Hydrides solution entropy

At the beginning stage of dehydrogenation, the substrate organic hydride is adsorbed onto the catalyst surface from the liquid phase directly and easily. Catalytic reaction processes will succeed it, until the surface sites are filled with the adsorbed reactant and products. Once product desorption starts to form and grow a bubble, product readsorption becomes unfavorable due to the increment of translational entropy of the product molecule in the bubble, if compared with that in the solution, shifting the adsorption equilibrium for the product and suppressing its effect of rate retardation. 

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 ... 

Many arguments are against this mechanism. As shown by Bar-Eli and Klein (1), the hydride ion from LiH is not a catalyst for this reaction. In the similar case of water, Schindewolf (16) has shown that the hydride ion does not appear in a solution of potassium hydroxide in water. The kinetic isotopic effect observed is also in contradiction to a dissociative mechanism (1). The largely negative value of the entropy of activation is an argument for a highly organized activated complex. 

The observation of intermolecular hydridic-protonic bonds in complexes in solution is more challenging because often these energies are of the same magnitude as the 5 kcal/mol energy penalty for working against entropy at room temperature (eq. 26, AS<0). Proton transfer and subsequent loss of Hj to form M-X is a common reaction that must be avoided to observe eq 26. 

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. 

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