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Electrochemistry
Lesson 2
Oxidation Numbers
Oxidation Numbers
HV2O4-

V6+
Is the above equation oxidation or reduction?
Oxidation numbers are assigned numbers that are used to
determine if oxidation or reduction has occurred.
Rules for Oxidation Numbers
1.
Oxidation #
All elements are zero:
N2
0
Pb
0
Na
0
O2
0
Rules for Oxidation Numbers
2.
Oxidation #
Monatomic Ions are their charge
Na+
+1
Br-
-1
MgSO4
+2
Al2S3
-2
Rules for Oxidation Numbers
3.
Oxidation #
O in a compound is -2
exception: peroxides -1
Na2O
CO2
O2
CH3COOH
HOOH
-2
-2
0
-2
-1
exception
Rules for Oxidation Numbers
4.
Oxidation #
H in a compound is +1
exception: metal hydrides -1 (metal + Hydrogen)
H2O
C12H22O11
H2
NaH
+1
+1
0
-1
Rules for Oxidation Numbers
5.
The sum of the oxidation numbers must equal the
charge
oxidation #s
+6 -2
SO4
Total charge
2-
+6 -8 = -2
-2 x 4
oxidation #s
+5 -2
PO4
Total charge
3-
+5 -8 = -3
oxidation #s
+6
-2
Cr2O7
Total charge
2-
+12 -14 = -2
12 / 2
Together:
Ca(NO3)2
Together:
Na2C4O8
Together:
CH3COOH
Try:
+2 - 1
Zn(OH)42+2 - 4
= -2
Try:
+1 +4 -1
H2PtCl6
+2 +4 -6 = 0
Try:
+1 +3 -2
H3PO3
+3 +3 -6 = 0
Together:
(Hint: treat the polyatomic ion as a whole)
Al(NO3)3
Try:
+1 -1
-2
Na2O2
+2
-4 = 0
-2
Exception - does not work, O must be -1
Exceptions!
+2 +1
-1
CaH2
+2 -2
+2
=0
Exception - does not work, H must be -1
Try:
Fe(H2O)5(OH)2+
0
-1
Fe(H2O)5(OH)2+
? + 0
-1
=
+2
+3
0
-1
Fe(H2O)5(OH)2+
+3 + 0
-1
=
+2
Oxidation Numbers
Increase
+6
+3
HV2O4-

V6+
Is the above equation oxidation or reduction?
If the oxidation number increases it is oxidation.
Using Oxidation Numbers
If the oxidation number of the central atom increases going
from left to right, its oxidation.
+3
Increase is oxidation
ClO2oxidation
reducing agent
4 electrons lost

+7
ClO4-
Using Oxidation Numbers
If the oxidation number of the central atom decreases going
from left to right, its reduction.
+5
decrease is reduction
NO3reduction
2 electrons gained
oxidizing agent

+3
HNO2
Label the oxidizing agent and the reducing agent.
+7
ReO4-
+
+1
IO-

Reduced
Oxidized
Ox Agent
Red Agent
+5
IO3-
+
0
Re
Label the oxidizing agent and the reducing agent.
+3
+5
+5
+2
3As2O3 + 4NO3- + 7H2O + 4H+  6H3AsO4 + 4NO
oxidized
reduced
red agent
ox agent
Homework
Page194
#3,4,5 ,6