C3H8O + CrO3 + H2SO4 
Cr2(SO4)3 + C3H6O + H2O
Balancing equations using
oxidation numbers
Review: balancing chemical equations
• Balance the following chemical reaction:
CuCl2 + Al  Cu + AlCl3
• In the past (e.g. in grade 11) we balanced
equations by “inspection”.
• Balancing equations relied on having equal
numbers of atoms on each side of the equation
• We can balance equations using oxidation #s.
• This relies on the idea that the number of
electrons lost by an element must be equal to
the number gained by a different element.
• In other words the total gain in oxidation
numbers must be equal to the total lost.
Using Oxidation Numbers
+2 -2
0
0
+3 -3
+2 -1
0
0
+3 -1
CuCl2 + Al  Cu + AlCl3
Notice: Cu has gained 2e– (oxidation #  by 2)
Notice: Al has lost 3e– (oxidation #  by 3)
But, number of e– gained must equal e– lost
Multiply Cu by 3, Al by 2: change is 6 for both
+6
-6
change
+6
0
0
+6
total
+2 -2
0
0
+3 -3
0
0
+3 -1
oxidation # +2 -1
3CuCl2 + 2Al  3Cu + 2AlCl3
total
oxidation #
•
•
•
•
Steps to balancing equations
1. Write the skeleton equation
2. Assign oxidation numbers to all atoms
3. Identify which atoms change oxidation number
4. Make the number of atoms that change
oxidation number the same on both sides by
inserting temporary coefficients
5. Compute the total change in oxidation number
6. Make the total increase in oxidation number
equal the total decrease by multiplication
using appropriate factors
7. Balance the remainder by inspection. Do not
change what has been balanced. Compounds
with elements that have changed in one case
but not in another are considered twice.
Example 1
Balance the following equation:
-2 x 3 = -6
+6
change
+6
0
0
total +2+6 -8
0
ox. # +1+6 -2
3H2SO4+3H2SO4 +2Al 
+6
+4
+6 +18-24 +4 -4 +2 -2
+3 +6 -2 +4 -2 +1 -2
Al2(SO4)3 + 3SO2 + 6H2O
Step 1:
2: Assign
3:
4:
5:
6:
7:
Identify
Compute
Make
Balance
Write
equation:
the
oxidation
which
the
the
number
total
remainder
total
atoms
increase
numbers
of
change
atoms
change
by
in inspection.
oxidation
inthat
oxidation
ox.change
#
oxidation
number
Note: only
equal
compounds
the
the
total
same
decrease
that
have
both
by
notsides
already
by
Snumber
already
(+6
todone
+4)
and
for
Al
uson
(0
to
+3)
inserting
multiplication
been balanced
temporary
using
need
appropriate
coefficients
to be balanced
factorshere.
Practice
Zn + HNO3  Zn(NO3)2 + NH4NO3 + H2O
PE 4c (pg. 453)
Balance the following equation:
+2 x 4 = +8
-8
0
+5
+2
-3
0 +1+5 -6 +1+5-6 +2+10-12 -3 +4+5 -6 +2 -2
0 +1+5-2 +1+5 -2 +2 +5 -2 -3 +1+5 -2 +1 -2
4 Zn+ HNO3+ 9HNO3  4Zn(NO3)2 + NH4NO3+ 3
H2O
Step 1:
4: Assign
6:
2:
7:
3:
5:
Make equation:
Balance
Identify
Compute
Write
the
oxidation
which
the
the
number
total
remainder
total
atoms
increase
numbers
of
change
atoms
change
by
in inspection.
oxidation
inthat
oxidation
ox.change
#
oxidation
number
Note: only
equal
number
compounds
the
total
same
decrease
that
on
have
both
not
already
by
Zn
already
(0 the
to done
+2)
and
for
N
us(+5
toby
-3)sides
inserting
multiplication
been balanced
temporary
using
need
appropriate
coefficients
to be balanced
factorshere.
Assignment:
PE 4b (pg. 453) & 12.21 (pg. 484)
PE 4b (pg. 453)
Balance the following equation:
-5 x 2 = -10
+2 x 5 = +10
+7
+2
+6
+4
1 7 -8 2 6 -8 2 6 -8 2 6 -8 2 6 -8 6 18 -24 2-2
1 7 -2 2 6 -2 1 6 -2 1 6 -2 2 6 -2 3 6 -2 1-2
2KMnO4+2FeSO4+8 H2SO4 K2SO4+2MnSO4+ 5Fe2(SO4)3+8H2O
10
Step 3:
7: Assign
4:
5:
1:
6:
2:
Balance
Compute
Write
Make
Identify
equation:
the
oxidation
which
the
the
number
total
remainder
total
atoms
increase
numbers
of
change
atoms
change
by
in inspection.
oxidation
inthat
oxidation
ox.change
#
oxidation
number
Note: only
equal
number
compounds
the
the
total
decrease
that
on
have
both
by
not
already
by
already
Mn
(+7
to
done
+2)same
for
and
usFe
(+2
tosides
+3)
inserting
multiplication
been balanced
temporary
using
need
appropriate
coefficients
to be balanced
factorshere.
Example 2
Balance the following equation:
-2 x 3 = -6
+6
+6
0
+6
+4
1 7 -8 2 6 -8
0
+6 +18 -24 +4 -4 +2 -2
1 7 -2 1 6 -2
0
+3 +6 -2 +4 -2 +2 -2
KMnO4+ H2C2O4+ H22SO4 CO2+ K2SO4+3 MnSO4+3 H2O
Step 1:
2: Assign
3:
4:
5:
6:
7:
Identify
Compute
Make
Balance
Write
equation:
the
oxidation
which
the
the
number
total
remainder
total
atoms
increase
numbers
of
change
atoms
change
by
in inspection.
oxidation
inthat
oxidation
ox.change
#
oxidation
number
Note: only
equal
compounds
the
the
total
same
decrease
that
have
both
by
notsides
already
by
Snumber
already
(+6
todone
+4)
and
for
Al
uson
(0
to
+3)
inserting
multiplication
been balanced
temporary
using
need
appropriate
coefficients
to be balanced
factorshere.
Try PE 4 a, b (p 453). Try 12.21 (p 484)
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