Graphite , thermodynamic properties

C14-0087. Calculate the standard entropy change at 298 K of each of the following reactions, which are important in the chemistry of coal. Assume that coal has the same thermodynamic properties as graphite. 

However, since the standard state of carbon is the condensed state, carbon graphite, the only partial pressure it exerts is its vapor pressure (pw), a known thermodynamic property that is also a function of temperature. Thus, the preceding formation expression is written as... 

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. 

Diamond, graphite, and the fullerenes differ in their physical and chemical properties because of differences in the arrangement and bonding of the carbon atoms. Diamond is the densest (3.51 vs 2.22 and 1.72 g cm-3 for graphite and Cw, respectively), but graphite is more stable than diamond, by 2.9 kJ mol-1 at 300 K and 1 atm pressure it is considerably more stable than the fullerenes (see later). From the densities it follows that to transform graphite into diamond, pressure must be applied, and from the thermodynamic properties of the two allotropes it can be estimated that they would be in equilibrium at 300 K under a pressure of —15,000 atm. Of course, equilibrium is attained extremely slowly at this temperature, and this property allows the diamond structure to persist under ordinary conditions. 

Here, C symbolizes coke in its final condensed form, which is close to graphite in its thermodynamic properties. Therefore, assume figraph = const and, as a consequence, the quantity... 

Mounet N, Marzari N (2005) First-principles determination of the structural, vibrational and thermodynamic properties of diamond, graphite, and derivatives. Phys Rev B 71 205214... 

The performance of the program was verified by reproducing the results presented in [11] for the thermodynamic properties associated with the adsorption of hydrocarbons on the homogeneous basal surface of graphite. The BC potential was used in the calculations, with the coefficients listed in Table 1. For the reasons explained... 

Bomchil, G., Harris, N., Leslie, M., et al. (1979). Structure and dynamics of ammonia adsorbed on graphitized carbon black. Part 1. Adsorption isotherms and thermodynamic properties. J. Chem. Soc. Faraday Trans. 1, 75, 1535-41. 

Davydov et al. [46] used IGC to determine several adsorption thermodynamic properties (equilibrium constants and adsorption heats) for the adsorption of organic compounds on C q crystals, and compared them with those obtained for graphitized carbon black. The adsorption potential of the surface of fiillerene crystals was much lower than that of a carbon black surface. The dispersive interaction of organic molecules with C q is much weaker than with carbon black. The adsorption equilibrium constant for alkanes and aromatic compounds is therefore lower in the case of fullerenes. Aliphatic and aromatic alcohols as well as electron-donor compounds such as ketones, nitriles and amines were adsorbed more efficiently on the surface of fiillerene crystals. This was taken as proof that fiillerene molecules have electron-donor and electron-acceptor properties, which is in agreement with the results of Abraham et al. [44]... 

The direction of a chemical reaction or a phase transformation can be determined from the equilibrium thermodynamic properties of the phases involved. Note, though, that the speed of any transformation is not accessible from thermodynamics. Thermodynamics clearly states that diamond will transform into graphite at room temperature, but the rate of the reaction is insignificant. This chapter is concerned mainly with the kinetics of reactions, the speed at which they occur. Marrying this aspect with thermodynamics lies outside the scope of this chapter, but some introductory notes are given in Section S3.2. 

Cho] The Knudsen cell-mass spectrometer for measurements of the activity of Cu in Fe rich C-Cu-Fe alloys. Alloys were prepared by melting together Cu (99.99% pure) and eutectic composition C-Fe prepared from Fe (99.99% pure) and high purity C. Carbon saturation was ensured by holding at 1660° C for 15 min. Isotherm at 1600°C. The thermodynamic properties of liquid alloys saturated with graphite. 

Investigators must take care to read the foreword of the particular table they use so that they know which standard state has been employed, because most thermodynamic properties are calculated with respect to convenient scales. For example, the standard enthalpy of formation of a compound, AHf, is almost always quoted for a temperature of 298.15 K, and the enthalpy of formation of an element in its standard state must by definition be zero. It is therefore practically useful to look at a table and find, for an element, where a zero entry occurs. For example, the following values might appear C(graphite), A/ff = 0.000 kcalthmol" C(diamond), AHf = 0.4532 kcalthmol". It is clear that C(graphite) is the standard state adopted for carbon in the table under consideration. Entropy, on the other hand, is usually defined by taking as zero the entropy, at T = 0, of the crystalline form in which all the molecules are orientated regularly. Because many of the extant tables have used thermochemical calories, care will also have to be taken in the future to see that values taken from different tables are corrected to the same units. 

The table of thermodynamic properties [2] presents the standard enthalpies and Gibbs energies of formation for all materials as that of an ideal gas at298.15 K and 1 atm. That table does not include common solid minerals like NaCl, CaCOj, diamonds, or graphite. What difficulties might we encounter while attempting to insert these materials in that table ... 

Compiled from Daubert, T. E., R. R Danner, H. M. Sibiil, and C. C. Stebbins, DIPPR Data Compilation of Pure Compound Properties, Project 801 Sponsor Release, July, 1993, Design Institute for Physical Property Data, AlChE, New York, NY and from Thermodynamics Research Center, Selected Values of Properties of Hydrocarbons and Related Compounds, Thermodynamics Research Center Hydrocarbon Project, Texas A M University, College Station, Texas (extant 1994). The compounds are considered to be formed from the elements in their standard states at 298.15 K and 101,325 P. These include C (graphite) and S (rhombic). Enthalpy of combustion is the net value for the compound in its standard state at 298.15K and 101,325 Pa. 

Solid carbon exists as graphite, diamond, and other phases such as the fullerenes, which have structures related to that of graphite. Graphite is the thermodynamically most stable of these allotropes under ordinary conditions. In this section, we see how the properties of the different allotropes of carbon are related to differences in bonding. 

---