The equations for dissolution reactions that are discussed on this page are also shown in Example 12 of your workbook.
When a nonelectrolyte dissolves into solution the change is a relatively simple one. The chemical changes from its original state (the solid or liquid or gas phase) and goes into solution becoming aqueous. No other changes take place. The molecules are separated from one another, surrounded by water molecules, and that's all that happens.
In the case of a strong electrolyte, something else happens. We know that for a solution to conduct electricity, there must be ions in that solution. So if a material (solid, liquid, or gas) goes into solution and allows the solution to conduct electricity, then it must have been ionized or split into ions in the process. So the general format for an equation for the dissolution of a strong electrolyte is that the material, AB (solid, liquid, or gas) will change to the aqueous A+ ion, plus the aqueous B- ion.
In other words, the material not only goes into solution and is surrounded by water molecules, it also is ionized, that is, it splits up into ions. Any time an ionic compound dissolves in water, this is the process that takes place. There are also some molecular materials that will ionize in water. The second equation is shown to make sure that you realize that sometimes an ionic compound will split into more than two ions. It might split into three or four or more.
When a weak electrolyte dissolves in water, a more complicated situation occurs. Here the molecule AB (solid, liquid, or gas) goes into solution and that same molecule is surrounded by water molecules. But something else takes place. Some, not all, but some of those molecules dissociate (split up) into ions.
The double arrow indicates that this is an equilibrium reaction, similar in some ways to the saturated solution that we worked with before. The AB molecules split up to form A+ ions and B- ions. Those ions in turn reassociate to form the AB molecules. An equilibrium is set up where the molecules are dissociating just as fast as the ions are recombining to form molecules. At any given time only a portion of the molecules will be dissociated. There will not be as many ions as possible, therefore this solution will not conduct electricity as well as a strong electrolyte which does dissociate completely into ions.
Next, let's consider the molecular changes that take place when a chemical dissolves in water.
E-mail instructor: Eden Francis
Clackamas Community College