Page 103 - Instant notes

Solubility 89

solubility of the salt would then be given by , as only the silver ions in solution
must have come from the silver chloride salt. Hence:

and

This confirms the shift to the left of the equilibrium with the solubility, s, decreasing as c
increases. This is the common ion effect.
It must be remembered that this equation only rigorously applies if c is sufficiently
−3
small (of the order of 0.001 mol dm or less) to ensure that there is no interaction
between the ions in solution. If c becomes larger than this, the energetically favorable
interactions between ions seen in the inert ion effect become increasingly important (see
Topic E1) and the effects of activity cannot be neglected. In this case, the solubility
equation becomes

which as γ<1 (see Topic E2) allows for the small increase in solubility due to the
electrostatic stabilization of the ions. However, this effect is relatively small and is
dominated by the common ion effect when adding a common ion and so even at higher
concentrations, an overall decrease in solubility is seen in this case.

The inert ion effect

When inert ions, which take no part in the solubility equilibrium, are added to the
solution, these tend to cause an increase in the solubility of the salt, called the inert ion
effect. An example is adding an NaNO 3 solution to the saturated AgCl solution. In this
case

and since