Calculation of AH from Relative Partial Molar Enthalpies

3b Calculation of AH from Relative Partial Molar Enthalpies 

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°. 

The results given in Table 11.1 can be used to calculate AH for various mixing processes. 

The enthalpy change for this process, in which we mix pure liquids, is known as an integral enthalpy of solution. 

With the choice of standard states used in Table 7.2, Hf = H°. Making this substitution and adding and subtracting n2H°2 gives 

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