Enthalpies of disproportionation

MgCl(s), however, is not energetically stable with respect to disproportionation. The following energy cycle enables the enthalpy of disproportionation to be calculated, i.e. 

We present the enthalpies of disproportionation of aminyl radicals with hydroxyalkyl and alkyl peroxyl radicals in Table 16.2 [22-24],... 

The cross-disproportionation of nitroxyl and hydroperoxyl radicals is an exothermic reaction. For example, the enthalpies of disproportionation of TEMPO radical with H02, Me2C(0H)02, and cydo-C(,Y 10(OH)O2 radicals are equal to 109, —92, and 82 kJ mol-1, respectively. The Ee0 value for the abstraction of an H atom from the O—H bond in ROOH by a nitroxyl radical is 45.6 kJ mol 1 and AHe min = —58 kJ mol-1. Since AHe < AHe min, (see Chapter 6), the activation energy of such exothermic reactions for these reactions is low (E 0.5RT), and the rate constant correspondingly is high [31 34]. Therefore, in the systems in which hydroperoxyl, hydroxyperoxyl, and aminoperoxyl radicals participate in chain propagation, the cyclic chain termination mechanism should be realized. 

In the case of Hg, the transition 2HgX HgzXa must be considered and in the case of indium and gallium the disproportionation to the trihalide will be considered. It is interesting to note in passing that the value A/7/ InCl = 41 kcal/mole is quite close to the thermochemical value quoted in the U. S. Bureau of Standards, circular 500. The enthalpies of disproportionation are given in Table XXXVIII. The conclusions that... 

Enthalpies of Disproportionation (kcal/mole) of the Monohai.ides... 

Troyanov et al. [74TRO/MAR] also measured the enthalpy of disproportionation of both ZrBr2(cr) and ZrBr3(cr) using the same techniques. For the reactions,... 

The absence of reliable thermodynamic data for the tetrafluorides has contributed to difficulties in defining the chemistry of the rare earth elements. The fact that only Ce, Pr, and Tb form stable Rp4(s) phases has been established (see section 2.4) however, the thermochemistry of these fluorides has remained uncertain. Insight is provided by the work of Johansson (1978), who has correlated data for lanthanide and actinide oxides and halides and derived energy differences between the trivalent and tetravalent metal ions. The results, which have been used to estimate enthalpies of disproportionation of RF4 phases, agree with preparative observations and the stability order Prp4< TbP4 < CeP4. However, the results also indicate that tetravalent Nd and Dy have sufficient stability to occur in mixed metal systems like those described by Hoppe (1981). 

These values together with the known AHf for LnXj have been used to assess the stability of the dihalides with respect to disproportionation. The results are summarized in Table 6 as enthalpies of disproportionation. Only the values for the endothermic reactions are quoted as these imply stable LnX2. The data in Table 6 broadly agree with the predictions of stability quoted above for LnCb based on lattice energies. ... 

FIGURE 29 Enthalpy of disproportionation reaction (22) as a function of the number of the lanthanide in the series , data from Johnson (1969) O, data from Kim and Oishi (1979) and A, our calculations. Reproduced from Chervonnyi and Chervonnaya (2005c) with permission from Pleiades Publishing, Ltd. 

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