Enthalpies of hydration

Figure 30.3 Variation with atomic number of some properties of La and the lanthanides A, the third ionization energy (fa) B, the sum of the first three ionization energies ( /) C, the enthalpy of hydration of the gaseous trivalent ions (—A/Zhyd)- The irregular variations in I3 and /, which refer to redox processes, should be contrasted with the smooth variation in A/Zhyd, for which the 4f configuration of Ln is unaltered.

In the second hypothetical step, we imagine the gaseous ions plunging into water and forming the final solution. The molar enthalpy of this step is called the enthalpy of hydration, AHhvd, of the compound (Table 8.7). Enthalpies of hydration are negative and comparable in value to the lattice enthalpies of the compounds. For sodium chloride, for instance, the enthalpy of hydration, the molar enthalpy change for the process... 

TABLE 8.7 per Mole Cation Enthalpies of Hydration, AWhyd, at 25°C, of Some Halides, Anion in Kilojoules... 

Because the enthalpy of solution is positive, there is a net inflow of energy as heat when the solid dissolves (recall Fig. 8.23b). Sodium chloride therefore dissolves endothermically, but only to the extent of 3 kj-mol-1. As this example shows, the overall change in enthalpy depends on a very delicate balance between the lattice enthalpy and the enthalpy of hydration. 

Enthalpies of solution in dilute solutions can be expressed as the sum of the lattice enthalpy and the enthalpy of hydration of the compound. 

Interpret enthalpies of solution in terms of lattice enthalpies and enthalpies of hydration (Section 8.12). 

The enthalpy of solution of ammonium nitrate in water is positive, (a) Does NH4N05 dissolve endothermically or exothermically (b) Write the chemical equation for the dissolving process, (c) Which is larger for NH4NO , the lattice enthalpy or the enthalpy of hydration ... 

The solubilities of the ionic halides are determined by a variety of factors, especially the lattice enthalpy and enthalpy of hydration. There is a delicate balance between the two factors, with the lattice enthalpy usually being the determining one. Lattice enthalpies decrease from chloride to iodide, so water molecules can more readily separate the ions in the latter. Less ionic halides, such as the silver halides, generally have a much lower solubility, and the trend in solubility is the reverse of the more ionic halides. For the less ionic halides, the covalent character of the bond allows the ion pairs to persist in water. The ions are not easily hydrated, making them less soluble. The polarizability of the halide ions and the covalency of their bonding increases down the group. 

The enthalpies of hydration plotted in this figure refer to the process in Eq. (8.8). 

Morris DFC (1968/1969) An Appendix to Structure and Bonding. 4 6 157-159 Morris DFC (1968) Ionic Radii and Enthalpies of Hydration of Ions. 4 63-82 Mortensen OS (1987) A Noncommuting-Generator Approach to Molecular Symmetry. 68 1-28... 

Oostenbrink, C., Villa, A., Mark, A. E., Van Gunsteren, W. F. A biomolecular force field based on the free enthalpy of hydration and solvation the GROM OS force-field parameter sets 53A5 and 53A5. J. Comput. Chem. 2004, 25, 1555-1575. 

If a substance is to be dissolved, its ions or molecules must first move apart and then force their way between the solvent molecules which interact with the solute particles. If an ionic crystal is dissolved, electrostatic interaction forces must be overcome between the ions. The higher the dielectric constant of the solvent, the more effective this process is. The solvent-solute interaction is termed ion solvation (ion hydration in aqueous solutions). The importance of this phenomenon follows from comparison of the energy changes accompanying solvation of ions and uncharged molecules for monovalent ions, the enthalpy of hydration is about 400 kJ mol-1, and equals about 12 kJ mol-1 for simple non-polar species such as argon or methane. 

The direct measurement of enthalpies of solution of lanthanide trifluorides in water is hardly practicable, in view of their very small solubilities in this solvent. Enthalpies of hydration of lanthanide trifluorides have been measured (217). It should be possible to derive enthalpies of solution, at least for the hemihydrates LnF3-y2H20, by measuring enthalpies of precipitation. 

Table 9 compares ionic enthalpies of hydration from the Bernal and Fowler,164 Latimer et al.165 and Rashin and Honig88 procedures. Given the inherent uncertainty, the latter two sets of data are remarkably similar, considering that they were obtained 46 years apart. A number of tabulations of the thermodynamic solvation properties of ions in various solvents have now appeared. It is important to keep in mind, however, that there is a degree of arbitrariness associated with the experimental AHsoivation and AGSoiVation of individual ions. 

Experimental Ionic Enthalpies of Hydration Obtained By Three Different Methods, in kcal/mole ... 

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