Dissolution Reactions
Home Up Dissolution Reactions Molecular Changes Identifying Acids,Bases and Salts Writing Ionic Equations

 

Dissolution Reactions

The equations for dissolution reactions that are discussed on this page are also shown in Example 12 of your workbook.

Nonelectrolytes

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.

Alcohol is a liquid. When it dissolves in water, the alcohol molecules separate and become surrounded by water molecules.

C2H5OH(l) rtarrow.gif (850 bytes) C2H5OH(aq)

Oxygen molecules exist in the gaseous state. When oxygen dissolves in water, the oxygen molecules become surrounded by water molecules. Oxygen is only slightly soluble in water and comes out of solution very easily. To show that the reverse process also happens, we can write a reverse arrow. Arrows going in both directions show that the reaction goes in both directions.

O2(g) <--> O2(aq)

 

Strong Electrolytes

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.

AB(s,l,g) rtarrow.gif (850 bytes) A+(aq) + B-(aq)
or
A2B(s,l,g) rtarrow.gif (850 bytes) 2 A+(aq) + B2-(aq)

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.

Hydrogen chloride is a gaseous molecular material that can be viewed as splitting up into ions when it dissolves in water. A better view is that it chemically reacts with water and we will explore that approach later in the lesson on acids and bases.

HCl(g) rtarrow.gif (850 bytes) H+(aq) + Cl-(aq)

When the ionic compound sodium chloride dissolves in water, the sodium ions and chloride ions from the salt crystal separate from each other and become surrounded by water molecules.

NaCl(s) rtarrow.gif (850 bytes) Na+(aq) + Cl-(aq)

In this next case, we have the compound Na2SO4 which splits into two Na+ ions for every one SO42- ion when it dissolves. You should remember that sulfate ion is a polyatomic ion and does not break up into separate sulfur and oxygen ions or atoms. If you don't remember that, you should take some time before you finish this lesson to review the names and formulas of the polyatomic ions that you learned during the previous course.

Na2SO4(s) rtarrow.gif (850 bytes) 2 Na+(aq) + SO42-(aq)

 

Weak Electrolytes

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.

AB(s,l,g) rtarrow.gif (850 bytes) AB(aq) <--> A+(aq) + B-(aq)

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.

This equation shows what happens to acetic acid, a liquid, when it dissolves in water. The acetic acid molecules separate from one another and are surrounded by water molecules. Some of these acetic acid molecules will react with water and dissociate into ions. The dissociated ions can also recombine to form molecules.

C2H3O2H(l) rtarrow.gif (850 bytes) C2H3O2H(aq) <--> C2H3O2-(aq) + H+(aq)

Next, let's consider the molecular changes that take place when a chemical dissolves in water.

 

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