Evaluation of Standard Molar Quantities

Suppose the solution is prepared from water and the aeid, and H from the dissociation of H2O is negligible compared to H+ from the acid dissociation. We may then write m+ = m- = aniB, where a is the degree of dissociation and me is the overall molality of the acid. The molality of the undissociated acid is wha = (1 — and the dissociation 

From this equation, we see that a change in the ionic strength that decreases y when T, p, and mB are held constant must increase the degree of dissociation (FYob. 12.17). 

Some of the most useful experimentally-derived data for thermodynamic calculations are values of standard molar reaction enthalpies, standard molar reaction Gibbs energies, and standard molar reaction entropies. The values of these quantities for a given reaction are related, as we know (Eq. 11.8.21), by 

The standard molar quantities appearing in Eqs. 12.10.1 and 12.10.2 can be evaluated through a variety of experimental techniques. Reaction calorimetry can be used to evaluate AfH° for a reaction (Sec. 11.5). Calorimetric measurements of heat capacity and phase-transition enthalpies can be used to obtain the value of Sf for a solid or liquid (Sec. 6.2.1). For a gas, spectroscopic measurements can be used to evaluate S° (Sec. 6.2.2). Evaluation of a thermodynanuc equilibrium constant and its temperature derivative, for any of the kinds of equilibria discussed in this chapter (vapor pressure, solubility, chemical reaction, etc.), can provide values of ArG° and AfH° through the relations AfG° = —RTln K and ArH° = -Rd aK/d /T). 

In addition to these methods, measurements of cell potentials are useful for a reaction that can be carried out reversibly in a galvanic cell. Section 14.3.3 will describe how the standard cell potential and its temperature derivative allow AtH°, ArG°, and ArS° to be evaluated for such a reaction. 

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CHAPTER 12 EQUILIBRIUM CONDITIONS IN MULTICOMPONENT SYSTEMS 12.10 EVALUATION OF STANDARD MOLAR QUANTITIES... 

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