Salts dissolve producing both cations and anions. It means that it is impossible to measure thermodynamic data for individual ion type. The solvation of ions from the gas phase is a thermodynamically favorable exothermic process, which increases the stability of the ions in solution and gives out energy. However, in order for the enthalpy of solution to be exothermic, this enthalpy of solution must be greater than the lattice enthalpy; otherwise the overall reaction is endothermic.
Fig. The conflict in symmetry
between bulk water and water in a
Qualitative treatment of ionic interaction:
At very low ionic concentration, the solvated cations and anions are so far apart that they do not interact significantly. At higher concentrations the ions are closer together and ion-ion interactions between the cations and anions are important. These are energetic-ally favorable, as the ions tend to cluster around other ions of opposite charge (counter-ions) and avoid ions of like charge (co-ions). This process further decreases the energy of the system and stabilizes the solution. That means although ions move dynamically in solution, on average each cation has as nearest neighbors more anions than cations and each anion more cations than anions. An ionic atmosphere of ion of overall opposite charge therefore surrounds each ion (Fig. 3).
Fig. The ionic atmosphere
surrounding a cation (...) and an anion
(---) in solution.