Standard enthalpy of solution

When ammonium nitrate, NH jNOj, dissolves in water, it absorbs heat. Consequently, its standard enthalpy of solution must be positive. This means that the entropy change caused by ammonium nitrate going from solid to solution must increase for the process to proceed spontaneously. This is exactly what one would expect based on the concept of entropy as a measure of randomness or disorder. 

The qualitative trend predicted by this equation is that, when the heat of solution is negative (the dissolution is exothermic, i.e., heat is evolved, the enthalpy of solvation is more negative than the lattice enthalpy is positive), the solubility diminishes with increasing temperatures. The opposite trend is observed for endothermic dissolution. An analogue of Eq. (2.58), with H replacing G, and the same tables [12] can be used to obtain the required standard enthalpies of solution of ionic solutes. No general analogues to Eqs. (2.53)-(2.55) are known as yet. 

Standard heat capacities of transfer can be derived from the temperature dependence of standard enthalpies of solution (8). While this technique can give general trends in the transfer functions from water to mixed solvents (9), it is not always sufficiently precise to detect the differences between similar cosolvents, and the technique is rather laborious. Direct measurements of the difference between heat capacities per unit volume of a solution and of the solvent a — gq can be obtained with a flow microcalorimeter (10) to 7 X 10 5 JK 1 cm-3 on samples of the order of 10 cm3. A commercial version of this instrument (Picker dynamic flow calorimeter, Techneurop Inc.) has a sensitivity improved by a factor oi about two. 

Table I. Standard Enthalpies of Solution in kj mol 1 of Tetra -n-butylammonium Bromide in Mixtures of Af,iV-dimethylformamide and Water at 5°, 25°, and 55°C as a Function of the Mole Fraction of Water, XHp...

Table II. Standard Enthalpies of Solution in kj mol-1 Tetra-n-propylammonium Bromide, of Rubidium Chloride, and...

The second step shown here combines the terms AHZ (for expanding the solvent) and AH3 (for solvent-solute interactions) and is called the enthalpy (heat) of hydration (AH d). This term represents the enthalpy change associated with the dispersal of a gaseous solute in water. Thus the standard enthalpy of solution for dissolving sodium chloride is the sum of AH and AH yd-... 

AH, is the standard enthalpy of solution of the crystalline salt to unit activity. 

Titanium(il).—Standard enthalpies of solution have been determined for the molten systems of TiCl3-TiCl2-NaCl and TiCl3-TiCl2. The system VCl3-TiCl2-KCI has been studied by thermoanalytical methods. ... 

Ti has been detected by e.p.r. spectroscopy in electrochemically reduced solutions of TiCU, and the standard enthalpies of solution have been determined for the systems TiClj-NaChTiClj-TiClj-NaCl, and TiCla-TiClj. The system VCI3-TiClj-KCl has been studied by thermoanalytical methods. TiCla reacts with alkaline K2Hgl4 to form a compound with the stoicheiometry K2[Hgl4],4TiCl3,18H20 which liberates Hgl2 when heated. ... 

The authors also measured the enthalpies of solution of samples of NiS04 6.000H20 and NiSO4-6.940H2O in water such that the final solutions had essentially the same compositions (0.1 M in NiS04). Thus, in that medium, the enthalpy of the dehydration reaction was (7.68 + 0.10) kJ-mof. The authors assumed that this enthalpy difference was unaffected by the final medium, and that it was equal to the difference at infinite dilution in water. This value is accepted in the present review, and used in the calculation of the standard enthalpy of solution of the heptahydrate. 

If the solubility and activity coeflOicient data exist over a range of temperatures, one can obtain the standard enthalpy of solution by dividing both sides of eqn. 2.3.4 by the temperature and differentiating with respect to temperature, at constant pressure... 

Comparison between thermodynamic values is generally made with standard state functions. To obtain the standard enthalpy of solution, A//g°in, it is necessary to extrapolate directly measured enthalpies of solution at finite concentrations to infinite dilution some form of the Debye-Huckel theory is generally used in this extrapolation (see sect. 2.5.2). 

In the vast majority of cases, dissolving a solute in a solvent produces measurable heat change. At constant pressure, the heat change is equal to the enthalpy change. The enthalpy (or heat) of solution (A// iJ is the heat absorbed when a certain amount of solute dissolves in a certain amount of solvent. When this quantity is measured at standard pressure (1 bar), it is called the standard enthalpy of solution (AWsoin)-The quantity AWsoin represents the difference between the enthalpy of the final solution and the enthalpies of its original components (that is, the solute and solvent) before they are mixed. Thus,... 

Fig. 7.6. Standard enthalpy of solution zl//s°(II) (in kcal/mole) of methane as a function of mole fraction of ethanol at two temperatures.

The difference between the standard energy and enthalpy of solution for process II (see Section 7.2) is quite small. As an example, for argon in water at 25°C, the standard enthalpy of solution is —2000 cal/mole. The value of P A at 1 atm... 

Cf compounds have been acquired recently (Fuger et al. 1993). A linear relationship exists between the heat of formation of these perovskite-type oxide systems and the Goldschmidt tolerance factor, which permits estimations for the enthalpies of formation of a number of homologous compounds of this type. From data of such Cf compounds, the enthalpy of formation of CfOj was estimated to be - 854-1- lOkJ mol in good agreement with a value of — 858 kJ mol derived from interrelationships between the molar volumes of dioxides and their standard enthalpies of solution (Morss 1986). It has not been feasible to carry out such direct experimental measurements with CfOj and these indirect approaches have provided the only data of this type. Data for other actinide oxides can similarly be obtained by these indirect approaches. 

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