Halliwell and Nyburg

A critical examination of the volta potential method has been made by Parsons (40) and by de Bethune (41) and some of the problems of this approach have been summarized by Halliwell and Nyburg (39). Randles (42) has obtained a value for AG° abs. hyd. for H+(g) by a procedure in this category and this combined with entropy estimates yields the value for Wn+ accepted by Rosseinsky (38) in his review, namely —269.7 kcal mole-1. 

Probably the most satisfactory approach which has been used to estimate absolute enthalpies of hydration of single ions is that of Halliwell and Nyburg (39). The method makes use of a simple model of aqueous solutions, and is based on the significance of differences between thermodynamic functions for ions of opposite charge rather than attempting to split the enthalpy of hydration of pairs of ions. Values of the differences between conventional enthalpies of hydration are plotted against (R -fa) - , where R is the effective radius of the ion and a is the effective radius of the water molecule. This plot yields the absolute enthalpy of hydration of the proton. 

In their investigation Halliwell and Nyburg (39) employed cation radii due to Ahrens (7) and anion radii due to Pauling (8) as the appropriate values for R. In the present work the calculation is repeated using the radii in col. 2 of Table 1. As in the original study, the effective radius of the water molecule has been taken as 1.38 A this is the magnitude found in ice. 

The three equations fit the experimental data well within the limits of experimental error. Moreover, the values obtained for the absolute enthalpy of hydration of the proton do not appear to be very dependent on the model chosen. Results for the quadrupole of the water molecule 0 in the direction of the ja axis obtained from the three equations are, respectively, 0.70, 0.58 and 0.81 Xl0-26 e.s.u. these magnitudes correspond more closely than the analogous values of Halliwell and Nyburg to a result of quantum mechanical calculation, namely 0 0.43 X 10-2a... 

According to Moore (as cited in Ref. (90)). Strictly, these values refer ro 0° K, but it has been judged unnecessary to introduce the small temperature correction. e) In the main according to Halliwell and Nyburg (91), or from Ref. (90), with the values recalculated with the value of A H = — 260.7 kcal, recommended by Halliwell and Nyburg. For a few ions, however, values have been compiled from other sources (for a comprehensive list of references, see Ref. (91)). For... 

Conway and Soloman corrected Halliwell and Nyburg s (1%3) value for the numerical value of the dipole moment of water and got -1117 kJ mol". Then, Lister, Nyburg, and Doyntz reattacked the calculation using a different series of ions of the same size. They got -1096 kJ mol . ... 

A special case, but one of seminal importance, is the heat of hydration of the proton because so much depends upon it. A rather clever method was set out many years ago by Halliwell and Nyburg and although their approach has been reexamined... 

Explain what is meant by the statement The relative heat of hydration of T is -1385.7 kJ mor. Halliwell and Nyburg s value for the heat of hydration of protons was -1087.8 kJ mol". Use this value to establish the absolute heat of hydration of T. 

The overall strength of interaction between a metal ion and its hydration shell, which will parallel the strength of bonding within the [M(OH2 ),]" , can be assessed from ion hydration enthalpies. The values listed in Table are based on the single ion assumption of Halliwell and Nyburg... 

There are several empirical relations for the lattice energy (cf. E7, E8) and a recent one proposed by Halliwell and Nyburg is given by... 

Halliwell and Nyburg ° argued that the division into individual ionic contributions of Ai/g iv could be achieved semi-empirically using Buckingham s concepts. They employed enthalpies of solvation relative to the hydrogen ion (conventional enthalpies) defined by... 

Somsen obtained individual AH iy(ion) data for the alkali metals and halides in the amides using the method of Halliwell and Nyburg. The quadrupole moments of the amides were obtained by modifying eqn. 2.11.21 to allow for repulsion effects. Considering a system of soft spheres , eqn. 2.11.21 becomes... 

Salomon has extended Halliwell and Nyburg s method for obtaining enthalpies of solvation by allowing for the possibility that the ion-dipole term is not equal for ions of equal charge and radius. Conway and... 

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