Thermochemistry state functions

In considering the thermochemistry of solid and liquid explosives, it is usually adequate, for practical purposes, to treat the state functions AH and A U as approximately the same. Consequently, heats, or enthalpy terms, tend to be used for both constant pressure and constant volume conditions. 

In Section 2.1, we remarked that classical thermodynamics does not offer us a means of determining absolute values of thermodynamic state functions. Fortunately, first-principles (FP), or ab initio, methods based on the density-functional theory (DFT) provide a way of calculating thermodynamic properties at 0 K, where one can normally neglect zero-point vibrations. At finite temperatures, vibrational contributions must be added to the zero-kelvin DFT results. To understand how ab initio thermodynamics (not to be confused with the term computational thermochemistry used in Section 2.1) is possible, we first need to discuss the statistical mechanical interpretation of absolute internal energy, so that we can relate it to concepts from ab initio methods. 

The importance of this new state function, enthalpy, will become apparent when we study thermochemistry, the branch of thermodynamics concerning the heat changes associated with chemical reactions. For the moment let us note that when we see... 

In the case of isobaric processes (p = constant) in closed systems the heat exchanged with the surroundings is equal to the change in enthalpy of the system. For this reason the enthalpy is an important state function in thermochemistry. 

The negative value of the enthalpy change for Equation 8 when n = 0 is defined (7) as the electron affinity (EA) of the oxidized species when the oxidized and reduced species are in their ground rotational, vibrational and electronic states (0 K). At any temperature for any value of n (0, positive, or negative), the thermodynamic state functions for l uation 8 are given by aX (X = G, H, or S), and the thermochemistry of electron attachment can be defined in the ion convention ("stationary electron convention") (7). The relationship between EA and aG is given by Equation 9. A similar relationship applies for adiabatic ionization energies. 

Sections 5.3 and 5.4 When a gas is produced or consumed in a chemical reaction cKcurring at constant pressure, the system may perform pressure-volume work against the prevailing pressure. For this reason, we define a new state function called enthalpy, H, vdiich is important in thermochemistry. In systems where only pressure-volume work due to gases is involved, the change in the enthalpy of a system, AH, equals the heat gained or lost by the system at constant pressure. For an endothermic process, AH > 0 for an exothermic process, AH < 0. 

The main purpose of these final comments is to show a few general trends in the thermochemistry of Group 14 organometallic compounds, helped by some (hopefully) reliable values. And one of the trends is revealed by a rather usual plot1,2, in which the mean bond dissociation enthalpies of the species MR4 (i.e. one-fourth of the enthalpy required to break all the M—R bonds) are represented as a function of the enthalpy of formation of M in the gaseous state. As observed in Figure 4, for R = H and Me, D(M—H) and D(M—Me) increase with the enthalpy of formation (or sublimation) of M. It is noted, on the other hand, that the differences D(M—H) — D(M—Me) vary from 47.7 kJmol-1... 

The dynamics of hydrogen abstraction reactions promoted by F, O, OH and OD with monogermane have been studied as a function of the vibrational and rotational state by infrared chemiluminescence While this technique provides enormous insight on the energy disposal in a reaction, it also led to a value of 326 4 kJmoP for the H3Ge—H bond dissociation energy at 0 K. This value is somewhat lower than the value of 346 10 kJmoP obtained from a kinetic study of reaction 44 and its thermochemistry . ... 

Zhao, Y Truhlar, D. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements two new functionals and systematic testing of four M06-class functionals and 12 other functionals, Theor. Chem. Acc. 2008,120, 215-241. 

Hess s law states that the heat changes of successive processes are additive if they are carried out at the same temperature and at constant pressure. This property follows from the fact that the enthalpy is a function of state and thus is a function only of the initial and final states of a system. Hess s law is extremely useful in thermochemistry since it permits the calculation of the heat change of a reaction difficult to perform from a series of reactions which are more easily carried out. 

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