Spontaneous change enthalpy changes

We consider how the relationship among free-energy change, enthalpy change, and entropy change provides insi into how temperature affects the spontaneity of a process. 

Both these everyday processes are spontaneous, but whereas one process is endothermic, the other is exothermic. The energy and enthalpy of the system increase in one process, but these quantities decrease in the other process. This simple example demonstrates that analyzing energy changes and enthalpy changes is not enough to predict whether a process will occur spontaneously. We need a property other than energy and enthalpy if we hope to use thermodynamics to determine when a process will be spontaneous. 

The decomposition of N2 O4 requires a bond to break. This is the reason why the decomposition has a positive A 77 °. At the same time, the number of molecules doubles during decomposition, which is the reason AS° has a positive value. The positive enthalpy change means that energy Is removed from the surroundings and constrained, whereas the positive entropy change means that matter is dispersed. At temperatures below 315 K, the enthalpy term dominates and decomposition is not spontaneous, but at temperatures above 315 K, the entropy term dominates and decomposition is spontaneous. 

As described in Section 14-1. when AR and ZlS have the same sign, the spontaneous direction of a process depends on T. For a phase change, enthalpy dominates AG at low temperature, and the formation of the more constrained phase is spontaneous, hi contrast, entropy dominates AG at high temperature, and the formation of the less constrained phase is spontaneous. At one characteristic temperature, A G = 0, and the phase change proceeds in both directions at the same rate. The two phases coexist, and the system is in a state of d Tiamic equilibrium. 

Solid ammonium nitrate is an orderly, crystalline substance, a state considerably less random than a solution of ions in water. In this case, the positive entropy change outweighs the enthalpy change. That is TAS > AH. The Gibbs free energy change is negative, so the process will proceed spontaneously. 

For many years, it was thought, purely on an empirical basis, that if the enthalpy change for a given reaction were negative, that is, if heat were evolved at constant pressure, the transformation could occur spontaneously. This rule was verified for many reactions. Nevertheless, numerous exceptions, exist such as the polymorphic transformation of a quartz to (3 quartz at 848 K and atmospheric pressure ... 

Thus, spontaneous changes under adiabatic, isobaric conditions are always accompanied by net decrease in the enthalpy, which serves as the governing potential under these conditions. 

Table 7.5 Factors That Favor Spontaneity Enthalpy change Entropy change...

Two factors determine the spontaneity of a chemical or physical change in a system a release or absorption of heat (AH) and an increase or decrease in molecular randomness (AS). To decide whether a process is spontaneous, both enthalpy and entropy changes must be taken into account ... 

Since some spontaneous reactions are exothermic and others are endothermic, enthalpy alone can t account for the direction of spontaneous change a second factor must be involved. This second thermodynamic driving force is nature s tendency to move to a condition of maximum randomness or disorder (Section 8.13). 

G decreases for a spontaneous process, like the energy of a mechanical system. Since AG incorporates both driving forces for spontaneity—enthalpy (energy) decrease and entropy (disorder) increase—an endothermic process may be spontaneous if the increase in disorder is big enough to counteract the unfavorable enthalpy change, and a process that leads to increased order (negative AS) may be spontaneous if the process is sufficiently exothermic (negative AH). 

The inequalities of the previous paragraph are extremely important, but they are of little direct use to experimenters because there is no convenient way to hold U and S constant except in isolated systems and adiabatic processes. In both of these inequalities, the independent variables (the properties that are held constant) are all extensive variables. There is just one way to define thermodynamic properties that provide criteria of spontaneous change and equilibrium when intensive variables are held constant, and that is by the use of Legendre transforms. That can be illustrated here with equation 2.2-1, but a more complete discussion of Legendre transforms is given in Section 2.5. Since laboratory experiments are usually carried out at constant pressure, rather than constant volume, a new thermodynamic potential, the enthalpy H, can be defined by... 

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