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Balancing Redox Equations
Balancing Redox
Equations
based on equal total increases and decreases in
oxidation #’s
Steps: page 645 in textbook
1.  Write equation and assign oxidation #’s.
2.  Determine which element is oxidized and which is reduced,
and determine the change in oxidation # for each.
3.  Connect the atoms that change ox. #’s using a bracket; write
the change in ox. # at the midpoint of each bracket.
4.  Choose coefficients that make the total increase in ox. # = the
total decrease in ox. #.
5.  Balance the remaining elements by inspection.
If needed, reactions that take place in acidic or
basic solutions can be balanced as follows:
Acidic:
Basic:
add H2O to the side
needing oxygen
balance as if in acidic sol’n
then add H+ to balance the add enough OH- to both
hydrogen
sides to cancel out each H
+ (making H O) & then
2
cancel out water as
appropriate
In balancing redox equations, the # of
electrons lost in oxidation (the increase in
ox. #) must equal the # of electrons
gained in reduction (the decrease in ox. #)
l 
There are 2 methods for balancing redox
equations:
1.  Change in Oxidation-Number Method
2.  The Half-Reaction Method
Example
1. Change in Oxidation-Number Method:
l 
l 
3 (+4) oxidation
3S
0
+ 4 HNO3 è
3 SO2 + 4 NO
+1 +5 -2
+4 -2
+2 -2
+ 2 H 2O
+1
-2
4 (-3) reduction
Step 1:
Step 2:
Oxidation
numbers
Which was oxidized? S. By how much?
0 to +4 = change of +4
N. By how much? +5 to +2 = change of -3
Which was reduced?
Example:
Balance the following equation,
assuming it takes place in acidic solution.
Page 648 in textbook
+ 8H+ +1
ClO4- + I+7
-2
Step 1:
Oxidation
numbers
è Cl- +
8
-1
-8
Step 2: Which was
oxidized?
Which was reduced?
Step 5: Balance acid soln with
water…
4
-1
I2 + H2O
4
0
Iodine, -1 to 0 = +1
Chlorine, +7 to -1 = -8
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2. The Half-Reaction Method:
Example:
Balance the following equation,
assuming it takes place in basic solution.
ClO4-
+
I-
è Cl- +
l 
I2
Steps:
1.  Write equation and assign oxidation #’s.
2.  Determine which element is oxidized and which is reduced,
and determine the change in oxidation # for each.
3.  Construct unbalanced oxidation and reduction half reactions.
4.  Balance the elements and the charges (by adding electrons
as reactants or products) in each half-reaction.
5.  Balance the electron transfer by multiplying the balanced halfreaction by appropriate integers.
6.  Add the resulting half-reaction and eliminate any common
terms to obtain the balanced equation.
Cancel H+ by adding OH- to both sides.
H+ + OH- = H2O
Cancel H2O
Example: Balance the following
using the half-reaction method:
HNO3 + H2S è
NO
+
S
+
separate and balance the oxidation and
reduction half-reactions.
Example: Balance the following
using the half-reaction method:
H 2O
2
HNO3 + 3 H2S è
+1 +5 -2
Step 1: oxidation
numbers
Step 2: Which was
oxidized?
S. -2 to 0 = +2
Which was
reduced?
N. +5 to +2 = -3
+1 -2
2
NO
+3 S
+2 -2
+
Step 3: unbalanced half-rxns
x3 S2- à S + 2 e-
3S2- à 3S + 6e-
x2 N5+ + 3 e-à N2+ 2 N5+ + 6e- à 2 N2+
Step 4: balance the
half – rxns by adding
electrons
---------------------------------3S2- + 2N5+ à 3S + 2N2+
Step 6: Add half-rxns and
Step 5: balance electron cancel any common terms
transfer by multiplying by to obtain a balanced eq.
appropriate integers
Now, balance the eq. w/
coefficients
If needed, reactions that take place in basic
solutions can be balanced as follows:
Acidic:
Basic:
2. 
3. 
4. 
5. 
Write separate eq’ns for the oxidation & reduction half-rxns
For each half-rxn:
a) 
Balance all the elements except H and O
b) 
Balance O using H2O
c) 
Balance H using H+
d) 
Balance charge using elections
If necessary, multiply one or both balanced half-rxns by an integer
to equalize the number of electrons transferred in the two half-rxns.
Add the half-reactions and cancel the identical species (those
appearing in reactants and products)
Check that the elements and charges are balanced
H 2O
+1 -2
If needed, reactions that take place in acidic
solutions can be balanced as follows:
1. 
4
0
1. 
Balance as if in acidic sol’n (follow ALL steps for acidic redox)
2. 
Add enough OH- to both sides to cancel out each H+ (making H2O)
& then cancel out water as appropriate
3. 
Check that the elements and charges are balanced
2
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HOMEWORK:
Balance the following using the
half-rxn method…
In acidic sol’n:
a) Cu + NO3- → Cu2+ + NO
b) Cr2O72- + Cl- → Cr3+ + Cl2
c) Pb + PbO2 + H2SO4 → PbSO4
In basic sol’n:
a) Al + MnO4- → MnO2 + Al(OH)4b) Cl2 → Cl- + OClc) NO2- + Al → NH3 + AlO2-
3