Transcript PowerPoint - Balancing Equations Using Oxidation Numbers

C3H8O + CrO3 + H2SO4 
Cr2(SO4)3 + C3H6O + H2O
13.3b Balancing equations
using oxidation numbers
Balancing chemical equations
• 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 with ox #’s
1.
2.
3.
4.
Write the skeleton equation
Assign oxidation numbers to all atoms
Identify which atoms change oxidation number
Make the number of atoms that change oxidation
number the same on both sides by inserting
temporary coefficients
5. Calculate 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 +3 SO2+ 6 H2O
Step 7:
2:
3:
4:
5:
6:
Identify
Compute
Make
Balance
the
oxidation
which
the
the
number
total
remainder
total
atoms
increase
numbers
of
change
atoms
change
by
in inspection.
oxidation
inthat
oxidation
ox.change
#
1: Assign
Write equation:
oxidation
number
Note: only
equal
number
compounds
the
the
total
same
decrease
that
have
both
by
notsides
already
by
S
(+6
todone
+4)
and
Al
(0
to
+3)
already
for
uson
inserting
multiplication
been balanced
temporary
using
need
appropriate
coefficients
to be balanced
factorshere.
Example 2
Zn + HNO3  Zn(NO3)2 + NH4NO3 + H2O
Balance the following equation:
+2 x 4 = +8
-8
0
+5
0 +1+5 -6 +1+5-6
0 +1+5-2 +1+5 -2
+2
-3
+2+10-12 -3 +4+5 -6 +2 -2
+2 +5 -2 -3 +1+5 -2 +1 -2
4Zn+HNO3+ 9 HNO3 4Zn(NO3)2 + NH4NO3+ 3H2O
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.
Example 3
Balance the following equation:
-5 x 2 = -10
+2 x 5 = +10
+7
+4
1 7 -8 2 6 -8
1 7 -2 2 6 -2
2 6 -8
1 6 -2
+2
2 6 -8 2 6 -8
1 6 -2 2 6 -2
+6
6 18 -24 2-2
3 6 -2 1-2
2KMnO4+2 FeSO4+8H2SO4 K2SO4+2MnSO4+ 5Fe2(SO4)3+8 H2O
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.
Homework
LSM 13.3E