Lattice Energy and Ion Solvation Enthalpy

Mass spectrometric measurements coupled with solution thermochemical results are the sources of solvation enthalpy values for anions and cations. These data are related to the lattice energy, which is a parameter used to assess the ionic character of solids and predict their standard enthalpies of formation. An introduction to that 

Consider figure 2.5, which is a thermochemical scheme involving crystalline lithium methoxide (LiOCI I3) as the starting substance. The lattice energy, Aiati/°(LiOCI I3), is the internal energy associated to the following process, at 298.15 K  

In other words, Aiatf/°(LiOCH3) is the energy required to destroy the crystalline network, yielding the gas-phase ions. As the cation and the anion are infinitely separated, the gas phase can be described by the ideal gas model and the enthalpy corresponding to the same process is given by 

Aiat72°(LiOCH3) = Aiatf/°(LiOCH3) + A(pV) A,at(/°(LiOCH3) + AvRT 

The enthalpy of reaction 2.45 cannot be determined directly. As shown in figure 2.5, it is calculated by using several experimental quantities the standard enthalpy of formation of the solid alkoxide, the standard sublimation enthalpy and the ionization energy of lithium, and the standard enthalpy of formation and the adiabatic electron affinity of gaseous methoxy radical (equation 2.47). 

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