Enthalpy of transfer

1 Enthalpies of Transfer Calorimetric measurements of the heats of solution of electrolytes in different solvents yield the most accurate data for their standard molar enthalpies of transfer between the solvents. Less accurate are the temperature derivatives of the solubilities and of the electromotive force (half-wave potential) of electrochemical cells. 

The splitting of the electrolyte values into the individual ionic contribniions can be made with the extrapolation method appUed for ion hydration (Section 4.2.2), as suggested by Somsen and Weeda [37]. This is implemented by the selection of a cation C and anion A of equal size (radius r), and then  

The temis on the left-hand side correspond to neutral species, and hence are available from measurements, and those on the right-hand side pertain to the ion-dipole and ion-quadrupole interactions. Plots of the left-hand side against extrapolated to infinitely large ion size, although not linear (due to the br term), then yield 2 H (Na, W S) as the ordinate intercept. 

A much more commonly used method for splitting electrolyte enthalpies of transfer into the individual ionic contributions is the TATB method, according to which, in analogy with its use for the Gibbs energies, (Ph As , W - S) = (BPh , W S), practiced by Arnett and McKelvey and by Friedman [38,39]. 

The uncertainty involved in the application of the TATB method was estimated as 1 kJ moF by Cox and Parker [40]. Indeed, if the TATB assumption is accepted for A,G at 25°C, at which temperature it is generally applied, there is no good reason for not accepting it at any other temperature, and hence the TATB assumption should be valid also for the A,// of ions. The TPTB assumption has in more recent years replaced to some extent the TATB one, but with hardly any effect on the results. 

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Estimates of the heat of solvation of various species in DMSO as compared to water have been made and can be expressed as enthalpies of transfer. Some data for some common ions are given below. Discuss their significance. 

It should be born in mind, however, that the activation parameters calculated refer to the sum of several reactions, whose enthalpy and/or entropy changes may have different signs from those of the decrystalUzation proper. Specifically, the contribution to the activation parameters of the interactions that occur in the solvent system should be taken into account. Consider the energetics of association of the solvated ions with the AGU. We may employ the extra-thermodynamic quantities of transfer of single ions from aprotic to protic solvents as a model for the reaction under consideration. This use is appropriate because recent measurements (using solvatochromic indicators) have indicated that the polarity at the surface of cellulose is akin to that of aliphatic alcohols [99]. Single-ion enthalpies of transfer indicate that Li+ is more efficiently solvated by DMAc than by alcohols, hence by cellulose. That is, the equilibrium shown in Eq. 7 is endothermic ... 

In the case of Kryptofix 221D, a cryptand able to complex the alkali metal cations [141-143], it has been observed that it is solubilized mainly in the palisade layer of the AOT-reversed micelles. And from an analysis of the enthalpy of transfer of this solubilizate from the organic to the micellar phase it has been established that the driving force of the solubilization is the complexation of the sodium counterion. In addition, the enthalpy... 

Enthalpies of transfer of trihalides from water into nonaqueous and mixed solvents can be obtained by simple arithmetic for all the cases where enthalpies of solution in nonaqueous and mixed aqueous media are known. As long as the enthalpies concerned have been measured in reasonably dilute conditions, or have been estimated for infinite dilu-... 

Enthalpies of Transfer of La3+ and of Yb3+ from Water into Organic Solvents (25°C)a... 

In what was intended as another experimental probe to ascertain whether a concerted or stepwise mechanism was involved in substitution reactions of arenesulfonyl chlorides, Rogne (1975) measured the enthalpies of transfer from propanol to acetonitrile for the transition states, SAH, for the reaction of imidazole with (a) benzoyl chloride and (b) benzenesulfonyl chloride. He found that SAH was considerably more negative for the reaction involving the sulfonyl chloride than for the one involving benzoyl chloride. This means that the transition state for attack of imidazole on benzenesulfonyl chloride is considerably better solvated by acetonitrile relative to its solvation by propanol... 

The important parameters for substituting a solvent S for water (W) follow from the thermodynamic cycle (6). Ks and Ky, are related through the free energy of transfer terms (7). The enthalpy of transfer is given by (8). 

