Reference and standard states

Thermodynamics deals with processes and reactions and is rarely concerned with the absolute values of the internal energy or enthalpy of a system, for example, only with the changes in these quantities. Hence the energy changes must be well defined. It is often convenient to choose a reference state as an arbitrary zero. Often the reference state of a condensed element/compound is chosen to be at a pressure of 1 bar and in the most stable polymorph of that element/compound at the 

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The problem discussed here is common to all partial molar quantities. It is for this and similar reasons, which we have already indicated, that reference and standard states are defined (see Chapter 8). 

In the following sections expressions are developed for the chemical potential of the components or species in solution in terms of the composition —first for an ideal solution and then for real solutions—with special emphasis on reference and standard states. 

Liquids and solids reference and standard states Equation (8.92)... 

In the previous sections concerning reference and standard states we have developed expressions for the thermodynamic functions in terms of the components of the solution. The equations derived and the definitions of the reference and standard states for components are the same in terms of species when reactions take place in the system so that other species, in addition to the components, are present. Experimental studies of such systems and the thermodynamic treatment of the data in terms of the components yield the values of the excess thermodynamic quantities as functions of the temperature, pressure, and composition variables. However, no information is obtained concerning the equilibrium constants for the chemical reactions, and no correlations of the observed quantities with theoretical concepts are possible. Such information can be obtained and correlations made when the thermodynamic functions are expressed in terms of the species actually present or assumed to be present. The methods that are used are discussed in Chapter 11. Here, general relations concerning the expressions for the thermodynamic functions in terms of species and certain problems concerning the reference states are discussed. 

There are four undetermined quantities Apfx, (Apf ), pf, and (pf) and two equations. We must, therefore, define two of the four quantities, which in turn determines the other two quantities and the relationship between them. We can define the reference states for the component and the species. The difference between the standard chemical potential of the component and that of the species is then expressed in terms of the mole fractions in the reference state. The problem is the determination of this difference. The different species may be known from our knowledge of the chemical system, or they may be assumed. However, a definite decision must be made concerning the species, and all calculations must be carried out based upon this decision. Several examples concerning reference and standard states are discussed here and in the following sections. 

The problems of the reference and standard states are slightly different when we use the infinitely dilute solution-of all solutes as the reference states. When we take the reference state of both the component and the species as the infinitely dilute solution, Equation (8.168) becomes... 

Solutions of electrolytes form a class of thermodynamic systems for which the concept of species is all-important. In this section we discuss the problems of reference and standard states for strong electrolytes as solutes dissolved in some solvent. 

Concentration Scales For other, more practical, concentration scales for mixtures, similar considerations arise regarding reference and standard states. For aqueous solutions, both the mole fraction scale and the molal scale are used for thermodynamic interpretations. The solvent water is described on the... 

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