The Foundations of Chemistry

Solution

I 2 
S 2 O 32 88nI
S 4 O 62 

I 2 88nI (red. half-reaction)

I 2 88n 2I

I 2 
 2 e 88n2I (balanced red. half-reaction)

S 2 O 32 88nS 4 O 62  (ox. half-reaction)

2S 2 O 32  88nS 4 O 62 

2S 2 O 32 88nS 4 O 62 
 2 e (balanced ox. half-reaction)

Each balanced half-reaction involves a transfer of two electrons. We add these half-reactions
and cancel the electrons.

I 2 
 2 e88n2I

2S 2 O 32 88nS 4 O 62 
 2 e

I 2 (s)
2S 2 O 32 (aq)88n2I(aq)
S 4 O 62 (aq)

ADDING H
, OH, OR H 2 O TO BALANCE OXYGEN

OR HYDROGEN

Frequently we need more oxygen or hydrogen to complete the mass balance for a reac-
tion or half-reaction in aqueous solution. We must be careful, however, not to introduce
other changes in oxidation number or to use species that could not actually be present in
the solution. We cannot add H 2 or O 2 to equations because these species are not present
in aqueous solutions. Acidic solutions do not contain significant concentrations of OH
ions. Basic solutions do not contain significant concentrations of H
ions.

In acidic solution: We add only H
or H 2 O (notOHin acidic solution).

In basic solution: We add only OHor H 2 O (notH
in basic solution).

The following chart shows how to balance hydrogen and oxygen.

11-6

Each I 2 gains 2e. I 2 is reduced; it is

the oxidizing agent.

Each S 2 O 32 ion loses an e. S 2 O 32 

is oxidized; it is the reducing agent.

11-6 Adding H
, OH, or H 2 O to Balance Oxygen or Hydrogen 417

In Acidic Solution:

In Basic Solution:

To balance O: and To balance H:

88n

Add H then

2 O Add H

To balance O: To balance H:

For eachO needed ,For eachH needed,

(1) add twoOHto side needing O (1) add oneH 2 O to side needing H

and and

(2) add oneH 2 O to other side (2) add oneOHto other side

and

88n

then