Balancing Equations
for Redox Reactions
Some redox reactions have equations that
must be balanced by special techniques.
MnO4- + 5 Fe2+ + 8 H+
Mn = +7
Fe = +2 ---> Mn2+ + 5 Fe3+ + 4 H2O
Mn = +2 Fe = +3
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Balancing Equations
Consider the reduction of Ag+ ions with
copper metal.
Cu + Ag+
--give--> Cu2+ + Ag
Balancing Equations
Step 1:
Divide the reaction into halfreactions, one for oxidation and the other for
reduction.
Ox
Cu ---> Cu2+
Red
Ag+ ---> Ag
Step 2:
Balance each for mass. Already
done in this case.
Step 3:
Balance each half-reaction for
charge by adding electrons.
Ox
Cu ---> Cu2+ + 2eRed
Ag+ + e- ---> Ag
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Balancing Equations
Step 4:
Multiply each half-reaction by a
factor so that the reducing agent supplies as
many electrons as the oxidizing agent
requires.
Reducing agent
Cu ---> Cu2+ + 2eOxidizing agent
2 Ag+ + 2 e- ---> 2 Ag
Step 5:
Add half-reactions to give the
overall equation.
Cu + 2 Ag+ ---> Cu2+ + 2Ag
The equation is now balanced for
both charge and mass.
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Reduction of VO2+ with Zn
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Balancing Equations
Balance the following in acid solution—
VO2+ + Zn ---> VO2+ + Zn2+
Step 1:
Write the half-reactions
Ox
Zn ---> Zn2+
Red
VO2+ ---> VO2+
Step 2:
Balance each half-reaction for
mass.
Ox
Zn ---> Zn2+
Red
2 H+ + VO2+ ---> VO2+ + H2O
Add H2O on O-deficient side and add H+
on other side for H-balance.
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Balancing Equations
Step 3:
Ox
Red
Step 4:
Ox
Red 2e-
Balance half-reactions for charge.
Zn ---> Zn2+ + 2ee- + 2 H+ + VO2+ ---> VO2+ + H2O
Multiply by an appropriate factor.
Zn ---> Zn2+ + 2e+ 4 H+ + 2 VO2+
---> 2 VO2+ + 2 H2O
Step 5:
Add balanced half-reactions
Zn + 4 H+ + 2 VO2+
---> Zn2+ + 2 VO2+ + 2 H2O
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Tips on Balancing Equations
• Never add O2, O atoms, or
O2- to balance oxygen.
• Never add H2 or H atoms to
balance hydrogen.
• Be sure to write the correct
charges on all the ions.
• Check your work at the end
to make sure mass and
charge are balanced.
• PRACTICE!