Oxidation-Reduction Reactions Chemistry Worksheet Page 11
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6.2 Oxidation Numbers
The oxidation numbers for the individual atoms in the second reaction are
below.
Oxidation number increases, oxidized
+1 −2
0
0
+1 +5 −2
+ 5O
+ 6H
P
O
4H
PO
4
2
2
3
4
Oxidation number decreases, reduced
Phosphorus atoms and oxygen atoms change their oxidation numbers, so
the reaction is redox. Each phosphorus atom changes its oxidation number
from zero to +5, so the phosphorus atoms in P
are oxidized, and P
is the
4
4
reducing agent. Each oxygen atom in O
changes its oxidation number from
2
zero to −2, so the oxygen atoms in O
are reduced, and O
is the oxidizing
2
2
agent.
c. Finally, let’s consider the third reaction.
Hydrogen in a molecule,
Hydrogen in a polyatomic ion,
oxidation number +1
oxidation number +1
+ NH
H
PO
(NH
)
PO
3
4
3
4
3
4
Combined oxygen, oxidation number −2
We know from part (b) that the oxidation number of the phosphorus atoms
is +5.
in H
PO
3
4
The oxidation number of the nitrogen atom in NH
is calculated below.
3
(ox # N) + 3(ox # H) = 0
(ox # N) + 3(+1) = 0
(ox # N) = −3
We can determine the oxidation number of each nitrogen atom in
(NH
)
PO
in two ways, either from the whole formula or from the formula
4
3
4
for the ammonium ion alone.
3(ox # N) + 12(ox # H) + (ox # P) + 4(ox # O) = 0
3(ox # N) + 12(+1) + (+5) + 4(−2) = 0
(ox # N) = −3
or (ox # N) + 4(ox # H) = +1
(ox # N) + 4(+1) = +1
(ox # N) = −3
The oxidation numbers for the individual atoms in this reaction are below.
+1
+5
−2
−3 +1
−3 +1
+5
−2
H
PO
+ NH
(NH
)
PO
3
4
3
4
3
4
None of the atoms change their oxidation number, so the reaction is not redox.
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