Thermodynamic data carbonate species

References (20, 22, 23, 24, 29, and 74) comprise the series of Technical Notes 270 from the Chemical Thermodynamics Data Center at the National Bureau of Standards. These give selected values of enthalpies and Gibbs energies of formation and of entropies and heat capacities of pure compounds and of aqueous species in their standard states at 25 °C. They include all inorganic compounds of one and two carbon atoms per molecule. 

Thermodynamic data on hydrocarbons show that at temperatures above 1300°C. ethylene, acetylene, and carbon are more stable than methane or other saturated hydrocarbons however, although data of this kind will indicate the species likely to be formed at high temperatures, the amounts of unsaturated hydrocarbons and carbon produced will depend on the rates of the reactions involved and on the temperature cycle used. 

Following the work of Garrels and Thompson (1962), many chemists have made calculations of the inorganic speciation of fresh and marine waters among these are Stumm and Brauner (1975), Turner et al. (1981), and, more recently, Byrne et al. (1988). The latter workers paid particular attention to the influence of temperature and pH and provided a useful compilation of relevant thermodynamic data (including enthalpies, from which temperature dependences can be estimated). Palmer and van Eldik (1983) have presented a detailed review of metal complexes with carbonic species. 

Table 1C. Summary of Revised Thermodynamic Data. m. Carbonate Species... 

The dissolved carbonate system can be described by a number of parameters. Most geochemical models require the pH, which indicates the distribution among the carbonate species, and either the total dissolved carbonate content, the total alkalinity, or the concentrations of bicarbonate and carbonate to indicate the amount of carbonate present. From these data, models calculate the concentrations and thermodynamic activities of the various dissolved carbonate species. The total dissolved carbonate content is the sum of these concentrations. The activity of the carbonate ion is used in calculating the saturation indices of carbonate minerals. 

Both quahtative and qnantitative data ate available. Only the qnantitative data can be nsed to obtain thermodynamic parameters, but the quahtative information can in some cases be nsed to corroborate the quantitative conclusions, e.g. concerning the stoichiometiy of complexes and the mode of coordination of the carbonate ligand. (Qnantitative determinations of the stoichiometiy and equilibrium constants are described in the following section. These studies are complicated by the formation of sparingly solnble sohd phases and the formation of ternary Th(lV)-hydroxide-carbonate species. This review will therefore begin with a short summary of the advantages and disadvantages of the varions methods nsed to deduce the stoichiometiy and eqnilibrinm constants in the temaiy Th(lV)-carbonate-water system. 

Tables 1.11 to 1.13 give the thermodynamic data of elements in the standard state and of hydrocarbon and oxygenated, molecular and radical species, containing at most 2 carbon atoms.

One fundamental aspect of carbocation chemistry is the large dependence of the energy of the cation on the substituents attached to the positively charged carbon atom. This dependence is most evident in the gas phase. Table 5.4 shows thermodynamic data for selected carbocations. The hydride ion affinity, HIA(R ), is defined as the negative of the AH for the attachment of a hydride ion to the cation in the gas phase. That is, the greater the H1A(R ), the more endothermic is the removal of a hydride ion from an alkane. We expect the trends to be the same for heterolytic dissociation of alkyl halides or other species that produce carbocations. 

In general, the hydrolytic behaviour of the actinides is very complicated and, in spite of much effort, is still poorly understood with respect to both thermodynamic and kinetic factors (Waters et al. 1991). The presence of mixed hydroxy-carbonate-actinide species has been shown in solutions from which carbon dioxide has not been excluded and so there is a strong possibility that some equilibrium hydrolysis data has been contaminated by the presence of carbonate (Bernkopf and Kim 1984, Kim 1986). For certain of the actinides, particularly uranium, it is probable that ternary hydroxy-carbonate species exist in vivo (Bulman 1980b). 

The situation, when a sparingly soluble soUd is placed in water, is much more complicated in principle, because of the successive reactions taking place spontaneously in the aqueous phase. This fact is obvious for carbonates, sulfates, where the dissolved amoimt cannot be calculated directly from the characteristic thermodynamic data (i.e., from the solubility products) because additional reactions take place simultaneously in the liquid phase depending on its composition, which influence the activity of dissolved species and, therefore, the amoimt of dissolved material. For example, a common carbonate, calcite (CaC03), with intermediate solubdity dissolves as bicarbonate [Ca(HC03)2] at favorable conditions with respect to pH and partial pressure of CO2 or the dissolved amount of a moderately soluble salt, gypsiun... 

Many simple systems that could be expected to form ideal Hquid mixtures are reasonably predicted by extending pure-species adsorption equiUbrium data to a multicomponent equation. The potential theory has been extended to binary mixtures of several hydrocarbons on activated carbon by assuming an ideal mixture (99) and to hydrocarbons on activated carbon and carbon molecular sieves, and to O2 and N2 on 5A and lOX zeoHtes (100). Mixture isotherms predicted by lAST agree with experimental data for methane + ethane and for ethylene + CO2 on activated carbon, and for CO + O2 and for propane + propylene on siUca gel (36). A statistical thermodynamic model has been successfully appHed to equiUbrium isotherms of several nonpolar species on 5A zeoHte, to predict multicomponent sorption equiUbria from the Henry constants for the pure components (26). A set of equations that incorporate surface heterogeneity into the lAST model provides a means for predicting multicomponent equiUbria, but the agreement is only good up to 50% surface saturation (9). 

Thermodynamic calculations for reactions forming carbon disulfide from the elements are compHcated by the existence of several known molecular species of sulfur vapor (23,24). Thermochemical data have been reported (12). Although carbon disulfide is thermodynamically unstable at room temperature, the equiHbtium constant of formation increases with temperature and reaches a maximum corresponding to 91% conversion to carbon disulfide at about 700°C. Carbon disulfide decomposes extremely slowly at room temperature in the absence of oxidizing agents. 

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