Molar enthalpies of solute formation

Molar integral enthalpies of solution and dilution are conveniently expressed in terms of molar enthalpies of formation. The molar enthalpy of formation of a solute in solution is the enthalpy change per amount of solute for a process at constant T and p in which the solute, in a solution of a given molality, is formed from its constituent elements in their reference states. The molar enthalpy of formation of solute B in solution of molality will be denoted by Afi/(B, hib). 

As explained in Sec. 11.3.2, the formation reaction of a solute in solution does not include the formation of the solvent from its elements. For example, the formation reaction for NaOH in an aqueous solution that has 50 moles of water for each mole of NaOH is 

Thermodynamics and Chemistry, second e6H on,version 3 2011 by Howard DeVoe. Latest version www.chein.umd.edu/thennobook 

Consider a solution process at constant T and p in which an amount b of pure solute (soUd, liquid, or gas) is mixed with an amount n of pure solvent, resulting in solution of molality ms. We may equate the enthalpy change of this process to the sum of the enthalpy changes for the following two hypothetical steps  

An amount b of the pure solute decomposes to the constituent elements in their reference states. This is the reverse of the formation reaction of the pure solute. 

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From tabulated values of molar enthalpies of formation, we can calculate molar integral enthalpies of solution with Eq. 11.4.11 and molar integral enthalpies of dilution with Eq. 11.4.12. Conversely, calorimetric measurements of these molar integral enthalpies can be combined with the value of AfH(B ) to establish the values of molar enthalpies of solute formation in solutions of various molalities. 

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