Graphite, free energy

For a free energy of fonnation, the preferred standard state of the element should be the thennodynamically stable (lowest chemical potential) fonn of it e.g. at room temperature, graphite for carbon, the orthorhombic crystal for sulfiir. 

The standard free energy changes for the process graphite and... 

Hammer, G.E. and Drzal, L.T. (1980). Graphite fiber surface analysis by X-ray photoelectron spectroscopy and polar/dispersive free energy analysis. Application of Surf. Sci.. 4, 340-355. 

The phenomenon of the orientation dependent surface free energy of metals is theoretically and experimentally well established [1-4]. An example from the experimental work of Heyraud and Metois for Pb is shown in fig. 1 [5]. Here the relative anisotropy of y(0) is derived from the ECS of Pb particles on graphite measured by scanning electron... 

Yu, Y.-F., Zettlemoyer, A. C., and Chessick, J. J., Free Energies, Heats and Entropies of Wetting of Graphite, presented before the Colloid Division, American Chemical Society, New York, Sept. 1957. 

Figure 2. Equilibrium concentrations in mole fractions of selected compounds at 500°K. and 1 atm. with composition of 40% oxygen, the indicated percentage of carbon, and the rest hydrogen. To this basic composition is added an amount of nitrogen equal to the amount of carbon. The nitrogen remains primarily as N2 but produces significant quantities of some interesting compounds. The free energy of carbon in the system equals that of graphite at the composition indicated by the arrow. At this point solid carbon would be precipitated if it could be formed there is no inflection of the curves at this point. The asphalt threshold is shown as a sharp inflection, sharpest of all for the aromatic and related heterocyclic compounds. If an atmosphere such as this were to condense, there would be about 1 molecule of glycine per droplet of condensate (6).

Values of AG°f at 25°C for some common substances are listed in Table 17.3, and additional values are given in Appendix B. Note that AG°f for an element in its most stable form at 25°C is defined to be zero. Thus, solid graphite has AG°f = 0 kj/mol, but diamond, a less stable form of solid carbon at 25°C, has AG°f = 2.9kJ/mol. As with standard enthalpies of formation, AH°f, a zero value of AG°f for elements in their most stable form establishes a thermochemical "sea level," or reference point, with respect to which the standard free energies of other substances are measured. We can t measure the absolute value of a substance s free energy (as we can the entropy), but that s not a problem because we are interested only in free-energy differences between reactants and products. 

The (p,T) equilibrium line for the graphite-diamond transition is obtained from equation (15.14) by expressing AH° and AS° as a function of temperature and the integral term as a function ofpressure and temperature. The free energy change A Gpr is then set equal to zero (the equilibrium condition) and the resulting expression... 

Figure 9.1 Difference of CH4 conversion and carbon formation in CH4 = C (graphite) + 2H2 and CH4 = C (carbon on nickel) + 2H2. The free energy of carbon on a nickel catalyst was derived from Ref. [12].

Based on these observations, the decision was taken to use the thermodynamic properties of graphite in the thermodynamic analysis in C02 reforming, because this reaction has applicable conversions only at temperatures above 973 K. At this point, the difference in free energy between graphite and carbon on catalysts becomes so small that it has a negligible effect on the thermodynamic analysis. 

The solubility of carbon in austenite. If there is true equilibrium with graphite, the solubility is lower because the point of tangency on the austenite free energy curve is lower than for metastable equilibrium with cementite. 

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