Because we have not studied periodic
trends in properties of transition
metals, it would be difficult for you
to predict that Cu is more active than
Ag. The fact that this reaction occurs
(see Figure 4-3) shows that it is.
136 CHAPTER 4: Some Types of Chemical Reactions
solutions. Some typical examples are included. Refer to Table 4-9 often as you study the
following sections.1.In formula unit equations,we show complete formulas for all compounds. When
metallic copper is added to a solution of (colorless) silver nitrate, the more active
metal—copper—displaces silver ions from the solution. The resulting solution
contains blue copper(II) nitrate, and metallic silver forms as a finely divided solid
(Figure 4-3):2AgNO 3 (aq)Cu(s)88n2Ag(s)Cu(NO 3 ) 2 (aq)Both silver nitrate and copper(II) nitrate are soluble ionic compounds (for solubility
guidelines see page 134 and Table 4-8).
2.In total ionic equations,formulas are written to show the (predominant) form in
which each substance exists when it is in contact with aqueous solution. We often
use brackets in total ionic equations to show ions that have a common source or
that remain in solution after the reaction is complete. The total ionic equation for
this reaction is2[Ag(aq)NO 3 (aq)]Cu(s)88n2Ag(s)[Cu^2 (aq)2NO 3 (aq)]Examination of the total ionic equation shows that NO 3 ions do not participate
in the reaction. Because they do not change, they are often called “spectator” ions.
3.In net ionic equations,we show only the species that react. The net ionic equa-
tion is obtained by eliminating the spectator ions and the brackets from the total
ionic equation.2Ag(aq)Cu(s)88n2Ag(s)Cu^2 (aq)Net ionic equations allow us to focus on the essenceof a chemical reaction in aqueous
solutions. On the other hand, if we are dealing with stoichiometric calculations we
frequently must deal with formula weights and therefore with the completeformulas of all
species. In such cases, formula unit equations are more useful. Total ionic equations provide
the bridge between the two.(a) (b)Figure 4-3 (a) Copper wire and
a silver nitrate solution. (b) The
copper wire has been placed in the
solution and some finely divided
silver has deposited on the wire. The
solution is blue because it contains
copper(II) nitrate.
Brackets are not used in net ionic
equations.
This is why it is important to know
how and when to construct net ionic
equations from formula unit equations.