The Foundations of Chemistry

Recall that in balanced equations we

show the smallest whole-number

coefficients possible.

4-9 Metathesis Reactions 155

Problem-Solving Tip:Salt Formation

The salt that is formed in a neutralization reaction is composed of the cation of the base

and the anion of the acid. The salt may be soluble or insoluble. If our goal were to obtain

the salt from the reaction of aqueous HCl with aqueous NaOH, we could evaporate the

water and obtain solid NaCl.

EXAMPLE 4-9 Neutralization Reactions

Predict the products of the reaction between HI(aq) and Ca(OH) 2 (aq). Write balanced formula
unit, total ionic, and net ionic equations.

Plan

This is an acid–base neutralization reaction; the products are H 2 O and the salt that contains
the cation of the base, Ca^2
, and the anion of the acid, I; CaI 2 is a soluble salt (solubility
guideline 4). HI is a strong acid (see Table 4-5), Ca(OH) 2 is a strong base (see Table 4-7), and
CaI 2 is a soluble ionic salt, so all are written in ionic form.

Solution

2HI(aq)
Ca(OH) 2 (aq)88nCaI 2 (aq)
2H 2 O(
)

2[H
(aq)
I(aq)]
[Ca^2 
(aq)
2OH(aq)]88n[Ca^2 
(aq)
2I(aq)]
2H 2 O(
)

We cancel the spectator ions.

2H
(aq)
2OH(aq)88n2H 2 O(
)

Dividing by 2 gives the net ionic equation

H
(aq)
OH(aq)88nH 2 O(
)

You should now work Exercise 71.

Reactions of weakacids with strong bases also produce salts and water, but there is a signif-
icant difference in the balanced ionic equations because weak acids are only slightlyionized.

EXAMPLE 4-10 Neutralization Reactions

Write balanced formula unit, total ionic, and net ionic equations for the reaction of acetic acid
with potassium hydroxide.

Plan
Neutralization reactions involving metal hydroxide bases produce a salt and water. CH 3 COOH
is a weak acid (see Table 4-6), and so it is written as formula units. KOH is a strong base (see
Table 4-7) and KCH 3 COO is a soluble salt (solubility guidelines 2 and 3), and so both are
written in ionic form.

Solution

CH 3 COOH(aq)
KOH(aq)88nKCH 3 COO(aq)
H 2 O(
)

CH 3 COOH(aq)
[K
(aq)
OH(aq)]88n[K
(aq)
CH 3 COO(aq)]
H 2 O(
)