ABSE_Solubility Equilibria Lesson

Solubility Equilibria

We have focused mostly on the equilibrium reactions of weak acids and bases so far. There is another very important category of equilibrium reactions involve precipitation reactions. Recall the solubility rules for salts that were studied earlier. When studying those rules, the scenarios were very black and white. A substance either dissolved or did not dissolve in a solution. In reality, none of the salts that were described as insoluble are totally insoluble. Consider silver chloride, AgCl. We know from the solubility rules that AgCl is insoluble. Actually, a small amount does dissolve, according to the equilibrium chemical equation below:

AgCl(s)   ⇌    Ag⁺(aq)   +     Cl^-(aq)

When equilibrium is established, there are very few ions in solution, but the equilibrium is very important. Below is the mass action expression for this equilibrium reaction. Notice that the solid is moved to the side with k_c to become k_sp.

Because this equation involves species in more than one state, this represents a heterogeneous equilibrium. Remember that a pure solid or liquid is removed from the mass action expression of a heterogeneous equilibrium and incorporated into the constant. For solubility equilibria, the new constant, K_sp, is called the solubility product. The value of the solubility product constant is dependent upon temperature.   A list of some k_sp values at 25°C is shown below. The colors of the precipitates are included for reference.

Ion Product and K_sp

The product of the concentrations of ions in the equilibrium expression is called the ion product.   In any diluted solution, the ion product is different. Once the solution is saturated, the ion product becomes constant, and is represented by the value of K_sp. This is similar to the relationship between Q and k_eq.

If a salt dissociates into more than one of any type of ion, the concentrations in the mass action expression must be raised to the power of the coefficient.   Here is an example:

Ag₃PO₄   ⇌  3Ag⁺  + PO₄^3-

K_sp  = [Ag⁺]³[PO₄^3-]

Determining K_sp From Solubility Data

The solubility of a salt is the maximum amount of the salt that will dissolve in a certain volume of solution at a certain temperature. In other words, it is the concentration of a saturated solution of the salt.

Using the "ice" method, you can calculate the value of K_sp from solubility data. Before you watch the presentation below to see how, let me point out what is often the trickiest part of these type problems. The solubility is usually presented at the molarity of the solid salt. However, the solid is not included in the ksp expression. So, students typically get confused about where to use this information. We use the molar solubility of the salt to determine the "change" row in the ice chart. Make sure that this row matches the stoichiometry of the balanced equation. Pay attention to these points as you follow along with the video.

The solubility of a salt can also be determined from K_sp. Watch this video to see how.

Remember to work on the module practice problems as you complete each section of content.

[CC BY-NC-SA 4.0 Links to an external site.] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION - INTENDED ONLY FOR USE WITHIN LESSON.