Balancing Redox Reactions

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Balancing Redox Reactions

• PreAP Chemistry
• McKinney H.S.
• Mr. Bruce McHam

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An oxidation-reduction reaction (redox) is one in
which one element loses electrons (is oxidized)
and another gains electrons (is reduced)
O.I.L.
R.I.G.
Oxidation Is Loss
Reduction Is Gain
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An oxidation-reduction reaction (redox) is one in
which one element loses electrons (is oxidized)
and another gains electrons (is reduced)
LEO
GER
says
Loss of Electrons is Oxidation
Gain of Electrons is Reduction
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ASSIGN OXIDATION NUMBERS TO EACH ELEMENT IN THE
REACTION
0
1
2
0
Zn Cu ? Zn2 Cu
The oxidation number of zinc increases from zero
to positive two by losing two electrons. ZINC IS
OXIDIZED
The oxidation number of copper decreases from
positive one to zero by gaining one
electron. COPPER IS REDUCED
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ASSIGN OXIDATION NUMBERS TO EACH ELEMENT IN THE
REACTION
1
7
-2
2
2
3
1
-2
H MnO4- Fe2 ? Mn2 Fe3 H2O
The oxidation number of Mn increases from 7 to
2 by gaining five electrons. MANGANESE IS REDUCED
The oxidation number of Fe increases from 2 to
3 by losing one electron. IRON IS OXIDIZED
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1
7
-2
2
2
3
1
-2
H MnO4- Fe2 ? Mn2 Fe3 H2O
MANGANESE IS REDUCED
IRON IS OXIDIZED
The compound that contains the element being
reduced is called the OXIDIZING AGENT
The compound that contains the element being
oxidized is called the REDUCING AGENT
7
1
7
-2
2
2
3
1
-2
H MnO4- Fe2 ? Mn2 Fe3 H2O
MANGANESE IS REDUCED
IRON IS OXIDIZED
MnO4- is the oxidizing agent
Fe2 is the reducing agent
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TO BALANCE A REDOX REACTION
Zn Cu ? Zn2 Cu
Zn ? Zn2
Cu ? Cu
BREAK IT INTO HALF-REACTIONS!!!
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THEN BALANCE EACH REACTION
Zn Cu ? Zn2 Cu
Zn ? Zn2 2e-
Cu ? Cu
FOR BOTH MASS AND CHARGE
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THEN BALANCE EACH REACTION
Zn Cu ? Zn2 Cu
Zn ? Zn2 2e-
1e- Cu ? Cu
FOR BOTH MASS AND CHARGE
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EQUILIZE ELECTRON TRANSFER
Zn Cu ? Zn2 Cu
Zn ? Zn2 2e-
2(1e- Cu ? Cu)
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AND ADD THE REACTIONS TOGETHER
Zn Cu ? Zn2 Cu
Zn ? Zn2 2e-

2(1e- Cu ? Cu)
Zn 2Cu ? Zn2 2Cu
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C2O42- Fe3 ? CO2 Fe2
C2O42- ? CO2
Fe3 ? Fe2
BREAK IT INTO HALF-REACTIONS!!!
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THEN BALANCE EACH REACTION
C2O42- Fe3 ? CO2 Fe2
C2O42- ? 2CO2
Fe3 ? Fe2
FOR BOTH MASS AND CHARGE
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THEN BALANCE EACH REACTION
C2O42- Fe3 ? CO2 Fe2
C2O42- ? 2CO2 2e-
Fe3 ? Fe2
FOR BOTH MASS AND CHARGE
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THEN BALANCE EACH REACTION
C2O42- Fe3 ? CO2 Fe2
C2O42- ? 2CO2 2e-
1e- Fe3 ? Fe2
FOR BOTH MASS AND CHARGE
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EQUILIZE ELECTRON TRANSFER
C2O42- Fe3 ? CO2 Fe2
C2O42- ? 2CO2 2e-
2(1e- Fe3 ? Fe2)
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AND ADD THE REACTIONS TOGETHER
C2O42- Fe3 ? CO2 Fe2
C2O42- ? 2CO2 2e-

