Relative Apparent Molar Enthalpy

We cannot obtain values for H since we cannot obtain absolute values for H or H. To overcome this problem, we define a quantity d>L, which we call the relative apparent molar enthalpy, by the equation... 

Thus, the relative apparent molar enthalpy is the difference between the apparent molar enthalpy in the mixture and the apparent molar enthalpy in the standard state. 

The relative apparent molar enthalpy, 4>L, is usually obtained from enthalpy of dilution measurements in which the moles of solute are held constant and additional solvent is added to dilute the starting solution. The process can be represented as... 

To show how we can calculate relative apparent molar enthalpies from enthalpies of dilution, consider as an example, a process in which we start with a HC1 solution of molality m = 18.50 mol-kg-1 and dilute it to a concentration of m = 11.10 mol-kg-1. The initial solution contains 3 moles of H20 per mole of HC1 (A = 3) while the final solution has A = 5. The enthalpy change for that process is measured. Then the m = 11.10 mol-kg-1 solution is diluted to one with m = 4.63 mol-kg-1 and its enthalpy of dilution measured. The series continues as illustrated below,... 

Figure 7.9 (a) Enthalpies of dilution starting with a solution containing 3 moles fEO per 1 mole HC1 (m = 18.50 or w1/2 = 4.30) and (b), relative apparent molar enthalpies (4>L) and relative partial molar enthalpies (L and L >) for the resulting mixtures. 

Randall, M. 1. 264. 265 Raoult s law 268-73 and phase equilbria 419. 423 standard state 289 Rectilinear diameters, law of 393 Redlich-Kister equation 220, 362 Redlick-K wong equation 256 relative apparent molar enthalpy of solutions 356-7... 

The enthalpies of dilution of BE were required to calculate the enthalpies of transfer (19). From these Integral enthalpies of dilution AHj.jj the relative apparent molar enthalpies were derived following the technique of Fortier et al (21). The values of AHjjj corresponding to the Initial and final molalities are given In Table 1 along with the parametric equation for. ... 

The relative apparent molar enthalpy L is given bymm... 

Figure 18.6 Thermal properties of aqueous NaCl solutions as a function of temperature, pressure and concentration, (a) activity coefficient (b) osmotic coefficient (c) relative apparent molar enthalpy and (d) apparent molar heat capacity. The effect of pressure is shown as alternating grey and white isobaric surfaces of 7 , , L, and Cp at p = 0.1 or saturation, 20, 30, 40, 50, 70, and 100 MPa, that increase with increasing p in (a), (b), and (d), and decrease with increasing P in (c).

Lange and Miederer determined the relative apparent molar enthalpy, of NiS04 and demonstrated that ... 

Fig. 4. Relative apparent molar enthalpy of KI in propanol (25 °C) from heat of dilation measurements, (a) measured curve (b) limiting law for explanation see text...

Because in the west of China some salt lake brines contain abundant boron and lithium, in which solute-solvent and solute-solute interactions are complex, studies on the ihermochemical properties for the systems related with the brines are essential to understand the effects of temperature on excess free energies and solubility, and to build a thermodynamic model that can be applied for prediction of the properties. Yin et al. [43] measured the enthalpies of dilution for aqueous Li2B407 solutions from 0.0212 to 2.1530 mol/kg at 298.15 K. The relative apparent molar enthalpies and relative partial molar enthalpies of the solvent and solute were also calculated, and the thermodynamic properties of the complex aqueous solutions were represented by a modified Pitzer ion-interaction model. 

FIGURE 8 Relative apparent molar enthalpy potassium... 

This series of papers contains an extensive array of correlated data on aqueous electrolyte solutions, much of It having been calculated using the system of equations given In paper I In this series. The contents of these papers have been summarized by Pitzer In a chapter in the book edited by Pytkowicz (see Item [123]). The data Include activity and osmotic coefficients, relative apparent molar enthalpies and heat capacities, excess Gibbs energies, entropies, heat capacities, volumes, and some equilibrium constants and enthalpies. Systems of Interest Include both binary solutions and multi-component mixtures. While most of the data pertain to 25 °C, the papers on sodium chloride, calcium chloride, and sodium carbonate cover the data at the temperatures for which experiments have been performed. Also see Items [48], [104], and [124]. 

This is a bibliography of sources of experimental data that can be used to calculate either relative apparent molar enthalpies or apparent molar heat capacities of aqueous electrolyte solutions. The compounds are arranged according to the standard thermochemical order of arrangement. There are approximately 300 references to the source literature. 

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