Endothermic process ionic bonds

Were we to simply add the ionization energy of sodium (496 kJ/mol) and the electron affin ity of chlorine (—349 kJ/mol) we would conclude that the overall process is endothermic with AH° = +147 kJ/mol The energy liberated by adding an electron to chlorine is msuf ficient to override the energy required to remove an electron from sodium This analysis however fails to consider the force of attraction between the oppositely charged ions Na" and Cl which exceeds 500 kJ/mol and is more than sufficient to make the overall process exothermic Attractive forces between oppositely charged particles are termed electrostatic, or coulombic, attractions and are what we mean by an ionic bond between two atoms... 

When an ionic compound dissolves in water, energy is needed to break the ionic bonds of the crystal. As the ions attach to the water molecules and become hydrated, energy is released. The process is endothermic if the energy needed to break the bonds is greater than the energy released when the ions attach to water. 

The first equilibrium shows an enthalpy change of —3.6 kJ/mol (1 M) and —6.3 kJ/mol (2 M), whereas for the second, the AH value is 17.1 kJ/mol. Although the overall effect of an increase in temperature would be to increase the ionic association because the second equilibrium is strongly endothermic, the exothermic nature of the first step is rather surprising because it involves the loss of two DMF molecules from the first solvation sphere of the lithium ion and the formation of only one Li —N03 bond. Furthermore, in the second step, the substitution of one molecule of DMF by one nitrate ion in the Li first solvation sphere has an enthalpy cost of 17.1 kJ/mol, which demonstrates that, as expected, the desolvation of the lithium ion is an endothermic process, which is partially compensated by the likely exothermic nature of the ion-pair formation. 

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