Module 4.3: Net Ionic Equations

Predicting Exchange Reactions

When ionic compounds are dissolved in water, they dissociate (or break apart) into the individual ions. If the predicted products of an exchange reaction are also soluble ionic compounds, an exchange reaction will not occur. Let's take a closer look. Suppose we mix sodium chloride (NaCl) with calcium sulfate (CaSO₄), both soluble ionic compounds. The predicted products of an exchange reaction would be sodium sulfate (Na₂SO₄) and calcium chloride (CaCl₂):

Is Na₂SO₄ soluble in water?	Yes	No	Click on buttons to review solubility rules.
Is CaCl₂ soluble in water?	Yes	No

Since both of the reactants and both of the products are soluble in water, the same individual ions are present before and after the reaction. Since there is no change in the species present, there is no chemical reaction.

When will an exchange reaction occur?

An exchange reaction will occur whenever a predicted product is not soluble in water. This may mean a precipitate (a solid) is formed. It also could mean a liquid (often water), or gas is produced.

Suppose sodium chloride (NaCl) is mixed with silver nitrate (AgNO₃).

What will the products be for this reaction?

NaAg + ClNO₃	NaNO₃ + AgCl	Na₂NO₃ + AgCl₂

The positive ion from one reactant will combine with the negative ion from the second reactant.

Good!

Since the sodium ion has a +1 charge and the nitrate ion has a -1 charge, Na₂NO₃ will not be electrically neutral. Likewise, since the silver ion has a +1 charge and the chloride ion has a -1 charge, AgCl₂ will not be electrially neutral.