Nernst hypothesis

It must be remembered that the Nernst hypothesis is one which we cannot verify by direct experiment, for the simple reason that the measurement of A and U in the neighbourhood of the absolute zero cannot be carried out We can, however, reach very low temperatures experimentally, and Nernst considers an extrapolation of such as justifiable... 

Fig. 8 Concentration profile of the electroactive species dissolved in the diffusion layer, a Species consumed by the electrochemical reaction, b species produced by the electrochemical reaction. The concentration profiles are represented in the Nernst hypothesis continuous curve), and in real values...

With the entropy, however, the case is quite otherwise, and we shall now go on to show that as soon as we are in possession of a method of determining the absolute value of the entropy of a system, all the lacunae of the classical thermodynamics can be completed. The required information is furnished by a hypothesis put forward in 1906 by W. Nernst, and usually called by German writers das Nernstsche Wdrmetheorem. We can refer to it without ambiguity as Nernsfs Theorem. ... 

A good deal of this work had no impact in the development of models of molecular structure and the elucidation of reaction mechanisms one reason was Perrin s own coolness to quantum wave mechanics. 108 Another, according to Oxford s Harold Thompson, who studied with Nernst and Fritz Haber, was that researchers like Lecomte "did not know enough chemistry he was a physicist." 109 Perrin, too, approached physical chemistry as a physicist, not as a chemist. He had little real interest or knowledge of organic chemistry. But what made his radiation hypothesis attractive to many chemists was his concern with transition states and the search for a scheme of pathways defining chemical kinetics. 

This remarkable influence is explained by Chapman f in terms of a degradation of the energy of the activated chlorine molecules by collision with oxygen molecules. On the basis of the Nernst chain hypothesis the effect can also be explained by supposing that the oxygen removes the atomic hydrogen by combining with it and thus interrupts the chain of reactions. J... 

The self-diffusion coefficients of CF and Na" in molten sodium chloride are, respectively, 33 x 10 exp(-8500// 7) and 8x10 exp(-4000// 7) cm s". (a) Use the Nernst-Einstein equation to calculate the equivalent conductivity of the molten liquid at 935°C. (b) Compare the value obtained with the value actually measured, 40% less. Insofar as the two values are significantly different, explain this by some kind of structural hypothesis. 

In deviations from the Nernst-Einstein equation in a molten salt, one hypothesis involved paired-vacancy diffusion. Such a model implies that holes of about twice the average size are available at about one-fifth the frequency of averagesized holes. Use the equation in the text for the distribution of hole size to test this model. 

We conclude that, in view of the unequivocal existence of the diffuse double layer at the ITIES, the Frumkin-type correction appears to be a plausible working hypothesis, though its unambiguous proof has not been provided yet. A more advanced approach, which relies on solving the Nernst-Planck equation in the space charge region, has been developed by Matsuda and Delahay [164]. 

Figure 2.8 compares corrected Nernst plots for C-lobe half-transferrin free in solution and bound to the transferrin receptor, at endosomal pH. These data clearly demonstrate that docking iron-loaded C-lobe transferrin at the transferrin receptor at pH 5.8 makes it energetically more favourable to reduce Fe " to Fe by 200 mV. Furthermore, receptor-docking places Fe reduction in a range accessible to NADH or NADPH cofactors, consistent with the hypothesis that reduction is the initial event in iron release from transferrin in the endosome. Fe " is bound by /zTf at least 14 orders of magnitude more weakly than Fe, so that reductive release of iron bound to HTi in the transferrin-transferrin receptor complex is then physiologically and thermodynamically feasible, and the barrier to transport across the endosomal membrane is lifted. The transferrin receptor, therefore, is more than a simple conveyor of... 

The original arguments of Nernst on this subject were based upon the view that AG and AH are equal not only at the absolute zero itself (as they must be from the general thermodynamic equation) but also in the immediate vicinity of this point, so that d AO)jdT and 8 AH)jdT vanish when T = 0. From these conditions it follows that J will be zero. Nemst put forward the hypothesis about the temperature coefficients as a fundamental thermodynamic principle (sometimes spoken of as the third law of thermodynamics). While there are reservations about the complete validity of this view, it is certainly one which is always nearly true, and may often be accurately so. 

Let us recall here (see section 4.2.1) that it is only possible to split the migration and diffusion contributions in quantitative terms if one first draws up a hypothesis based on their respective properties. The simplest assumption is that the two phenomena have an identical microscopic mechanism, and that the activity and concentration values are equal. This leads to the Nernst-Einstein equation, as used hete. 

In these calculations, the authors do not assume that the electric field strength U is constant. Rather, they examine this hypothesis, using the Nernst-Planck equation, which relates the fluxes of charged species to the electric field gradient. Even assuming local electric neutrality, that is, [A " "]-I-[T" ] — [C ], they find nonnegligible spatial variations in the electric... 

His three early Leipzig papers (5-7) represent a synthesis of concepts that he was well qualified to make. Working in Ostwald s laboratory, he must have absorbed some of the mass of electrochemical information which appeared a few years later in Ostwald s two-volume work on the history and theory of electrochemistry (H). He was thoroughly familiar with the second-law thermodynamics of Thomson and Clausius, and with the more recent pronouncements of van t Hoff and Helmholtz. Nernst was also imbued with the atomism of Dalton and Boltzmann, in v hich respect he differed from Ostwald and Helmholtz, and he had accepted Arrhenius s recently published (12,13) hypothesis of the complete dissociation of strong electrolyses in solution. However, his conductance work in Kohlrausch s laboratory had given him a lively appreciation of the effects of incomplete ionization of weak electrolytes. 

The membrane is treated as the same simple system as described above (Figure 1). We assume, besides the electroneutrality inside the membrane (constant field hypothesis), a continuity of the electrical potential ip from the bulk of the surrounding medium to the center of the cell. We are compelled, therefore, to assume that the potential at the outer Helmholtz plane, which is induced by the effective surface charge (dissociated functional groups or strongly adsorbed ions) is screened to a major extent by counterion-site pairing in the Nernst layer. We consider the three classes of transferable chemical species quoted in the introduction. Among the species not directly involved in chemical reactions, the calculation takes into account an anion (Cl ), two monovalent cations (K, Na+) and two divalent cations (Mg2+, Ca ). As for the species involved in chemical reactions, we assume that they do not interact inside the membrane (independent channels). However, their flux balance must take into account the chemical reactions in both lateral phases. We handle only here two types of equilibria the water dissociation and the first dissociation of the carbonic acid. The three flux balances of the five chemical species which are interconnected by the conditions for the equilibria are as follows ... 

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