Formation of ion pairs from free ions

An ion pair is a physical entity with no specific chemical interactions between the ions. The ions of the ion pair move together as a single unit and are held together by electrostatic forces of the coulomb type acting over the short distances that the ions are apart in the ion pair. These coulombic forces impose a certain degree of cohesion on the unit and this is sufficiently great to overcome the tendency for normal thermal motion to cause the ions to move around as separate particles each with its own translational degrees of freedom. 

Because the forces holding the ions together are of this physical nature, they depend on three factors  

It is very important to realise that these forces are therefore independent of the chemical nature of the ions. On this basis, it would be expected that electrolytes which have ions of the same charge and are of similar size would have equihbrium constants of similar magnitude if the associated species is an ion pair. This is found for some 2 2 sulphates, and for some cations of similar size which associate with Cl (aq) or I (aq). 

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The combining of two or more substances or molecular entities to yield a single substance or molecular entity, a process that involves either covalent or noncovalent bonding. Included in this definition is the formation of ion pairs from free ions, the noncovalent aggregation of monomers to form polymeric structures or complexes, as well as colligation. The opposite of association is dissociation. 

Nevertheless, the situation is by no means as clear cut as it may seem, since addition of common ion salts R4N+BF4, to these systems reduces the value of rc in a given solvent Such salts would be expected to favour the formation of ion pairs from free ions, and it seems likely therefore that some contribution from free ions must also be accounted for in the original reactions. 

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