Activity of an Uncharged Species

The activity at of uncharged species i (i.e., a substance) is defined by the relation 

The quantity at is sometimes called the relative activity of i, because it depends on the chemical potential relative to a standard chemical potential. An important application of the activity concept is the definition of equilibrium constants (Sec. 11.8.1). 

For convenience in later applications, we specify that the value of at is the same in phases that have the same temperature, pressure, and composition but are at different elevations in a gravitational field, or are at different electric potentials. Section 9.8 10.1 will describe a modification of the defining equation /li = 11° + RT nat for a system with phases of different elevations, and Sec. 10.1 will describe the modification needed for a charged species. 

The standard states of different kinds of mixture components have the same definitions as those for reference states (Table 9.3), with the additional stipulation in each case that the pressure is equal to the standard pressure p°. 

When component i is in its standard state, its chemical potential is the standard chemical potential p,°. It is important to note from Eq. 9.7.2 that when jXi equals /r°, the logarithm of at is zero and the activity in the standard state is therefore unity. 

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Experimental measurements show that up to an ionic strength of 0.1 mol/L, the activity value of an uncharged species does not differ from its concentration value by an error higher than 1%. In the range of ionic strengths 0.1-5 mol/L, the following... 

All quantities in Eq. (12.6) are measurable The concentrations can be determined by titration, and the combination of chemical potentials in the exponent is the standard Gibbs energy of transfer of the salt, which is measurable, just like the mean ionic activity coefficients, because they refer to an uncharged species. In contrast, the difference in the inner potential is not measurable, and neither are the individual ionic chemical potentials and activity coefficients that appear on the right-hand side of Eq. (12.3). 

Common molecules have an even number of electrons. Stable radicals are rare exceptions, such as NO. In classical chemistry, we most often meet active species that are ions with an even number of electrons, or radicals, an uncharged species with an odd number of electrons. In mass spectrometry, we observe ions with an even number of electrons, but we also often meet radical ions, a species uncommon in solution chemistry and having specific characteristics. 

Because H4Si04 is an uncharged species, we can assume that its activity coefficient is near 1, and because the activity of quartz and water are 1 we can set... 

Although not a solvent, urea is an uncharged species that has long been known to have profound effect upon systems hydrophobically bound. If such bonds are operative in generating new asymmetric or electronic centers giving rise to Pfeiffer rotations, it is reasonable that urea would alter their rotations just as it alters the nature of and rotation of certain biologically optically active systems (e.g., proteins hydrophobically bound by solvent). Table 3 gives data for two Pfeiffer active systems in which BCS" and strychnine are the optically active species and [Zn(phen)3] the resolvable complex." ... 

In electrolyte solutions, nonideality of the system is much more pronounced than in solutions with uncharged species. This can be seen in particular from the fact that electrolyte solutions start to depart from an ideal state at lower concentrations. Hence, activities are always used in the thermodynamic equations for these solutions. It is in isolated instances only, when these equations are combined with other equations involving the number of ions per unit volume (e.g., equations for the balance of charges), that concentrations must be used and some error thus is introduced. 

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