Activity coefficient neutral molecule

As an example, take the molecule aminoazobenzene, one of the solutes listed in Table 39. When colorimetric measurements were made at room temperature on very dilute aqueous solutions of HC1, containing a trace of this substance, it was found that neutral molecules and (BH)+ ions were present in equal numbers when the concentration of the HCl was 0.0016 molal.1 At this low concentration the activity coefficient of the HCl is very near unity, and we may use (216) to find how far the vacant proton level provided by the aminoazobenzene molecule in aque-... 

The approximation of taking concentrations instead of activities was justified by the authors on the basis of the small degree of dissociation owing to the low e, and by the assumption that activity coefficients of neutral molecules and ion pairs are not significantly different from unity in the low range of concentrations studied. From eqns. 4.54 and 4.55 one obtains for the overall dissociation constant of HX... 

Activity coefficients in the aqueous phase, yiw, of neutral molecules are set equal to one because of the zero charge, and under the assumption that the activity coefficient of the infinitely diluted solution equals the actual activity coefficient. The activity coefficients of the charged species can be approximated with the Davies equation ... 

Equations (7), (8), (9a) thus permit the basicity constants to be determined if the concentrations and activity coefficients of the ions and neutral molecules present in the solution are known. 

Figure 8,11 Activity coefficients for neutral gaseous molecules dissolved in aqueous solutions of various ionic strengths. All values are for T = 25 °C and P = 1 bar, except for hydrogen (T = 15 P = I bar). Reprinted from Garrels and Christ (1965), with kind permission from Jones and Bartlett Publishers Inc., copyright 1990.

It is relatively easier for a small neutral molecule (such as Fie or H2) to move through a liquid or solid structure, than for a large molecule (such as Xe). Hence, the activation energy for diffusion of small molecules is small and the diffusion coefficient is large. 

Neutral molecules, such as benzene and acetic acid, have no ionic atmosphere because they have no charge. To a good approximation, their activity coefficients are unity when the ionic strength is less than 0.1 M. In this book, we set 7 = 1 for neutral molecules. That is, the activity of a neutral molecule will be assumed to be equal to its concentration. 

At low ionic strength, y I for neutral compounds. At high ionic strength, most neutral molecules can be salted out of aqueous solution. That is, when a high concentration (typically > 1 M) of a salt such as NaCl is added to an aqueous solution, neutral molecules usually become less soluble. Does the activity coefficient, -yclher, increase or decrease at high ionic strength ... 

Considering just acid-base chemistry, not ion pairing and not activity coefficients, find the pH and concentrations of species in 1.00 L of solution containing 0.040 mol benzene-1,2,3-tricarboxyl ic acid (H3A), 0.030 mol imidazole (a neutral molecule, HB), and 0.035 mol NaOH. 

Adler and Albert have reported the partition coefficients for the parent azaindoles. The oleyl alcohol-water solubility ratio for the neutral molecules at 25° are 53.2, 36.8,17.3, and 13.1 for 7-, 6-, 5-, and 4-azaindole, respectively. For comparison, indole, which is not basic, is more lipid-soluble, with a ratio of 85.7. The order for the azaindoles does not parallel their basic strength. The per cent of ionization at pH 7. 4 was also calculated and for the above order is 0.15,78.02, 87.87, and 27.75%, respectively. The effects of these properties on the biological activity of the azaindoles is discussed also (see below). 

It is not certain that the theoretical arguments, which led to the introduction of the term C t, are completely satisfactory, but it seems to be established that the experimental data require a term of this type. The aggregation of solvent molecules in the vicinity of an ion is the factor responsible for the so-called salting-out effect, namely, the decrease in solubility of neutral substances frequently observed in the presence of salts the constant C is consequently called the salting-out constant. The activity coefficient of a non-electrolyte, as measured by its solubility in the presence of electrolytes, is often given by an expression of the form log / = CV this is the result to which equation (62) would reduce for the activity of a non-electrolyte, i.e., when z+ and z arc zero, in a salt solution of ionic strength... 

The estimation of transfer activity coefficients was reviewed by Popovych. One method for measurement of transfer activity coefficients for electrolytes or neutral molecules is by measurement of solubility. When the solubility is low, the effects of contamination by traces of water can be profound. Furthermore, reliable solubility values even in water are difficult to obtain. Nevertheless, if saturated solutions of a substance in water and another solvent are considered, and if each solution can be shown to be in equilibrium with the same solid, the value of y, is given by the ratio of the solubility products ( sp)water/( sp)soivent electrolyte producing n ions. For... 

Neutral molecules have activity coefficients essentially equal to unity in solutions of less than 10 mM ionic strength. At higher salt concentrations, most neutral molecules are increasingly salted out of water that is, the activity coefficient > 1, so that a, /c, < 1 for molecules in higher ionic strength solutions. In our discussion of dilute aqueous acids and bases, we will assume ideal behavior of the neutral species. The importance of salting out of dissolved CO2 will be reflected in considering dissolved carbonic species in seawater (Chapter 4). 

In the region in which the Debye-Hiickel limiting law is applicable, the activity coefficient of the molecules of undissociated MA is probably very close to unity, as may be inferred from the known variation of the activity coefficient of a neutral molecule in the presence of added electrolytes (cf. 39m). It follows, therefore, upon taking logarithms of equation (41.17) that... 

For a symmetrical electrolyte, the ion pair is taken to behave as though it were a neutral species. It is, however, a dipole and as such will interact with the other ions. Consideration of this would lead to an activity coefficient for the ion pair which is not unity. There is also the question as to whether the ions of the ion pair are in contact, or are separated by one or more water molecules. Bjerrum himself recognised these limitations to his treatment the distinction between free and associated ions is not a chemical one, but only a mathematical device . 

Nonelectrolytes—dissolved gases, organic molecules, neutral ion pairs, and undissociated weak acids and bases—are also nonideal soiutes in water and are-common constituents of soil solutions. Their activities also vary nonlinearly with concentration, particularly at high concentrations. The activity coefficients of nonelectrolytes at low concentrations are approximated by... 

The determination of the activity coefficients of species that exist dominantly as neutral molecules, such as Si02(ag), H2S(ag) and C02(ag), is much simpler. In these cases it is usually possible to establish a two-phase equilibrium between the substance in its pure state (solid or gaseous) and the substance in its aqueous or dissolved state. This leads to a simple and rigorous determination of the activity coefficient in solutions of varying composition. 

It is assumed that the activity coefficient of HOCl is unity, since the ionic strength at which activity coefficients for neutral molecules deviate significantly from unity is of the order of O.I M. 

Medium activity coefficients, cf. Section 5.5, can be used to discuss these effects. Fig. 15 provides a summary of the changes in solvation energy of ions and neutral molecules of various types in solvents which are representative for the solvent classes of Table I. The energy scale, RTln j y, with methanol as the reference solvent is taken from Ref. The non-measurable medium activity coefficient of the activated complex can be estimated from similar stable mol niles or ions. 

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