2(1e- Fe3 ? Fe2)
C2O42- 2Fe3 ? 2CO2 2Fe2
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BALANCING IN ACIDIC SOLUTION
Ni2 Cr2O72- ? Cr3 Ni4
Ni2 ? Ni4
Cr2O72- ? Cr3
BREAK IT INTO HALF-REACTIONS!!!
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BALANCING IN ACIDIC SOLUTION
Ni2 Cr2O72- ? Cr3 Ni4
Ni2 ? Ni4 2e-
Cr2O72- ? Cr3
THEN BALANCE EACH REACTION
FOR BOTH MASS AND CHARGE
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BALANCING IN ACIDIC SOLUTION
Ni2 Cr2O72- ? Cr3 Ni4
Ni2 ? Ni4 2e-
Cr2O72- ? 2Cr3
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BALANCING IN ACIDIC SOLUTION
Ni2 Cr2O72- ? Cr3 Ni4
Ni2 ? Ni4 2e-
Cr2O72- ? 2Cr3 7H2O
WE CAN BALANCE OXYGEN WITH WATER MOLECULES
BECAUSE THE REACTION HAPPENS IN SOLUTION
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BALANCING IN ACIDIC SOLUTION
Ni2 Cr2O72- ? Cr3 Ni4
Ni2 ? Ni4 2e-
14H Cr2O72- ? 2Cr3 7H2O
WE CAN BALANCE HYDROGEN WITH H IONS SINCE IT IS
AN ACIDIC SOLUTION
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BALANCING IN ACIDIC SOLUTION
Ni2 Cr2O72- ? Cr3 Ni4
Ni2 ? Ni4 2e-
6e- 14H Cr2O72- ? 2Cr3 7H2O
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BALANCING IN ACIDIC SOLUTION
Ni2 Cr2O72- ? Cr3 Ni4
3(Ni2 ? Ni4 2e-)
6e- 14H Cr2O72- ? 2Cr3 7H2O
EQUILIZE ELECTRON TRANSFER
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BALANCING IN ACIDIC SOLUTION
Ni2 Cr2O72- ? Cr3 Ni4
3(Ni2 ? Ni4 2e-)

6e- 14H Cr2O72- ? 2Cr3 7H2O
14H Cr2O72- 3Ni2 ? 3Ni4 2Cr3 7H2O
AND ADD THE REACTIONS TOGETHER
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BALANCING IN ACIDIC SOLUTION
Ag NO3- ? Ag NO
Ag ? Ag
NO3- ? NO
SEE IF YOU CAN PREDICT EACH STEP BEFORE ADVANCING
THE SLIDE
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BALANCING IN ACIDIC SOLUTION
Ag NO3- ? Ag NO
Ag ? Ag 1e-
NO3- ? NO
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BALANCING IN ACIDIC SOLUTION
Ag NO3- ? Ag NO
Ag ? Ag 1e-
NO3- ? NO 2H2O
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BALANCING IN ACIDIC SOLUTION
Ag NO3- ? Ag NO
Ag ? Ag 1e-
4H NO3- ? NO 2H2O
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BALANCING IN ACIDIC SOLUTION
Ag NO3- ? Ag NO
Ag ? Ag 1e-
3e- 4H NO3- ? NO 2H2O
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BALANCING IN ACIDIC SOLUTION
Ag NO3- ? Ag NO
3(Ag ? Ag 1e-)
3e- 4H NO3- ? NO 2H2O
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BALANCING IN ACIDIC SOLUTION
Ag NO3- ? Ag NO
3(Ag ? Ag 1e-)
3e- 4H NO3- ? NO 2H2O

4H NO3- 3Ag ? 3Ag NO 2H2O
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BALANCING IN BASIC SOLUTION
To balance a redox reaction in a basic solution,
where hydroxide (OH-) ions outnumber hydrogen
(H) ions, FIRST BALANCE IT AS IF IT WERE IN
ACIDIC SOLUTION
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BALANCING IN BASIC SOLUTION
In the previous example
4H NO3- 3Ag ? 3Ag NO 2H2O
is balanced in acidic solution
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BALANCING IN BASIC SOLUTION
To balance in basic solution
4H NO3- 3Ag ? 3Ag NO 2H2O
4OH-
4OH-
add an amount of hydroxide equal to the amount
of H to each side of the reaction
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BALANCING IN BASIC SOLUTION
To balance in basic solution
4H NO3- 3Ag ? 3Ag NO 2H2O
4OH-
4OH-
Combine the H and OH- to make water
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BALANCING IN BASIC SOLUTION
To balance in basic solution
4H2O NO3- 3Ag ? 3Ag NO 2H2O
4OH-
Combine the H and OH- to make water
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BALANCING IN BASIC SOLUTION
To balance in basic solution
2
4H2O NO3- 3Ag ? 3Ag NO 2H2O
4OH-
since there is water on both sides, we can
cancel some out
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BALANCING IN BASIC SOLUTION
To balance in basic solution
2H2O NO3- 3Ag ? 3Ag NO 4OH-
and rewrite the final balanced equation