Strong electrolytes standard states

For electrolytes where dissociation is extensive, but not complete, the classification is somewhat arbitrary, and the electrolyte can be considered to be either strong or weak. Thermodynamics does not prevent us from treating an electrolyte either way, but we must be careful to designate our assignment because the choice of standard state is different for a strong electrolyte and a weak electrolyte. Assuming that an electrolyte is weak requires that we have some nonthermodynamic procedure for distinguishing clearly between the dissociated and undissociated species. For example, Raman spectroscopy... 

This equation is the basis of the strong electrolyte standard state. Substitution into equation (6.122) gives ArG° = 0, which when substituted into equation (6.123) gives... 

Comparing this equation with equation (6.125) demonstrates that K = I for the dissociation of an electrolyte when the strong electrolyte standard state is chosen. 

The definition of the strong electrolyte standard state gives a2 = a+a so that... 

With the choice of a pure substance standard state for the solid, and a strong electrolyte standard state for the dissolved AgCl, we get... 

This last example provides a demonstration of the flexibility inherent in the choice of standard states. A strong electrolyte standard state is chosen for NaA(aq) and NaCl(aq) so that... 

We should keep in mind that a strong electrolyte standard state could have been chosen for HOAc, in which case,... 

When activity data for a strong electrolyte such as HCl are plotted against 1712/m°), as illustrated in Figure 19.1, the initial slope is equal to zero. Thus, an extrapolation to the standard state yields a value of the activity in the standard state equal to zero, which is contrary to the definition of activity in Equations (16.1) and (16.3). Therefore, it is clear that the procedure for determining standard states must be modified for electrolytes. 

Optical and nuclear magnetic resonance methods apphcable to moderately strong electrolytes have been made increasingly precise (14). By these methods, it has proved feasible to determine concentrations of the undissociated species and hence of the dissociation constants. Thus, for HNO3 in aqueous solution (14) at 25°C, K is 24. However, in dehning this equilibrium constant, we have changed the standard state for aqueous nitric acid, and the activity of the undissociated species is given by the equation... 

SOLUTION and MIXTURE - There is some confusion between these two terms in geological literature. According to the I.U.P. A.C. (International Union for Pure and Applied Chemistry), the term mixture must be adopted whenever all components are treated in the same manner , whereas solution is reserved for cases in which it is necessary to distinguish a solute from a solvent. This distinction in terminology will be more evident after the introduction of the concept of standard state. It is nevertheless already evident that we cannot treat an aqueous solution of NaCl as a mixture, because the solute (NaCl) in its stable (crystalline) state has a completely different aggregation state from that of the solvent (H2O) and, because NaCl is a strong electrolyte (see section 8.2), we cannot even imagine pure aqueous NaCl. 

In the previous chapter, we described the thermodynamic properties of nonelectrolyte solutions. In this chapter, we focus on electrolytes as solutes. Electrolytes behave quite differently in solution than do nonelectrolytes. In Chapter 11, we described the strong electrolyte standard state and summarized relationships between the activity of the solute ai, the mean activity coefficient 7 , and the molality m in Table 11.3. 

It is important to remember that reactions such as the above do not invalidate our choice of a strong electrolyte standard state. We repeat that thermodynamics does not tell us what is happening in solution on a molecular (ionic) level. The observation we make is that j deviates more for mixtures of 2 2 electrolytes than for solutions of 1 1, 1 2, and 1 3 electrolytes, and we attribute this deviation to ion association. But we can handle these deviations perfectly well in our treatment without assuming ion association. We can stay with our strong electrolyte standard state and accept as a fact of life that, at a given low concentration, 7 deviates more from unity than for other mixtures. 

In a similar manner, standard states can be chosen for electrolytes that take into account molecular association. We call this the weak electrolyte standard state, and some method must be employed to determine the extent of association. As an example, we usually treat nitric acid HNO3 as a strong electrolyte so that in solution... 

Association, even in NaCl solutions, contributes to the decrease in 7 (based on the strong electrolyte standard state) with increasing temperature shown in Figures 18.5 and 18.6. 

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. 

We consider only aqueous solutions here, but the methods used are applicable to any solvent system. The standard Gibbs energy of formation of a strong electrolyte dissolved in water is obtained according to Equation (11.28). In such solutions the ions are considered as the species and we are concerned with the thermodynamic functions of the ions rather than the component itself. We express the chemical potential of the electrolyte, considered to be MVtAv, in its standard state as... 

For dissolved species the standard state is defined as an ideal solution with a concentration of 1 M (this is obtained in practice by extrapolating the dilute solution behavior up to this concentration). A special comment is in order on the standard enthalpies of formation of ions. When a strong electrolyte dissolves in water, both positive and negative ions form it is impossible to produce one without the other. It is therefore also impossible to measure the enthalpy change of formation of ions of only one charge. Only the sum of the enthalpies of formation of the positive and negative ions is accessible to calorimetric experiments. Therefore, chemists have agreed that AH° of H (aq) is set to zero. 

Similarly, the standard state of the strong electrolyte as a whole may be chosen so that its chemical potential in that state is equal to the sum of the chemical potentials of the ions in their respective standard states hence,... 

Relationships analogous to those given above may be derived in an exactb similar manner for the activities referred to mole fractions or molarities. As seen in 37c, the activities for the various standard states, based on the ideal dilute solution, can be related to one another by equation (37.7). The result is, however, applicable to a single molecular species the corresponding relationships between the mean ionic activity coefficients of a strong electrolyte, assumed to be completely ionized, are found to be... 

Ac Choice of Standard States for Strong Electrolyte Solutes... 

Solubility of a Pure Component Strong Electrolyte. The calculation of the solubility of a pure component solid in solution requires that the mean ionic activity coefficient be known along with a thermodynamic solubility product (a solubility product based on activity). Thermodynamic solubility products can be calculated from standard state Gibbs free energy of formation data. If, for example, we wished to calculate the solubility of KCI in water at 25 °C,... 

In this case it proved possible to obtain 6° values for reaction (2I) involving both the undissociated HCI(DMF) and the completely dissociated HCltoMF) standard states. Even though HCI(DMF) is a weak electrolyte = 2.7 x lO"" at 25°c), Petrov and Unanskii (l2) were able to establish a strong electrolyte standard state by... 

The equilibrium constant for reaction (89), based on nole-fraction-composltion, strong-electrolyte standard states fOr HC1[I 0] and DGl[DoO]. is 0-715 at 298.I5 K. Because this value of K for reaction C89) refers to strong electrolytes,we can rewrite equation (89) as... 

To be precise, the standard-state conditions are those in which the activities (ideal concentrations) of solutes and gases equal I.This is equivalent to 1 M only for ideal solutions. Because of the strong attractions between ions, substantial deviations from ideal conditions exist in electrolyte solutions, except when very dilute. We will ignore these deviations here. 

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