Hydrobromic acid activity coefficients

The standard emf E° of the cell was determined by means of an extrapolation technique involving a function of the measured emf E (which was measured experimentally), taken to the limit of zero ionic strength /. A linear function of I was observed when the Debye-Hiickel equation (in its extended form) (12) was introduced for the activity coefficient of hydrobromic acid over the experimental range of molalities m. With this type of mathematical treatment, the adjustable parameter became a0, the ion-size parameter, and a slope factor / . This procedure is essentially the same as that used in our earlier determinations (7,10) although no corrections of E° for ion association were taken into account (e = 49.5 at 298.15°K). 

The mean ionic activity coefficients of hydrobromic acid at round molalities (calculated by means of Equation 2) are summarized in Tables XI, XII, and XIII for x = 10, 30, and 50 mass percent monoglyme. Values of —logio 7 at round molalities from 0.005 to 0.1 mol-kg-1 were obtained by interpolating a least squares fit to a power series in m which was derived by means of a computer. These values at 298.15° K are compared in Figure 2 with those for hydrochloric acid in the same mixed solvent (I) and that for hydrobromic acid in water (21). The relative partial molal enthalpy (H2 — Hj>) can be calculated from the change in the activity coefficient with temperature, but we have used instead the following equations ... 

The activity coefficients of hydrobromic acid in the mixed solvents are lower, as expected, than those in water (20). Hydrobromic acid completely dissociates in the mixed solvents (e = 49.5 at 298.15° K for the 50 mass percent monoglyme) under investigation. Figure 2 clearly indicates that at a particular molality, the stoichiometric activity coefficient of hydrochloric acid is lower than that of hydrobromic acid in the same mixed solvent, and the heat capacity changes (Cp — Cp) also suggest that there are no ion-pair formations. 

Table XI. Activity Coefficients 7+ of Hydrobromic Acid in 10 Mass...

The first evidence for the involvement of these nitrosyl halides in nitrosation came from kinetic studies of diazotization. The rate equation for diazotization in aqueous hydrochloric and hydrobromic acids includes the kinetic term given in (15) where S stands for the free amine (Schmid, 1937 Schmid and Muhr, 1937). It was later recognized that this can be interpreted as the attack of the nitrosyl halides on the free amine (Hammett, 1940) and the kinetic term was rewritten as (16).7 The corresponding rate coefficients and activation energies calculated from the equilibrium concentration of the nitrosyl halides are given in Tables 3 and 4 together with later results (Williams, 1977). 

To overcome this difficulty the Debye-Hiickel theory was expanded for symmetrical valence-type electrolytes, and the complex functions in the expansion (1/2 X3 — 2Y3) and (1/2 X5 — 4Y5) calculated and published (26). The result of these expansions is to add a term Eext to the equation for the activity coefficient given in Equation 2. For symmetric valence-type electrolytes such as hydrobromic acid this term is... 

Table IV contains the calculated values of —log y , where y is the activity coefficient of hydrobromic acid for the solvent systems investigated here.

Using the value of obtained above, determine the activity coefficients of hydrobromic acid at the various molalities. 

With more concentrated solutions of hydrobromic acid the following results were obtained for the e.m.f. of the cell referred to in the preceding exercise [Keston and Donelson, /. Am. Chem. Soc. 58, 989 (1936)]. Using the value of E obtained there, determine the activity coefficients of hydrobromic acid at the various molalities. 

The activity coefficients of uncharged species were assumed to be unity in the evaluation of the equilibrium constants. The activity coefficients of all monovalent ions were set equal and taken as the mean activity coefficient of hydrochloric or hydrobromic acid, respectively, at the concentration of the hydrogen ions in the equilibrium... 

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