Balancing of Redox Reactions
OR
For reactions in a basic medium, balance O and H atoms as balanced in acidic medium. Then addOH– OH– OH–
Rules for
Balancing Redox Reactions by Ion Electron Method
1. Convert molecular equation into ionic
equation. Split ionic equation into two half-reactions or equations (one for
the species that is oxidized and its products and for the species that is
reduced and its products).
2. Balance the atoms other than O and H by
inserting suitable coefficients in each half reaction separately.
3 (a). For reactions in an acidic medium, add one H2O for
each O atom to balance O and one H+ for each H atom to balance H. The O atoms are
balanced first.
(b) For reactions in a basic medium, to balance O atom add two OH– OH–
OR
For reactions in a basic medium, balance O and H atoms as balanced in acidic medium. Then add
4.Balance the charge in each half
equations by inserting e¯ (electrons) as a reactant or product.
5. Equalize the loss and gain of electrons
in both half equations by multiplying one or both half reactions by appropriate
co-efficients.
6. Add the two half equations after
canceling electrons. If H+, OH–
Balance the following equation in acidic medium by I.E.M.
Solution
First
molecular equation splits into ionic form and then oxidation number of only
those species have been written who have undergone any change (the species that
do not undergo any change in oxidation no. do not appear in equation).
Separate ionic equation into two
half-reactions:-
Assignment
Equations
Q1. Balance the
following equations by I.E.M. in Acidic Medium.
1.
|
MnO4¯
|
+
|
Fe2+
|
¾®
|
Mn2+
|
+
|
Fe3+
|
|
2.
|
MnO4¯
|
+
|
Cl¯
|
¾®
|
Mn2+
|
+
|
Cl2
|
|
3.
|
MnO4¯
|
+
|
I¯
|
¾®
|
Mn2+
|
+
|
I2
|
|
4.
|
MnO4¯
|
+
|
C2O42¯
|
¾®
|
Mn2+
|
+
|
CO2
|
|
5.
|
MnO4¯
|
+
|
NO2¯
|
¾®
|
Mn2+
|
+
|
NO3¯
|
|
6.
|
MnO4¯
|
+
|
H2O2
|
¾®
|
Mn2+
|
+
|
O2
|
|
7.
|
MnO4¯
|
+
|
SO2
|
¾®
|
Mn2+
|
+
|
H2SO4
|
|
8.
|
MnO4¯
|
+
|
SO32¯
|
¾®
|
Mn2+
|
+
|
SO42¯
|
|
9.
|
Cr2O72¯
|
+
|
Fe2+
|
¾®
|
Cr3+
|
+
|
Fe3+
|
|
10.
|
Cr2O72¯
|
+
|
Zn
|
¾®
|
Cr3+
|
+
|
Zn2+
|
|
11.
|
Cr2O72¯
|
+
|
Cl¯
|
¾®
|
Cr3+
|
+
|
Cl2
|
|
12.
|
Cr2O72¯
|
+
|
I¯
|
¾®
|
Cr3+
|
+
|
I2
|
|
13.
|
Cr2O72¯
|
+
|
S2¯
|
¾®
|
Cr3+
|
+
|
S
|
|
14.
|
Cr2O72¯
|
+
|
I2
|
¾®
|
Cr3+
|
+
|
IO31¯
|
|
15.
|
Cr2O72¯
|
+
|
Cl2
|
¾®
|
Cr3+
|
+
|
ClO31¯
|
|
16.
|
Cr2O72¯
|
+
|
SO2
|
¾®
|
Cr3+
|
+
|
SO42¯
|
|
17.
|
Cr2O72¯
|
+
|
SO32¯
|
¾®
|
Cr3+
|
+
|
SO42¯
|
|
18.
|
Cr(OH)3
|
+
|
H2O2
|
¾®
|
CrO42¯
|
+
|
H2O
|
|
19.
|
BrO31¯
|
+
|
I¯
|
¾®
|
Br¯
|
+
|
I2
|
|
20.
|
NO3¯
|
+
|
H2S
|
¾®
|
NO
|
+
|
S
|
[For
balanced equation see next page]
Q2. Balance the
following equations in basic medium by I.E.M.
