Thermodynamic free energy data

The perspective of simple phases and phase equilibria is adequate for many corrosion problems. This is particularly true for the cases where thermodynamic free energy data really describe what phases are present. 

Thermodynamic data are available only for the lower alkylamines, mainly estimates based on a few experimental deterrninations (3,4). Many manufacturing processes appear to be limited by thermodynamic equiUbria. The lack of accurate free energy data for these amines limits the appHcation of thermodynamic considerations, in contrast to the situation in hydrocarbon technology. 

Before leaving the subject of thermodynamics it should be pointed out that most of the arguments in this section are based on standard free energy data rather than the actual free energies which would require knowing the concentrations of reactants and products in the mitochondria. This is a serious limitation we must live with until there is a reliable means for estimating these concentrations. 

A more detailed picture of hydroisomerization of n-octane and n-nonane is given in Table II in which product distributions are listed for different degrees of conversion along with thermodynamic equilibrium values. The latter have been calculated from Gibbs free energy data available in literature (F7) the accuracy of which, however, is not known. From Table II the following conclusions may be drawn ... 

For low pressures of hydrogen and for vacuum reactions the equilibrium pressure of methane is very low and the reaction is not feasible from a thermodynamic point of view. The reverse reaction of methane, forming hydrogen with the deposition of carbon, is feasible and may occur if a proper mechanism exists for the reaction. The free-energy data for methane are given by Kelley (13). 

Although we refer throughout the book to thermodynamic information and equilibrium constants that, in our opinion, have been well documented in the literature, it has not been the authors objective to critically select the best available data. There are various compilations available that recommend a set of equilibrium constants and/or free energy data. We mention some here. 

Numerous applications of standard electrode potentials have been made in various aspects of electrochemistry and analytical chemistry, as well as in thermodynamics. Some of these applications will be considered here, and others will be mentioned later. Just as standard potentials which cannot be determined directly can be calculated from equilibrium constant and free energy data, so the procedure can be reversed and electrode potentials used for the evaluation, for example, of equilibrium constants which do not permit of direct experimental study. Some of the results are of analjrtical interest, as may be shown by the following illustration. Stannous salts have been employed for the reduction of ferric ions to ferrous ions in acid solution, and it is of interest to know how far this process goes toward completion. Although the solutions undoubtedly contain complex ions, particularly those involving tin, the reaction may be represented, approximately, by... 

The presence of the monomere, volatile oxide hydrate [WO3 H2O resp. W02(OH)2] was proven by mass spectroscopy [3.28], and thermodynamic data are available for all of the above phase equilibria [3.10,3.29]. Based on these free energy data as well as on those of the solid oxides [3.23], the equilibrium partial pressure of the volatile compound can be calculated as a function of humidity. The result of such a calculation is shown in Fig. 3.2 for a temperature of 1000 °C [3.32]. In addition, the equilibrium pressures of the other volatile tungsten compoimds are also presented. From these calculations it is evident, that the oxide hydrate is by far the most volatile tungsten compound in the W-O-H system. 

In a thermodynamic analysis of reaction (9), structures of the gas-phase CHs(CH4) clusters have been predicted. The enthalpy and free-energy data are compatible with a three-centre-bond structure for CH5 with Q symmetry (19) ... 

The problems at lower temperatures happen because reactions are slow and systems are often far out of equilibrium. (The thermodynamically stable state for all the flora and fauna in any well-aerated lake should be H2O and CO2, for example). Reaction rates increase exponentially with temperature however, and at temperatures of 350°C or higher, even reactions involving molecular oxygen can equilibrate in several days time. This is fortunate, because free energy data are sparse for aqueous species at elevated temperatures and it becomes difficult to measure or calculate Eh much above 100°C. Instead, we can use other reactions between redox-sensitive species to calculate redox-related parameters at higher temperatures. This is the subject of the following section. 

REDUCTION OF OTHER METAL HALIDES. The reduction of several other metal halides with sodium dispersions proved very successful, although higher temperatures were required for the reduction of some of the metals. In most instances, reduction did not occur imtil a specific threshold, or trigger, temperature was reached no attempt was made to determine whether this phenomenon was due to a potential barrier which required a high activation energy to overcome, or to some other thermodynamic and/or kinetic properties of the system. Attempts to correlate free energy data with threshold temperatures were unsuccessful. 

If one uses tabulated free-energy data, this reaction has a 6G>, of —8.8 kcal/mol at 1200 K and thus would provide the additional thermodynamic driving force for wetting given by Equation 11.2. 

In the emerging theory of interfacial thermodynamics, the properties of the thermodynamic functions (PVT or free energy quantities) at densities corresponding to thermodynamically unstable states play a crucial role. For example, the Interfacial tension is computed from a formula calling for Helmholtz free energy data at every density (composition) between the compositions of the bulk phases in equilibrium. A particular implication of the theory is that Method (a) of Professor Prausnltz s classification is useful for interfacial calculations of vapor-liquid systems, but Method (b) is not. For those interested in the interfacial theory to which I refer, the following papers and references therein may be useful ... 

The most useful concept that biochemists have obtained from thermodynamics is that of free energy. By considering the free energy change one can tell whether a reaction may proceed spontaneously or whether it must be driven by other reactions. Further, one can calculate the amoimt of energy given off by a reaction or required by it, and this is a most important feature of many reactions. From free energy data one can easily calculate equilibrium constants and electromotive forces. 

Parks, G. S., Some Free Energy Data for Typical Hydrocarbons. . . , Symposium on Fundamental Chemical Thermodynamics, ACS Cincinnati Meeting, April, 1940. 

Thermodynamic data on H2, the mixed hydrogen—deuterium molecule [13983-20-5] HD, and D2, including values for entropy, enthalpy, free energy, and specific heat have been tabulated (16). Extensive PVT data are also presented in Reference 16 as are data on the equihbrium—temperature... 

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