Relative partial molal enthalpy

The mean ionic activity coefficients of hydrobromic acid at round molalities (calculated by means of Equation 2) are summarized in Tables XI, XII, and XIII for x = 10, 30, and 50 mass percent monoglyme. Values of —logio 7 at round molalities from 0.005 to 0.1 mol-kg-1 were obtained by interpolating a least squares fit to a power series in m which was derived by means of a computer. These values at 298.15° K are compared in Figure 2 with those for hydrochloric acid in the same mixed solvent (I) and that for hydrobromic acid in water (21). The relative partial molal enthalpy (H2 — Hj>) can be calculated from the change in the activity coefficient with temperature, but we have used instead the following equations ... 

Table XIV. Relative Partial Molal Enthalpy (//2 — H°) and Relative Partial Molal Heat Capacity (Cp — Cp°) of HBr in 50 Mass Percent Aqueous Monoglyme and in Water at 298.15°K (cal = 4.184/)...

To calculate the heats of mixing it is necessary to know the relative partial molal enthalpies of the salt and the water in the sea salt solutions. This quantity for the water, Llf is arrived at as follows ... 

Relative partial molal enthalpies of both components... 

Activity coefficients at temperatures other than that at which they were determined may be obtained using the relative partial molal enthalpy, and heat capacity, /j Differentiation with respect to temperature of the equation (j2 — 2 = In a2 for the partial molal free energy of the... 

R. K. Agarwal, D. K. Sahu, and B. Nayak, Ind. J. Chem., 9,978 (1971). Relative partial molal enthalpy and relative partial molal heat capacity of HCl from EMF measurements. 

Relative partial molar enthalpies can be used to calculate AH for various processes involving the mixing of solute, solvent, and solution. For example, Table 7.2 gives values for L and L2 for aqueous sulfuric acid solutions7 as a function of molality at 298.15 K. Also tabulated is A, the ratio of moles H2O to moles H2S(V We note from the table that L — L2 — 0 in the infinitely dilute solution. Thus, a Raoult s law standard state has been chosen for H20 and a Henry s law standard state is used for H2SO4. The value L2 = 95,281 Tmol-1 is the extrapolated relative partial molar enthalpy of pure H2SO4. It is the value for 77f- 77°. 

Robinson and Baker,18 and Wai and Yates 19 below 71 wt% they can be converted to other temperatures using partial molal enthalpies calculated from the relative enthalpies given by Bidinosti and Biermann.47 Similar values for HC1 in Tables 7 and 8 are obtained from Randall and Young48 and Akerlof and Teare,49 quoted by Bunnett,50 and can be converted to other temperatures using coefficients supplied by Liu and Gren.51 The author uses computer programs, written in Microsoft Basic for the Macintosh, that compute these quantities and others. 

Relative partial molar enthalpies can also be obtained from measurements of enthalpies of dilution. Humphrey et al. [4] have used enthalpy of dilution measurements to calculate relative partial molar enthalpies in aqueous solutions of amino acids. Their data for AH n of aqueous solutions of serine are shown in Table 18.2, where mj is the initial molality of the solution, rrif is the molality after addition of a small amount of solvent, and is equal to the measured AH divided by ti2, which is the number of moles of solute in the solutions. 

It should be mentioned that since the hydrogen solubility of hydrogen in the metal phase is relatively low, the contribution from the enthalpies (and entropies) of solution pf hydrogen in the metal phase is neglected. For completeness, however, eqs. (26.5) and (26.6) should include a term containing the partial molal enthalpy (and entropy) of hydrogen in the metal phase. 

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