Balancing Redox Equations:

Many redox equations can be balanced through trial
and error! You have been doing this for a least two
years without even knowing it! : )

Do the following:

___Al + ____HCl  ____ AlCl3 + ___ H2


2
Done!
6
2
3
Balancing Redox Reactions:

Another Example: (spectator ions have not been included)

Sn
+ Ag+  Sn2+ + Ag

1.
Assign oxidation numbers.

0

Sn
+1
+2
0
+ Ag+  Sn2+ + Ag

2. Oxidation occurs when the oxidation number increases.
Reduction occurs when the oxidation number decreases.
Write the two half reactions.

Oxidation: Sn  Sn2+

Reduction: Ag+  Ag

3. Use electrons to balance the charges in the half
reactions. In oxidation the electrons appear on the right. In
reduction the electrons appear on the left.

Oxidation: Sn
 Sn2+ + 2e-

Reduction: Ag+ + 1e-  Ag

4. If the number of electrons transferred is not equal
multiple by a whole number so that the number of electrons
lost equals the number gained.

Oxidation: Sn

Reduction: (Ag+ + 1e- Ag) x2
 Sn2+ + 2e-

5.

Oxidation: Sn
 Sn2+ + 2e-

Reduction: 2Ag+ + 2e-
 2Ag

Net Balanced Redox Reaction (notice that the electrons cancel!)

Sn + 2 Ag+  Sn2+ + 2Ag
Add the half reactions:
Steps for Balancing More:

Step 1: Identify and write the two half-reactions.

Step 2: Balance the elements and charges for each half-reaction:

2 a) Balance all the elements except hydrogen and oxygen.

2b)
Balance the oxygen atoms by adding H2O to the appropriate side.

2c)
Balance the hydrogen by adding H+ to the appropriate side.

2d)
Balance the charges by adding electrons to the appropriate side.
Step 3:
Multiply one or both of the half-reactions by a whole number so that the
number of electrons gained and lost is equal.
Step 4:
Combine the half-reactions. Eliminate anything common to the product
and reactant sides.
Step 5:
Check that all of the elements are balanced and that the total charge on
each side is the same.
Example:

Balance: MnO4- + Fe2+  Mn2+ + Fe3+

Step 1:

Reduction:
MnO4-  Mn2+
+7, -2
Oxidation:
+ 2

Step 2:

b)
MnO4-
 Mn2+ + 4H2O

c)
MnO4- + 8H+
 Mn2+ + 4H2O

d)
MnO4- + 8H+ + 5e- Mn2+ + 4H2O
Fe2+  Fe3+
+2
+3

Oxidation:

Fe2+  Fe3+ +1e-

Step 3:

MnO4- + 8H+ + 5e- Mn2+ + 4H2O

(Fe2+
 Fe3+ +1e-) x5

Step 4:

Reduction:
MnO4- + 8H+ + 5e- Mn2+ + 4H2O

Oxidation:
5Fe2+

Net Reaction: MnO4- + 8H+ +5Fe2+ Mn2+ + 4H2O + 5 Fe3+

(electrons are cancelled!)

Step 5:

Notice that the final redox equation is balanced by atom and by charge.
 5 Fe3+ +5e-

Redox reactions are usually too complex to use trial
and error method.

Balance the following example that is in an acidic
solution (assume the presence of H2O and H+):

HNO3 + Fe2+  Fe3+ + NO2

1.

+1+5-2

HNO3 + Fe2+  Fe3+ + NO2
Assign oxidation numbers.
+2
+3
+4 -2

2.

Oxidation: Fe 2+  Fe3+

Reduction: HNO3  NO2

3.
Balance the half reactions. Use water to balance the
oxygen (a), then hydrogen ions to balance the hydrogen(b),
then electrons to balance the charges(c).

a)


Write the half reactions:
Oxidation: Fe 2+  Fe3+
Reduction: HNO3  NO2 + H2O

b)

Oxidation: Fe 2+
 Fe3+

Reduction: HNO3 + H+
 NO2 + H2O

c)

Oxidation: Fe 2+

Reduction: HNO3 + H+ + 1e-  NO2 + H2O
 Fe3+ + 1e-

4.
Multiply the oxidation and reduction equations by
whole numbers so that the number of electrons transferred
is equal.

Oxidation: Fe 2+

Reduction: HNO3 + H+ + 1e-  NO2 + H2O

5.

Net: Fe 2++ HNO3 + H+  Fe3+ + NO2 + H2O

6.
 Fe3+ + 1e-
Add the reactions so that electrons cancel.
Check the equation is balance by charge and atom.
Videos to review the concept:

Watch the following videos to help you balance
redox equations:

http://www.youtube.com/watch?v=KIyGr1snMY&feature=relmfu

http://www.youtube.com/watch?v=-B3RWeC_7oI

http://www.youtube.com/watch?v=TBmwhTzc41o
&feature=fvwrel
Chapter 20.3- Read pages
645  654
CHAPTER 20

Page 647 #17 & #18

Page 649 #19 & #20

Page 652 #21

Page 654 #22,23,24 & 25

Review the study guide on page
656

Complete the odd numbered
questions on page 658

Complete the Standardized Test
Prep

Worksheet Redox - 3