Exchange Forces and Electrostatic Forces Between Ions in Solution

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Abstract
Though the treatment of ionic dissociation by thermodynamic methods has been fruitful, the mechanism can be better understood by the use of statistical mechanics. For this purpose one needs to know the potential energy of the ions as a function of temperature. The work D required to dissociate a molecule or molecular ion consists of two parts, one part Del due to electrostatic forces, and the other Dnon due to nonelectrostatic forces. As the former varies rapidly with temperature, for each species the value of dD/dT is a guide to the value of Del. It is shown that for acid molecules and molecular ions in aqueous solution the relative values of dD/dT for different types are in the order predicted. Further, the characteristic temperature θ at which the dissociation constant passes through a maximum should depend, not on the value of D, but on the relative magnitudes of Dnon and Del. It is found that the known values of θ fall into line in accordance with this rule. Between two Hg+ ions, or two Te— ions, there will be exchange forces of attraction similar to those which give rise to the molecules Na2 and Br2; thus in aqueous solution the ions (Hg2)++ and (Te2)—— are stable, although for them Del is negative.