Barrons SAT Subject Test Chemistry, 13th Edition

In the NaCl solution, the concentrations are:

[Na+] = 1.00 × 10−2 mol/L and [Cl−] = 1.00 × 10−2 mol/L

When 1 liter of one of these solutions is mixed with 1 liter of the other
solution to form a total volume of 2 liters, the concentrations will be halved.
In the mixture then, the initial concentrations will be:

For the Ksp of AgCl,

This is far greater than 1.7 × 10−10, which is the Ksp of AgCl. These

concentrations cannot exist, and Ag+ and Cl− will combine to form solid AgCl

precipitate. Only enough Ag+ ions and Cl− ions will remain to make the product of

the respective ion concentrations equal 1.7 × 10−10.

COMMON ION EFFECT

When a reaction has reached equilibrium, and an outside source adds more of one
of the ions that is already in solution, the result is to cause the reverse reaction to
occur at a faster rate and reestablish the equilibrium. This is called the common
ion effect. For example, in this equilibrium reaction:

NaCl(s) Na+(aq) + Cl−(aq)

the addition of concentrated HCl (12M) adds H+ and Cl− both at a concentration

of 12 M. This increases the concentration of the Cl− and disturbs the equilibrium.
The reaction will shift to the left and cause some solid NaCl to come out of
solution.
The “common” ion is the one already present in an equilibrium before a
substance is added that increases the concentration of that ion. The effect is to
reverse the solution reaction and to decrease the solubility of the original
substance, as shown in the above example.

DRIVING FORCES OF REACTIONS

Relation of Minimum Energy (Enthalpy) to Maximum Disorder

(Entropy)