Standard states conventional

In Equation 50 the chemical potential of non-electrolyte A depends on the usual choice of standard-state conventions described above, and the chemical potentials of both H2(g) and H+(sod are taken to be zero (this defines e.s.s., the electrolyte standard state). By setting the standard-state free energy of the solvated proton equal to zero, this standard-state convention... 

Enthalpies of reaction can also be calculated from individual enthalpies of formation (or heats of formation), AHf, for the reactants and products. Because the temperature, pressure, and state of the substance will cause these enthalpies to vary, it is common to use a standard state convention. For gases, the standard state is 1 atm pressure. For a substance in an aqueous solution, the standard state is 1 molar concentration. And for a pure substance (compound or element), the standard state is the most stable form at 1 atm pressure and 25°C. A degree symbol to the right of the H indicates a standard state, AH°. The standard enthalpy of formation of a substance (AHf) is the change in enthalpy when 1 mol of the substance is formed from its elements when all substances are in their standard states. These values are then tabulated and can be used in determining A//°rxn. 

A point of occasional confusion arises with respect to units. In Eq. (15.22), all portions are unitless except for k T/h, which has units of sec , entirely consistent with the units expected for a unimolecular rate constant. In Eq. (15.23), the same is true with respect to the r.h.s., but a bimolecular rate constant has units of concentration" sec", which seems paradoxical. The point is diat, as with any thermodynamic quantity, one must pay close attention to standard-state conventions. Recall that die magnitude of die iranslational partition function depends on specification of a standard-state volume (or pressure, under ideal gas conditions). Thus, a more complete way to write Eq. (15.23) is... 

Thus AG0 indicates a change for a reaction in which [H+] is standardised at 10 7 mol dm-3 to distinguish it from the case of AG° under normal standard state convention (when [H+] = 1 mol dm-3). 

Standard-State conventions for chemical elements and dissolved solutes are summarized in Table si. 1. Note that the Standard states for gases and for solutes are hypothetical, ideal states and not actual states. For gases, this choice of Standard State is useful because the ideal gas represents a good limiting approximation to the real behavior of gases and possesses equations of state that are mathematically tractable in applications. For solutes, the choice of a hypothetical Standard State is of value because the alternative choice, consisting simply of the pure solute at unit mole fraction, is not very relevant to a solution component whose concentration must always remain small. Moreover, by... 

Table sl.l Summary of Standard-State Conventions for Chemical Elements, Pure Compounds and Dissolved Solutes at T° = 298.15 K19... 

An excellent discussion of Standard-State conventions is given by D. D. Wagman et al., The NBS tables of chemical thermodynamic properties, J. Phys. Chem. Ref. Data 11 Supplement No. 2 (1982). 

Table 2.3 summarizes the essential relationships for pressure effects on chemical equilibrium for the variable-pressure standard-state convention. Note, that these relationships can apply to any consistent choice of standard part ial molar volumes, for example, one for which an ionic medium such as seawater is adopted as the solute reference state. For detailed discussion of applications to seawater see, for example, Millero (1969) and Whitfield (1975). A compie-... 

The change in free energy of a reaction in the standard state (conventionally, all reactants and products at IM) is related to the equilibrium constant for the reaction by the following relation ... 

Here, subscripts 1 and 2 refer to the components, superscripts a and 0 refer to the equilibrated liquid phases, and the y are activity coefficients. In these etpiations, for each component, the Raoult s Law standard-state convention is used for both phases. 

In the unsymmetric standard state convention, where the solvent is referenced to its pure state (Raoult s law reference state) and the solutes to the infinite dilution state (Henry s law reference state), the standard partial molar volume is equal to the pressure derivative of the chemical potential at... 

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