1.
|
Cr(OH)3
|
+
|
H2O2
|
¾®
|
CrO42¯
|
+
|
OH¯
|
|
2.
|
Cr(OH)3
|
+
|
SO42¯
|
¾®
|
CrO42¯
|
+
|
SO32¯
|
|
3.
|
ClO¯
|
+
|
I¯
|
¾®
|
Cl¯
|
+
|
I2
|
|
4.
|
S2¯
|
+
|
I2
|
¾®
|
SO42¯
|
+
|
I¯
|
|
5.
|
Al
|
+
|
H2O
|
¾®
|
Al(OH)4¯
|
+
|
H2
|
|
6.
|
Cr2O3
|
+
|
Br2
|
¾®
|
CrO42¯
|
+
|
Br¯
|
|
7.
|
MnO4¯
|
+
|
I¯
|
¾®
|
MnO2
|
+
|
IO3¯
|
|
8.
|
MnO4¯
|
+
|
H2O2
|
¾®
|
MnO2
|
+
|
O2
|
|
9.
|
MnO4¯
|
+
|
SO32¯
|
¾®
|
MnO2
|
+
|
SO42¯
|
|
10.
|
MnO4¯
|
+
|
SO32¯
|
¾®
|
MnO42¯
|
+
|
SO42¯
|
|
11.
|
CrO42¯
|
+
|
I¯
|
¾®
|
Cr3+
|
+
|
IO3¯
|
[For
balanced equation see below]
ANSWERS OF Q1 (IN
ACIDIC MEDIUM)
1.
|
MnO4¯
|
+
|
8H+
|
+
|
5Fe2+
|
¾®
|
Mn2+
|
+
|
5Fe3+
|
+
|
4H2O
|
(+17 = +17)
|
2.
|
2MnO4¯
|
+
|
16H+
|
+
|
10Cl¯
|
¾®
|
2Mn2+
|
+
|
5Cl2
|
+
|
8H2O
|
(+4 = +4)
|
3.
|
2MnO4¯
|
+
|
16H+
|
+
|
10I¯
|
¾®
|
2Mn2+
|
+
|
5I2
|
+
|
8H2O
|
(+4 = +4)
|
4.
|
2MnO4¯
|
+
|
16H+
|
+
|
5C2O42¯
|
¾®
|
2Mn2+
|
+
|
10CO2
|
+
|
8H2O
|
(+4 = +4)
|
5.
|
2MnO4¯
|
+
|
6H+
|
+
|
5NO2¯
|
¾®
|
2Mn2+
|
+
|
5NO3¯
|
+
|
3H2O
|
(–1 = –1)
|
6.
|
2MnO4¯
|
+
|
6H+
|
+
|
5H2O2
|
¾®
|
2Mn2+
|
+
|
5O2
|
+
|
8H2O
|
(+4 = +4)
|
7.
|
2MnO4¯
|
+
|
6H+
|
+
|
5SO2 + 2H2O
|
¾®
|
2Mn2+
|
+
|
5H2SO4
|
(+4 = +4)
|
||
8.
|
2MnO4¯
|
+
|
6H+
|
+
|
5SO32¯
|
¾®
|
2Mn2+
|
+
|
5SO42¯
|
+
|
3H2O
|
(–6 = –6)
|
9.
|
Cr2O72¯
|
+
|
14H+
|
+
|
6Fe2+
|
¾®
|
2Cr3+
|
+
|
6Fe3+
|
+
|
7H2O
|
(+24 = +24)
|
10.
|
Cr2O72¯
|
+
|
14H+
|
+
|
3Zn
|
¾®
|
2Cr3+
|
+
|
3Zn2+
|
+
|
7H2O
|
(+12 = +12)
|
11.
|
Cr2O72¯
|
+
|
14H+
|
+
|
6Cl¯
|
¾®
|
2Cr3+
|
+
|
3Cl2
|
+
|
7H2O
|
(+6 = +6)
|
12.
|
Cr2O72¯
|
+
|
14H+
|
+
|
6I¯
|
¾®
|
2Cr3+
|
+
|
3I2
|
+
|
7H2O
|
(+6 = +6)
|
13.