Tab. 7 Solvent parameters [dielectric constant (e), dipole moment (/x), acceptor number (AN) and free enthalpy of transfer (DG°tr) of the chloride ion from N, N-dimethylformamide as reference solvent (taken from Ref 34)...

Fig. 8 Relationship between the reduction peak potential, Epc, of a-K6P2Wig062 or a-K4SiWi2O40 versus the reduction peak potential, EpcFc+, of ferricinium and the free enthalpy of transfer of the chloride ion, ACtr(Cl ), taking DMF as the reference solvent. Abbreviations are the same as in Tables 6 and 7. The solid line is the best linear regression fit to all the experimental points, including water as a solvent, (a) a-K6P2Wig062 correlation coefficient for the solid line 0.996. The correlation coefficient for the best fit to nonaqueous solvents only is 0.999. (b) a-K4SiWi204o correlation coefficient for the solid line 0.999. The correlation coefficient for the best fit to nonaqueous solvents only is also 0.999 (taken from Ref 34).

The enthalpies of dilution of BE were required to calculate the enthalpies of transfer (19). From these Integral enthalpies of dilution AHj.jj the relative apparent molar enthalpies were derived following the technique of Fortier et al (21). The values of AHjjj corresponding to the Initial and final molalities are given In Table 1 along with the parametric equation for. ... 

The enthalpies of transfer of BE (0.025 mol kg" -) from water to NaDec solutions are shown In Figure 5. With no adjustable parameters curve A Is predicted with the model of Roux et al (. The relation Is analogous to Equation 3. By adjusting Kp and AH3 a nearly quantitative fit can again be obtained as shown In curve B of Figure 5. The value of Kp used In this simulation Is about twice as large as the one used for volumes and heat capacities. If was fixed as the same value as that used for other properties the maximum In Figure 5 Is then underestimated. 

Figure 5 Enthalpies of transfer of 2-butoxyethanol from water to sodium decanoate solutions at 25°C. Simulations (curves A and B) with a chemical equilibrium model.

The AHj are not reliable in these simulations since high m, data are not available. Still the lack of agreement between the predicted and experimental enthalpies of transfer in Figure 6 again supports self-association of NaDec Induced by BE as with other functions. The deviation is not as pronounced as with volumes and heat capacities since nij was maintained at a lower value in the enthalpy experiments. 

The general tendency in aqueous organic solvents (11) is for the free energies of transfer to be monotonic functions of solvent composition. Often the enthalpy of transfer shows a maximum in the water-rich region, and for salts of small ions there may be a further reversal in the organic-rich region. 

We shall discuss now the variation of the three main thermodynamic functions with solvent composition for the case of n-Bu4NBr-water-acetone system and shall extend this discussion to the n-Bu4NBr-water-THF system. Figure 4 and Table IV present the results obtained. The figure was constructed as follows first the standard enthalpy of transfer AH°t, obtained by Ahluwalia and co-workers (12) from pure water to Z2 = 0.30, was used in order to get the standard entropy of transfer function from the relation ... 

As indicated by Eq. 6-9, the standard enthalpy of transfer of a compound i from water to air is given by ... 

Table 6.3 Experimental Standard Enthalpies of Vaporization (AvapH,) and Standard Enthalpies of Transfer from Water to Air (Aaw//,) of Selected Organic Compounds at 25 °C ... 

Describe in general terms in which cases you would expect that the enthalpy of transfer of an organic compound from a bulk liquid phase (including water) to air (AaeHt) is (a) larger, (b) about equal, and (c) smaller than the enthalpy of... 

As for any partition constant, over a temperature range narrow enough that the enthalpy of transfer may be assumed nearly constant, we may express the temperature dependence of Kuw by ... 

ENTHALPIES OF TRANSFER FROM METHANOL TO AQUEOUS METHANOL OF TETRA-ETHYLTIN, MERCURIC CHLORIDE, AND THE TETRAETH YLTIN-MERCURIC CHLORIDE... 

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