Computed standard state enthalpies

Our computed standard state enthalpies and free energies at 298 K and 2000 K are listed in Tables 2-4. The energy minima at 0 K upon which these are based can be found in our earlier papers, as can also the optimized geometries of the boron- [32, 35] and aluminum-... 

The computed standard state enthalpies of these surface reactions are 0.47 eV and -0.09 eV. If we compare these surface reactions to the gas-phase species, we can see the effect of the catalyst on the energetics. The reactions are less favorable on the surface than they are in the gas phase. This means that the surface binds the reactants more strongly than it does the products. We also observe that the surface reaction that produces nitrogen is more favorable than the one producing ammonia, contrary to the gas-phase trend. This illustrates the ability of a catalyst to influence selectivity. 

The experimental data and the calculations involved in the determination of a reaction enthalpy by isoperibol flame combustion calorimetry are in many aspects similar to those described for bomb combustion calorimetry (see section 7.1) It is necessary to obtain the adiabatic temperature rise, A Tad, from a temperaturetime curve such as that in figure 7.2, to determine the energy equivalent of the calorimeter in an separate experiment and to compute the enthalpy of the isothermal calorimetric process, AI/icp, by an analogous scheme to that used in the case of equations 7.17-7.19 and A /ibp. The corrections to the standard state are, however, much less important because the pressure inside the burner vessel is very close to 0.1 MPa. 

Steele et al. [7. Phys. Chem. Soc.. 96, 4731 (1992)] used bomb calorimetry to compute the standard enthalpy of combustion of solid buckminsterfullerene (Cao) at 298.15 K to be 26 033 kJ/mol. Calculate the standard state Aff of transition from graphite to buckminsterfullerene and its standard enthalpy of formation. 

Compute the standard-state Gibbs energy change, enthalpy change, and entropy change for this- reaction for the temperature range in the table. 

In route IB, also shown in Figure 6.1, the required experimental data include mixture volumes, enthalpies, and some amoxmt of phase-equilibrium data. From those data, values for excess properties are extracted and fit to a model for g. However, before excess properties can be found, we must define the ideal solution that is, we must choose the standard state for each component. With the excess-property model plus values for ideal-solution properties, we can then compute property differences for the substance of interest. 

The internal energy U of a. substance is the total energy residing in the substance owing to the motion and relative position of the constituent atoms and molecules. Absolute values of internal energy are not known, but numerical values relative to some arbitrarily defined standard state for the substance can be computed. The sum of the internal energy and the product of pressure and volume of the substance, when both quantities are expressed in the same units, is defined as the enthalpy of the substance. 

The energy equivalent of the calorimeter, e, and the enthalpy of the isothermal calorimetric process, A//icp, were derived from equations 8.2 and 8.4, respectively. The standard enthalpy of reaction 8.5 was computed as Ar//°(8.5) = AZ/icp/n, where n is the amount of substance of Mo(ri5-C5H5)2(C2H4) used in the experiment. The data in table 8.1 lead to a mean value Ar//°(8.5) = — 186.0 2.1 kJ mol-1, where the uncertainty is twice the standard deviation of the mean (section 2.6). This value was used to calculate the enthalpy of reaction (8.6), where all reactants and products are in their standard reference states, at 298.15 K, from... 

The transition state approach leads in a natural way to the Butler-Volmer equation, but is relatively weak in its predictive properties regarding the exchange current, ( 0, which is proportional to the frequency factor kr(., i and to exp(—AG J). The latter is quite closely related to the enthalpy and standard entropy of formation of the adsorbed reduction product or intermediate, and this is one main reason for the very intense modern efforts to develop predictive theoretical tools for the ab initio computation of adsorption energies at... 

FIG. 2-5 Pressure-enthalpy diagram for dry air. Properties computed with the NIST REFPROP Database, Version 7.0 (Lemmon, E. W., McLinden, M. O., and Huber, M. L., 2002, NIST Standard Reference Database 23, NIST Reference Fluid Thermodynamic and Transport Properties—REFPROP, Version 7.0, Standard Reference Data Program, National Institute of Standards and Technology), based on the equation of state of Lemmon, E. W., Jacobsen, R. T., Penoncello, S. G., and Friend, D. G., Thermodynamic Properties of Air and Mixtures of Nitrogen, Argon, and Oxygen from 60 to 2000 K at Pressures to 2000 MPa, /, Phys. Chem. Ref. Data 29 331-385, 2000. 

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