The Foundations of Chemistry

Solution

Total ionic equation

10[Fe^2 
(aq)
SO 42 (aq)]
2[K
(aq)
MnO 4 (aq)]
8[2H
(aq)
SO 42 (aq)]88n

5[2Fe^3 
(aq)
3SO 42 (aq)]
2[Mn^2 
(aq)
SO 42 (aq)]
8H 2 O(
)


[2 K
(aq)
SO 42 (aq)]

Balanced formula unit equation

10FeSO 4 (aq)
2KMnO 4 (aq)
8H 2 SO 4 (aq)88n

5Fe 2 (SO 4 ) 3 (aq)
2MnSO 4 (aq)
K 2 SO 4 (aq)
8H 2 O(
)

You should now work Exercise 62.

Bleaches sold under trade names such as Clorox and Purex are 5% solutions of sodium
hypochlorite. The hypochlorite ion is a very strong oxidizing agent in basic solution. It
oxidizes many stains to colorless substances.

EXAMPLE 11-17 Balancing Redox Equations (HR method)

In basic solution, hypochlorite ions, ClO, oxidize chromite ions, CrO 2 , to chromate ions,
CrO 42 , and are reduced to chloride ions. Write the balanced net ionic equation for this
reaction.

Plan

We are given the formulas for two reactants and two products; we write as much of the equa-
tions as possible. The reaction occurs in basic solution; we can add OHand H 2 O as needed.
We construct and balance the appropriate half-reactions, equalize the electron transfer, add the
half-reactions, and eliminate common terms.

Solution

CrO 2 
ClO88nCrO 42 
Cl

CrO 2 88nCrO 42  (ox. half-rxn)

CrO 2 
4OH 88nCrO 42 
2H 2 O

CrO 2 
4OH88nCrO 42 
2H 2 O
 3 e (balanced ox. half-rxn)

ClO88nCl (red. half-rxn)

ClO
H 2 O 88nCl
2OH

ClO
H 2 O
 2 e 88nCl
2OH (balanced red. half-rxn)

11-6 Adding H
, OH, or H 2 O to Balance Oxygen or Hydrogen 419

These common household

chemicals, ammonia and bleach,

should never be mixed because they

react to form chloramine (NH 2 Cl), a

very poisonous volatile compound.

NH 3 (aq)
ClO(aq)88n

NH 2 Cl(aq)
OH(aq)