Heats of ionization

A wide range of caibocation stability data has been obtained by measuring the heat of ionization of a series of chlorides and cafbinols in nonnucleophilic solvents in the presence of Lewis acids. Some representative data are given in Table 5.4 These data include the diarylmediyl and triarylmethyl systems for which pX R+ data are available (Table 5.1) and give some basis for comparison of the stabilities of secondary and tertiary alkyl carbocations with those of the more stable aryl-substituted ions. 

In Eq. (6-35), A/Z is the molar heat of ionization of the buffer acid at the conditions (temperature, solvent composition) of the kinetic studies. It happens that for many commonly used acidic buffers this quantity is small. Hamed and Owen give A//2 = —0.09 kcal/mol for acetic acid at 25°C, for example. The very important buffer of dihydrogen phosphate-monohydrogen phosphate is controlled by pK2 of phosphoric acid at 25°C its heat of ionization is —0.82 kcal/mol. 

The quantities AH and AH require consideration. These are molar heats of ionization at the conditions of the kinetic measurements. The thermodynamic heat of ionization of water in pure water, A//°, is a function of temperature Hamed and Owen - pp give for this quantity... 

These same dependencies will, in general, apply to the heat of ionization of the buffer acid, AH. Thermodynamic quantities, namely, AH°, have been reported for some buffer substances, and it is found that A//° is temperature dependent. Bates and Hetzer studied the temperature dependence of for the important buffer tris(hydroxymethyl)aminomethane (TRIS), finding... 

To compare the present results with those of DeCarvalho and Choppin, our results in 1 M acid must be corrected for the heat and entropy of HSOiJ ionization. Zebroski et al. (12) determined Ka = 0.084, while Zielen (9) gives the heat of ionization as AHa = +23.2 kj/m. The calculated entropy is ASa - +98 j/m-°K. 

Additional evidence for the SnI mechanism, in particular, for the intermediacy of carbocations, is that solvolysis rates of alkyl chlorides in ethanol parallel carbocation stabilities as determined by heats of ionization mea.sured in superacid solutions (p. 219). It is important to note that some solvolysis reactions proceed by an Sn2 mechanism." ... 

In the case of non-HBD solvents, such as DMSO, the measured pK values are absolute (that is, free from ion pairing) and can be directly compared with gas-phase acidities6 in addition, knowledge of the heats of ionization in DMSO7 allows the evaluation of a possible entropy effect when the two phases are compared. The mechanism of proton transfer between oxygen and nitrogen acids and bases in aqueous solution has been reviewed8. 

Two issues associated with thermodynamic standard states bear some further attention. The first is associated with the enthalpy of ions. Ion heats of formation may be defined based on the heats of ionization of neutral molecules (or electron attachments thereto). For example, one might consider a reaction like... 

The electro-affinity of lithium is smaller than that of any of the other alkali metals, and it exhibits a greater tendency than the other alkali metals to form complex salts—e.g. the solubility of ammonia in water is raised by the addition of a lithium salt, which presumably unites with the ammonia the solubility curves of the lithium salts in water usually show more breaks than the corresponding salts of the other alkali metals owing to the formation of hydrates. Potassium, rubidium, and caesium seem to have a smaller and smaller tendency to form complex salts as the at. wt. of the element increases otherwise expressed, the electro-affinity, or the ionization tendency of the alkali metals increases as the at. wt. increases. This is illustrated by the heats of ionization. According to W. Ostwald,27 the heat of ionization per gram-atom iB... 

The existence of a measurable heat of dilution is taken to indicate the presence of complexes which are formed or decomposed by dilution. The reversible molar heats of dilution L, calculated from the relation d log (p/p0)j(dt=L/RT2> where p and Pq respectively denote the v.p. of the soln. and of the pure solvent at the same temp., do not agree closely with the observed results. Since L is negative, pjp0 decreases with a rise of temp., and this agrees with an increased degree of ionization. A. A. Noyes (vide infra) found decreased ionization with rise of temp. Hence, the observed effects are taken to show that complexes existing in soln. are decomposed by dilution. H. Lunden estimates that the heat of ionization of... 

According to A. A. Noyes (1907), the degree of ionization decreases with a rise of temp. G. N. Lewis and G. A. Linhart have compared the degrees of ionization calculated by thermochemical data and by the ratio A/A. S. Arrhenius gives for the heat Of ionization at 35°, with W-soln., LiCl, —399 cals. NaCl, —454 cals. and KC1, —362 cals. 

Heat of ionization in aqueous solution HC2H302 (oo) = H+ (oo) + C2H302- (oo). 

IO- (aq.). This value is computed from that for HIO (aq.), assuming the heat of ionization to be the same as that for HCIO (aq.). 

A given soln. has a definite temp, for the maximum conductivity, and above that temp., the conductivity decreases. The increasing mobility of the ions with rise of temp, may be compensated by the decreasing ionization. A. A. Noyes found for a 3x0-98 mol. soln., at 12°, 76°, and 93°, respectively, the mol. conductivities 50-2, 71-0, and 69-4 with the temp, coeff. 0-60, 0, and —0-12 respectively. The negative temp, coeff. corresponds with the negative heat of ionization. 

CH3O (CH3OH). Wolfenden, Jackson, and Hartley1 reported the heat of ionization of liquid methyl alcohol, in liquid methyl alcohol, to be -11.2. 

NH2CH2COO" (aq.). Louguinine1 found Q=2.97 for the reaction of aqueous aminoacetic acid with aqueous OH- to form aqueous glycinate, or aminoacetate, ion. Branch and Miyamoto,1 from conductivity measurements, calculated the heat of ionization of aqueous aminoacetic acid into aqueous H+ and aqueous glycinate ion to be —10.70. 

H2PdCle (aq.). We have assumed the heat of ionization to be zero. 

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