropy is typically greater for heavier atoms or more complex molecules. Entropy increases when a system is heated and when solutions form. Using these guidelines, the sign of entropy changes for some chemical reactions and physical changes may be reliably predicted.

16.3  The Second and Third Laws of Thermodynamics

The second law of thermodynamics states that a spontaneous process increases the entropy of the universe, S_univ > 0. If ΔS_univ < 0, the process is nonspontaneous, and if ΔS_univ = 0, the system is at equilibrium. The third law of thermodynamics establishes the zero for entropy as that of a perfect, pure crystalline solid at 0 K. With only one possible microstate, the entropy is zero. We may compute the standard entropy change for a process by using standard entropy values for the reactants and products involved in the process.

16.4  Free Energy

Gibbs free energy (G) is a state function defined with regard to system quantities only and may be used to predict the spontaneity of a process. A negative value for ΔG indicates a spontaneous process; a positive ΔG indicates a nonspontaneous process; and a ΔG of zero indicates that the system is at equilibrium. Free energy changes can be calculated using the standard free energy of formation (ΔG_f​°) or the equation ΔG = ΔH − TΔS.

16.5  Temperature-Dependence of Spontaneity

Since the value of ΔG depends on temperature, spontaneity of reactions are also temperature dependent. Depending on if ΔH and ΔS are positive or negative, reactions can be spontaneous at all temperatures, low temperatures, high temperatures or at no temperatures.

16.6  Free Energy and Equilibrium

Since ΔG and reaction quotient, Q, both predict the direction of reaction spontaneity, these two values are related through the equation ΔG = ΔG° + RT ln Q.  This equation also illustrates the relationship between Gibbs free energy and equilibrium since at equilibrium, ΔG = 0 and Q = K.