Hydrides solution enthalpy

Integral thermodynamic values are derived for the a-phase by Allen (1991). The pressure-composition data are unanticipated and raise questions about the nature of the solution. Enthalpies of formation for the solid and liquid solutions are comparable with changes of about — 84 J for each 0.01 mol of H dissolved in the metal. As evidenced by the absence of a resolvable temperature dependence for the InP versus H/Pu isotherms, the derived entropies of formatiom for the solutions are essentially zero. This result is inconsistent with a ASf value near — 130 + 20 J/mol K Hj observed for condensed hydride phases (see tables 4a, 7 and 8) and implies that the disorder of H atoms in solution with plutonium is comparable to that for gaseous H2. 

There is an excellent correlation between these data and the gas-phase data, in terms both of the stability order and the energy differences between carbocations. A plot of the gas-phase hydride affinity versus the ionization enthalpy gives a line of slope 1.63 with a correlation coefficient of 0.973. This result is in agreement with the expectation that the gas-phase stability would be somewhat more sensitive to structure than the solution-phase stability. The energy gap between tertiary and secondary ions is about 17kcal/mol in the gas phase and about 9.5 kcal/mole in the SO2CIF solution. 

Since the nature of the hydride chemical shifts, particularly in transition metal hydride complexes, is not simple [32], there is no reliable correlation between Sh and the enthalpy of dihydrogen bonding. Nevertheless, the chemical shifts of hydride resonances and their changes with temperature and the concentration of proton-donor components, for example, can be used to obtain the energy parameters for dihydrogen bonding in solution. As earlier, the enthalpy (A/f°) and entropy (AS°) values can be obtained on the basis of equilibrium constants determined at different temperatures. Let us demonstrate some examples of such determinations. 

Figure 5.23 Pressure composition isotherms for critical temperature 7. The construction of the hydrogen absorption in atypical metal (left). The van t Hoff plot is shown on the right. The slope of solid solution (a-phase), the hydride phase the line is equal to the enthalpy of formation (p-phase) and the region ofthe coexistence ofthe divided by the gas constant and the intercept with two phases. The coexistence region is the axis is equal to the entropy of formation...

The thermodynamic aspects of hydride formation from gaseous hydrogen are described by means of pressure-composition isotherms in equilibrium (AG = 0). While the solid solution and hydride phase coexist, the isotherms show a flat plateau, the length of which determines the amount of H2 stored. In the pure P-phase, the H2 pressure rises steeply vfith increase in concentration. The two-phase region ends in a critical point T, above which the transition from the a- to the P-phase is continuous. The equilibrium pressure peq as a function of temperature is related to the changes AH° and AS° of enthalpy and entropy ... 

Intermolecular hydride transfers between t-alkyl centres are observed under stable ion solution conditions. These have very low activation enthalpies (Dirda et al., 1979) and accurate rate data are scarce. The simplest reaction, transfer from isobutane to the t-butyl cation in sulphur dioxide, has been shown to be first order in each component, and to have Ea = 15.1 kJ mol 1 and AS = — lBJK moP1 (Brownstein and Bornais, 1971). Adaman-tane catalyses solution hydride transfer between acyclic tertiary centres such as t-butyl, and it is believed that this reflects higher efficiency of hydride transfers to and from bridgehead 1-adamantyl cation. With its non-planar geometry, the non-bonded interactions between alkyl substituents on donor and acceptor are likely to be less than those between two acyclic reactants. If locking of rotation about the C- H- C axis between the reactants does not... 

As seen from Fig. 6 at 305°C the values of partial molar enthalpy for different runs of hydrogen desorption have a large deviation probably because of a proximity to critical temperature, thus it is difficult to determine the phase boundaries. The critical temperature for the existence of ZrMn2 hydride phase estimated by different authors is 277-327 °C [8] and 318°C [15], The plot of the AHdes. -C could be divided into three parts the hydrogen a-solid solution region (0pi transition (0.6p2 transition (1.0[Pg.353]

The enthalpy of formation of Th" is derived from measurements of the enthalpy of solution in HCl solutions. Unless some fluorosilicate ion is present, this reaction leaves variable amounts of a black residue, probably an oxide hydride containing chloride and hydroxide ions, ThO(Cl,OH)H [1962KAT/KAP], [1973ACK/RAU3] (see Appendix A). 

The partial enthalpies of solution of hydrogen in sohd thorium hydrides with... 

---