The values of the heats of solution (2131 solutions) of different liquid solutes in organic and inorganic solvents were obtained from the literature data on the heat of mixing ( ) in the wide range of concentrations. The limit values of the heat of solution of a solute ( ) in a solvent ( ) ( ) were calculated from the limit data of the dependence / at and the values of that of a solute ( ) in a solvent ( ) ( ) from the limit data of the dependence / at , respectively. 1. Introduction At the present, there are a lot of data on the heats of mixing of binary liquid systems [1–5] and considerably less data on the heats of solution from direct calorimetric measurements [6–11]. In two handbooks [1, 2] about 2500 tables of data on the heats of mixing ( ) of the different liquid solutes in organic and inorganic solvents have been collected, whence the values of the enthalpies of solution were calculated. It is generally known that the enthalpy of solution is the enthalpy change associated with the dissolution of a substance in a solvent at constant temperature and pressure, resulting in infinite dilution. This process can mentally be separated on three steps: Here, is the energy of breaking of all the solute-solute interactions in molecular liquid (A), equal to the enthalpy of evaporation (invariably endothermic process), is the enthalpy of optimal destruction and rearrangement of the solvent-solvent part of interactions (invariably endothermic process), and is the enthalpy of interaction of the isolated molecule of solute (A) placed in the prepared hole of the solvent (S) (invariably exothermic process). The value of the overall enthalpy change is the sum of the individual enthalpy changes of each of these steps and can be positive (endothermic process) or negative (exothermic process). In the case of ideal solution, there is a complete compensation of breaking and forming energy in (1) with zero value of . In overwhelming majority of measurements, the values of the heat of solution differ from zero and supply with the additional information for analysis of the nature of these steps [10, 11]. Enthalpy transfer from the gas phase to solution is the enthalpy of solvation ( ) and can be calculated as follows: From (2) follows that the relative change of the enthalpy of solvation ( ) of compound (A) in the series of solvents is equal to the difference in the enthalpy of solution ( ). For all enthalpy cyclic processes, there is the possibility to calculate the unknown values of the enthalpy transfer on the base of relative changes of the enthalpy of solution. As an
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