All of the equilibria we've been talking about so far have chemical species in the same phase. For solutions, they're all dissolved, and for gaseous equilibria, they're all gases. Equilibria in which all species are in the same phase are called homogeneous equilibria.
However, we can also talk about equilibria in which not all of the species are in the same phase. These equilibria are referred to as heterogeneous equilibria. An example of a heterogeneous equilibrium would be when an ionic compound partially dissolves in water.
A homogeneous equilibrium occurs when all reagents and products are found in the same phase (solid, liquid, or gas) and a heterogeneous equilibrium is when they are in different phases.
To demonstrate a heterogeneous equilibrium, we'll discuss the equilibrium expression for when calcium carbonate dissolves in water (a process known as "dissociation"). The equation for this process is:
In our example, the equilibrium constant is called "Ksp" for "solubility product constant." Any time you study how one compound dissolves in another, you'll need a Ksp value.
Let's write the equilibrium expression for this process:
However, there's a twist. Recall that any time we put something in the square brackets, this means that we need its concentration. Because CaCO3 is a solid in this process, it doesn't really have a clearly defined concentration. As a result, we just leave it out of this expression. (I bet you wish you could do that with everything that didn't make sense!) Likewise, whenever you have a pure solid or a pure liquid (but not a solution) in an equilibrium expression, you leave it out of the expression for Keq or Ksp.
Leaving the [CaCO3] term out of the Ksp expression leaves us with the following expression:
Anytime you have a pure solid present in an equilibrium, leave it out of the equilibrium expression entirely.
The Ksp value for an ionic compound describes the degree to which the ions are present in a saturated aqueous solution. As with other equilibria, in saturated solutions the salt will dissolve and precipitate out at the same rates, causing no net change in ionic concentrations.
Example: What's the concentration of Ca+2 ions in a saturated solution of CaCO3? Ksp (CaCO3) = 4.5 × 10-9.
Problem 2: What is the equilibrium concentration of bromide ion when enough PbBr2 dissociates in water to make a saturated solution? Ksp (PbBr2) = 2.1 × 10-6.
Solution: In the equation for the dissociation of CaCO3, we see that the concentration of Ca+2 and CO32- must be the same in a saturated solution because one of each ion is formed by the breakup of each CaCO3. Because we don't know what this concentration will be, let's call it "x."
Putting this into our Ksp expression for CaCO3, we find that:
The concentration of both the calcium and carbonate ions is 6.7 × 10-5 in a saturated CaCO3 solution.
Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.