Page 145 - Theory and Problems of BEGINNING CHEMISTRY

CHAPTER 9

Net Ionic

Equations

9.1. INTRODUCTION
The chemist must know many thousands of facts. One way to remember such a wide variety of information
is to systematize it. The periodic table, for example, allows us to learn data about whole groups of elements
instead of learning about each element individually. Net ionic equations give chemists a different way of learning
a lot of information with relatively little effort, rather than one piece at a time. In this chapter we will learn:
1. What a net ionic equation means
2. What it does not mean
3. How to write net ionic equations
4. How to use net ionic equations
5. What they cannot tell us

9.2. WRITING NET IONIC EQUATIONS
When a substance made up of ions is dissolved in water, the dissolved ions behave independently. That is,
they undergo their own characteristic reactions regardless of what other ions may be present. For example, silver
−
ions in solution, Ag , always react with chloride ions in solution, Cl , to form an insoluble ionic compound,
+
AgCl(s), no matter what other ions are present in the solution. If a solution of sodium chloride, NaCl, and a
solution of silver nitrate, AgNO 3 , are mixed, a white solid, silver chloride, is produced. The solid can be separated
from the solution by filtration, and the resulting solution contains sodium nitrate, just as it would if solid NaNO 3
were added to water. In other words, when the two solutions are mixed, the following reaction occurs:
AgNO + NaCl −→ AgCl(s) + NaNO 3
3
−
+
−
or Ag + NO 3 + Na + Cl −→ AgCl(s) + Na + NO 3 −
+
+
Written in the latter manner, the equation shows that, in effect, the sodium ions and the nitrate ions have not
changed. They began as ions in solution and wound up as those same ions in solution. They are called spectator
ions. Since they have not reacted, it is really not necessary to include them in the equation. If they are left out, a
net ionic equation results:
−
+
Ag + Cl −→ AgCl(s)
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