Thermodynamic data exchange

The separation of niobium from tantalum tluough the gaseous chlorides is carried out at higher temperature, about 900 K, and it is therefore to be expected, as is the case, that the thermodynamic data will provide a useful guide. These metals form a number of chlorides, mainly the ui- tetra- and pentachlorides. These latter are much more volatile than the tetrahalides, and the exchange reaction at 900 K... 

The pre.sent account follows a Journey in this arena from solution calorimetric studies dealing with nucleophilic carbene ligands in an organometallic system to the use of these thermodynamic data in predicting the feasibility of exchange reactions to applications in homogeneous catalysis. 

Thermodynamic data for alkali metal cation exchange at 25 °C (in kJ equiv ) Camp Berteau and Wyoming Bentonite... 

Exchange of complex cations. Complexation of transition metal cations with uncharged ligands such as with amines and with amino acids results in a selectivity enhancement compared to the selectivity of the aqueous metal cation (27, 65-72). Fig. 3 shows an example for the Cu(ethylenediamine) adsorption in montmorillonites of different charge density. Standard thermodynamic data for other cases are given in table IV. In all cases the free ligand concentration in equilibrium solution was... 

TABLE IV. A. Thermodynamic data for the exchange of transition metal ions and transition metal ion... 

In general, the (scarce) thermodynamic data for exchanges involving complexes leads us to conclude that the selectivity enhancement upon complexing is enthalpy driven and may be ascribed to enhanced charge dependent (primarily coulombic) interactions with the surface as compared with the aqueous ions. 

Tin K and P in kbar) however, its application to other thermobaric conditions is not so appropriate. The most satisfactory formulations for the exchange reaction 5.216 are those of Slavinskiy (1976), Ganguly (1979), Saxena (1979), and Dahl (1980), which are based on a large body of thermodynamic data, petrologic studies, and natural observations. 

Saxena S. K. (1972). Retrival of thermodynamic data from a study of intercrystalline and intracrystalline ion-exchange equilibrium. Amer. Mineral, 57 1782-1800. 

Approximate equilibrium constants for the gas-phase reaction at 300° T have been determined for the trimethylborane-diborane exchange from equilibrium experiments and from thermodynamic data (164,261) with the computed constants listed in parentheses in Eqs. (68)-(71). 

As some intermediate thermodynamic data are missing or not well known (enthalpy of mixing of CuCl and CuCl2 in HC1/H20 mixtures for example) and some heat exchanges or separation are not linear (for instance, HC1/H20 mixture has an azeotrope which cannot be crossed), we decided to proceed to a global exergy analysis instead of a flow sheet analysis. 

Table 22 Thermodynamic data for the exchange of bridging and terminal methyl groups in Ln/Al heterobimetallic complexes ...

EXCHANGE MASTER SPECIES defines the interrelation between the name of an exchanger and its master species. Based on this, EXCHANGE SPECIES describes a half-reaction and requires a selectivity coefficient for each exchanger species. In contrast to stability constants or dissociation constants, these selectivity coefficients are dependent on the respective solid phase with the specific features of their inner and outer surfaces (see also chapter 1.1.4.2). Therefore, within thermodynamically data collections they are only to be seen as placeholders that have to be changed according to site specific exchange constants. 

Modeling hydrogeochemical processes requires a detailed and accurate water analysis, as well as thermodynamic and kinetic data as input. Thermodynamic data, such as complex formation constants and solubility products, are often provided as data sets within the respective programs. However, the description of surface-controlled reactions (sorption, cation exchange, surface complexation) and kinetically controlled reactions requires additional input data. 

Some indications in the form of thermodynamic data are expected from a series of hydride-exchange reactions, e.g., with 1,3-dithiole (84) or tropilidene (85). In view of the large volume of numerical... 

Therefore, when studying interfacial reactions on rocks and soils, it must always be determined what the mechanism of the interfacial reaction is, and what kind of processes take place. Also, it must identify the dominant processes responsible for surface excess concentration. If this is not done, and the resultant process is evaluated without knowing it in conventional ways, incorrect thermodynamic data are obtained. The concepts of adsorption, ion exchange, and surface precipitation have to be clearly differentiated, as done previously. When the character of the process can be neglected, only surface accumulation is considered, and we can speak about sorption, including all of the surface processes. In this case, only aphenomenological description can be given, and no thermodynamics can be applied. 

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