Page 147 - Theory and Problems of BEGINNING CHEMISTRY

136 NET IONIC EQUATIONS [CHAP. 9

Ans. (a)K + Cl − (b) BaSO 4 (insoluble) (c)SO 2 (not ionic) (d)Ca 2+ + 2 HCO 3 −
+
The insoluble compound is written as one compound even though it is ionic. The covalent compound is written
together because it is not ionic.

EXAMPLE 9.4. Each of the following reactions produces 56 kilojoules (kJ) per mole of water produced. Is this just a
coincidence? If not, explain why the same value is obtained each time.
KOH + HCl −→ KCl + H 2 O LiOH + HBr −→ LiBr + H 2 O
NaOH + HNO 3 −→ NaNO 3 + H 2 O RbOH + HI −→ RbI + H 2 O
Ans. The same quantity of heat is generated per mole of water formed in each reaction because it is really the same reaction
in each case:
OH + H −→ H 2 O
+
−
It does not matter whether it is a K ion in solution that undergoes no reaction or an Na ion in solution that undergoes
+
+
no reaction. As long as the spectator ions undergo no reaction, they do not contribute anything to the heat of the
reaction.
EXAMPLE 9.5. Write a net ionic equation for the reaction of aqueous Ba(OH) 2 with aqueous HNO 3 .
Ans. The overall equation is
Ba(OH) 2 (aq) + 2 HNO 3 −→ Ba(NO 3 ) 2 + 2H 2 O
In ionic form:
−
Ba 2+ + 2OH + 2H + 2NO 3 −→ Ba 2+ + 2NO 3 + 2H 2 O
−
−
+
Leaving out the spectator ions yields
2OH + 2H −→ 2H 2 O
+
−
Dividing each side by 2 yields the net ionic equation
OH + H −→ H 2 O
+
−
The net ionic equation is the same as that in Example 9.4.
Writing net ionic equations does not imply that any solution can contain only positive ions or only negative
ions. For example, the net ionic equation

Ba 2+ + SO 4 2− −→ BaSO 4 (s)

does not imply that there is any solution containing Ba 2+ ions with no negative ions, or any solution containing
2−
SO 4 ions with no positive ions. It merely implies that whatever negative ion is present with the barium ion
and whatever positive ion is present with the sulfate ion, these unspecified ions do not make any difference to
the reaction that will occur.
Net ionic equations must always have the same net charge on each side of the equation. (The same number
of each type of spectator ion must be omitted from both sides of the equation.) For example, the equation

+
Cu + Ag −→ Cu 2+ + Ag (unbalanced)
has the same number of each type of atom on its two sides, but it is still not balanced. (One cannot add just one
nitrate ion to the left side of an equation and two to the right.) The net charge must also be balanced:

Cu + 2Ag −→ Cu 2+ + 2Ag
+

There is a net 2+ charge on each side of the balanced equation.