|
Cr2O72¯
|
+
|
14H+
|
+
|
3S2¯
|
¾®
|
2Cr3+
|
+
|
3S
|
+
|
7H2O
|
(+6 = +6)
|
14.
|
5Cr2O72¯
|
+
|
34H+
|
+
|
3I2
|
¾®
|
10Cr3+
|
+
|
6IO3¯
|
+
|
17H2O
|
(+24 = +24)
|
15.
|
5Cr2O72¯
|
+
|
34H+
|
+
|
3Cl2
|
¾®
|
10Cr3+
|
+
|
6ClO3¯
|
+
|
17H2O
|
(+24 = +24)
|
16.
|
Cr2O72¯
|
+
|
2H+
|
+
|
3SO2
|
¾®
|
2Cr3+
|
+
|
3SO42¯
|
+
|
H2O
|
(0 = 0)
|
17.
|
Cr2O72¯
|
+
|
8H+
|
+
|
3SO32¯
|
¾®
|
2Cr3+
|
+
|
3SO42¯
|
+
|
4H2O
|
(0 = 0)
|
18.
|
2Cr(OH)3
|
+
|
3H2O2
|
¾®
|
2CrO42¯
|
+
|
4H2O
|
+
|
4H+
|
(0 = 0)
|
||
19.
|
BrO31¯
|
+
|
6H+
|
+
|
6I¯
|
¾®
|
Br¯
|
+
|
3I2
|
+
|
3H2O
|
(–1 = –1)
|
20.
|
2NO3¯
|
+
|
2H+
|
+
|
3H2S
|
¾®
|
2NO
|
+
|
3S
|
+
|
4H2O
|
(0 = 0)
|
ANSWERS OF Q2 (IN BASIC MEDIUM)
1.
|
2Cr(OH)3
|
+
|
4OH¯
|
+
|
3H2O2
|
¾®
|
2CrO42¯
|
+
|
8H2O
|
(–4 = –4)
|
||||
2.
|
2Cr(OH)3
|
+
|
4OH¯
|
+
|
3SO42¯
|
¾®
|
2CrO42¯
|
+
|
5H2O
|
+
|
2SO32¯
|
(–10 = –10)
|
||
3.
|
ClO¯
|
+
|
H2O
|
+
|
2I¯
|
¾®
|
Cl¯
|
+
|
2OH¯
|
+
|
I2
|
(–3 = –3)
|
||
4.
|
S2¯
|
+
|
8OH¯
|
+
|
4I2
|
¾®
|
SO42¯
|
+
|
4H2O
|
+
|
8I2
|
(–10 = –10)
|
||
5.
|
2Al
|
+
|
2OH¯
|
+
|
6H2O
|
¾®
|
2Al(OH)41¯
|
+
|
3H2
|
(–2 = –2)
|
||||
6.
|
Cr2O3
|
+
|
10OH¯
|
+
|
3Br2
|
¾®
|
2CrO42¯
|
+
|
5H2O
|
+
|
6Br¯
|
(–10 = –10)
|
||
7.
|
2MnO4¯
|
+
|
H2O
|
+
|
I¯
|
¾®
|
2MnO2
|
+
|
2OH¯
|
+
|
IO3¯
|
(–3 = –3)
|
||
8.
|
2MnO4¯
|
+
|
3H2O2
|
¾®
|
2MnO2
|
+
|
2OH¯
|
+
|
3O2
|
+
|
2H2O
|
(–2 = –2)
|
||
9.
|
2MnO41¯
|
+
|
H2O
|
+
|
3SO32¯
|
¾®
|
2MnO2
|
+
|
2OH¯
|
+
|
3SO42¯
|
(–8 = –8)
|
||
10.
|
2MnO41¯
|
+
|
2OH¯
|
+
|
SO32¯
|
¾®
|
2MnO42¯
|
+
|
H2O
|
+
|
SO42¯
|
(–6 = –6)
|
||
11.
|
2CrO42¯
|
+
|
5H2O
|
+
|
I¯
|
¾®
|
2Cr3+
|
+
|
IO3¯
|
+
|
10OH¯
|
(–5 = –5)
|
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