Virtual Substance thermodynamic properties

Virtual Substance simulations can be run using either periodic boundary conditions or fixed walls. In a fixed wall calculation, the simulation box has physical walls. The resulting system is a droplet , albeit a rectangular droplet, and as a result, the thermodynamic properties differ from those of the bulk substance. One can create a bulk substance by using periodic boundary conditions where the simulation box interacts with copies of itself repeated in three dimensions. (5) This approximates the extended nature of a bulk material, making it appear infinite, and results in better accuracy in the calculation of thermodynamic properties. 

Equation 18.42 is certainly complicated. However, we note two things. First, virtually all of the information required to evaluate equation 18.42 for a molecule is available experimentally, mostly using various spectroscopic techniques. Second, because equation 18.42 is independent of the identity of the molecule, it is a relatively simple task to write a computer program to evaluate Qsys for a given system. As we will see in the next section, once we have an expression for Qjyj, we will be able to derive expressions for various thermodynamic properties. These expressions can also be evaluated by calculator or computer program. In fact, an important use of these equations of statistical thermodynamics is not to verify that they yield numbers that agree with experiment, but to predict the thermodynamic properties at different conditions or for new substances whose thermodynamic properties have not been measured. 

Virtually all chemical reactions in soils are studied as isothermal, isobaric processes. It is for this reason that the measurement of the chemical potentials of soil components involves the prior designation of a set of Standard States that are characterized by selected values of T and P and specific conditions on the phases of matter. Unlike the situation for T and P, however, there is no strictly Ihermodynamic method for determining absolute values of the chemical potential of a substance. The reason for this is that p represents an intrinsic chemical property that, by its very conception, cannot be identified with a universal scale, such as the Kelvin scale for T, which exists regardless of the chemical nature of a substance having the property. Moreover, p cannot usefully be accorded a reference value of zero in the complete absence of a substance, as is the applied pressure, because there is no thermodynamic method for measuring p by virtue of the creation of matter. 

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