Kimi bir değerlikli iyonların sınır eşdeğer iletkenliklerinin mutlak hidratasyon entropilerinin mutlak hidratasyon entalpilerinin kuramsal olarak hesaplanması
Küçük Resim Yok
Tarih
1991
Yazarlar
Dergi Başlığı
Dergi ISSN
Cilt Başlığı
Yayıncı
Ege Üniversitesi
Erişim Hakkı
info:eu-repo/semantics/closedAccess
Özet
l 1 l SUMMARY This study covers the theoretical calculations of absolute hydration entropies and enthalpies of some univalent ions. When the related literatüre is thoroughly examined, a controversial situation about the values of ionic radii used in hydration calculations is noteworthy. it is a remarkable feature that "crystal ionic radii" have been employed for ions in the solution phase and different scales have been proposed. Upon a careful deliberation of the subject, therefore, a set of radii computed in a study involving the ions in the solution phase has been preferred and used throughout our calculations. The second point debatable in literatüre is the "hydration number"; both the definitions and the values deduced differ significantly. Our objection to the concept of hydration number as given in literatüre, in combination with the lack of agreement about certain values, has led us to recalculate "the number of water molecules surrounding the ion" dictated by the geometry. The third point disputable in literatüre is that what value the dielectric constant of water molecules would have in the vicinity of the ion. The first postulate of the two, on which our model is based, copes with this problem; the dielectric constant of "medium" is taken as "1" vvithin both the primary and the secondary hydration shells, that is, vvater molecules have been assumed to orient themselves completely. in our study, it has not been aimed to calculate the hydration entropies and enthalpies of tetraalkylammonium cations (R4N + ). These ions were indirectly associated with the study, as the experimental nature of the limiting equivalent ionic conductances was examined at the beginning. The variance in the values of ionic radiüs encountered in literatüre for alkali and halide ions is also a matter of concern for tetraalkylammonium ions. Therefore, the results of any öne of the six different studies in literatüre have not been employed; but, instead, a new set of radii has been calculated by considering ali of them in an original way. ı112 Since the limiting equivalent ionic conductance values constitute a reliable set of data obtained experimentally for individual ions, their experimental nature as a function of ionic size has been taken as the basis of our model. Two equations which satisfy the boundary conditions defined upon the examination of the experimental nature and express the viscosity as the probability function of the network energy of the water molecules under consideration in the bulk and the maximal electrostatic (ion-dipole) energy they can gain in the force field of the ion and two separate combinations of these two energies have been written. The main idea lying behind these equations actually constitutes the second postulate of the model which states that the orientations of water molecules in the process are subject to the Boltzmann statistical distribution. The viscosities of the water molecules within the primary shell surrounding the alkali, halide and R^N + ions have been calculated via the equations written. By inserting these viscosity values and the ioinc radii of the ions in question into the equation pertinent to the Stokes law, theoretical limiting equivalent conductances have been calculated. It has been established that the model is in accordance with experiment by comparing the experimental limiting equivalent conductances with the theoretically found values. In the light of the same basic ideas and the postulates, an equation which let us calculate the standard partial entropy of the ion "i" in solution has been derived. The entropy of the same ion in the gas phase has been found through the Sackur-Tetrode equation, and the absolute hydration entropy of the ion "i" has been calculated and compared with the experimental values. It was stated previously that mainly the change of the positions and energetics of water molecules has been taken into consideration in energy calculations involving the hydration reaction. In that case the enthalpy change relevant to the hydration reaction should equal the difference between the energy gained by water molecules upon hydration and the energy they had at the beginning. The absolute ionic hydration enthalpy of the ion T has been calculated from an equation written by the above reasoning and has been compared with the experimental values.113 In addition, the sums of the absolute ionic hydration enthalpies of ion combinations which form definite salts have been compared with the experimental values belonging to the salts. After the calculations regarding the enthalpy and entropy have been finished, the absolute Gibbs free energies of the ionic hydration reactions have been found through the fundamental equation relating the three thermodynamic functions, and the results obtained have been compared with the experimental values. Upon the evaluation of the results, it has been established that only the Li+ ion is hydrated in solution; others being bare, that is, they are not surounded with immobilized water molecules, which, together with the ion, behave like a single kinetic entity.
Açıklama
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Anahtar Kelimeler
Kimya, Chemistry, Entalpi, Enthalpy, Entropi, Entropy, Fizikokimya, Physical chemistry, İyonik hidratasyon, Ionic hydration
