Oxidation-Reduction
1+
2+
3+
...etc.
4+/- 3-
2-
1-
0
Recognize these!
NO31-
nitrate
NO21-
nitrite
OH1-
hydroxide
ClO21-
chlorite
ClO31-
chlorate
HCO31SO42-
hydrogencarbonate
(bicarbonate)
sulfate
SO32-
sulfite
CO3 2-
carbonate
PO43-
phosphate
NH41+
ammonium
Introduction
• oxidation and reduction can be considered
in terms of…
1. oxidation- substance gains oxygen
reduction- substance loses oxygen
2. oxidation- the loss of electrons
reduction- the gain of electrons
3. oxidation- the oxidation state/# increases
reduction- the oxidation state/# decreases
O
I
L
- oxidation
- is
- loss of electrons
R
I
G
- reduction
- is
- gain of electrons
Oxidation states
Note: oxidation states must be
written with the sign in FRONT:
+2 not 2+
• the oxidation state (number) is the
apparent or theoretical charge of an free
element, molecule, or ion
• oxidation
– a process where the number increases (more
positive because loses neg. electrons)
• Ex: Mg  Mg2+ (aq) + 2e-
• reduction
– a process where the number decreases (less
positive/more negative because gains neg.
electrons)
• Ex: O + 2 e-  O2- (g)
Rules for assigning oxidation states
1
The oxidation number of an free
element is always 0
O2, H2,
Ne, Zn
2
The oxidation number of Hydrogen is
usually +1
HCl,
H2SO4
3
The oxidation number of Oxygen is
usually -2.
H2O, NO2,
etc.
4
Group 1 metals are always +1
Group 2 metals are always +2
Aluminum is always +3
Li, Na…
Mg, Ba..
Al
5
Fluorine is always -1
Other group 17 (halogens) are often
-1
HF, OF2
HI, NaCl,
KBr
6
Oxidation numbers of ions follow the charge
of the ion
S2-, Zn2+
7
The SUM of oxidation numbers is
zero for a neutral compound.
LiMnO4…
8
For polyatomic ions, the SUM is their
charge.
SO42-,
NO31-
Practice Assigning Oxidation Numbers
O2
0
N2O5
O is -2 x 5 = -10
N must equal +10/2 = +5
HClO3
(+1) + (x) + 3(-2) = 0;
x = (+5)
HNO3
O is -2 x 3 = -6; H is +1;
N must equal +5
Ca(NO3)2
O is -2 x 3 = -6 x 2 = -12;
Ca is +2;
N must equal +10/2 = +5
KMnO4
O is -2 x 4 = -8; K is +1;
Mn must equal +7
Fe(OH)3
O is -2 x 3 = -6; Fe is +3;
H is +1 x 3 = +3
K2Cr2O7
O is -2 x 7 = -14;
K is +1 x 2 = +2;
Cr must equal +12/2 = +6
CO32-
x + 3(-2) = -2
x = +4
CN-
N is -3; Charge = -1 means
that C must be +2
K3Fe(CN)6
N is -3 x 6 = -18;
C is +2 x 6 = +12;
K is +1 x 3 =+3; Fe must be +3
CH4
H is +1 x 4 = +4;
C must be -4
Using Oxidation Numbers
• an increase in the oxidation number (more +)
indicates that an atom has lost electrons and
therefore is oxidized
• a decrease in the oxidation number (less +)
indicates that an atom has gained electrons and
therefore reduced
• Example
Zn
+ CuSO4  ZnSO4 + Cu
0
+2 +6-2
+2+6-2
0
Zn: 0  + 2  Oxidized
Cu: +2  0  Reduced
S and O are unchanged
NO31NO21OH1-
ClO21ClO31-
HCO31-
SO42SO32-
CO3 2PO43-
NH41+
Recogniz
Exercise
For each of the following reactions (not
balanced to simplify) find the element
oxidized and the element reduced
Cl2
Cu
+
KBr

KCl +
Br2
+ HNO3  Cu(NO3)2 + NO2 + H2O
HNO3 +
I2
 HIO3 +
NO2
Exercise
For each of the following reactions find
the element oxidized and the element
reduced
Cl2
0
+
KBr
+1-1

KCl +
+1-1
Br loses an electron -- oxidized
Cl gains an electron -- reduced
K remains unchanged at +1
Br2
0
Exercise
For each of the following reactions find
the element oxidized and the element
reduced
Cu + HNO3  Cu(NO3)2 + NO2 + H2O
0
+1+5-2
+2 +5 -2
+4 –2 +1 -2
• Cu increases from 0 to +2. It is oxidized
• Only part of the N in nitric acid changes from +5
to +4. It is reduced
• The nitrogen that ends up in copper nitrate
remains unchanged
Recognize these!
NO31-
nitrate
NO21-
nitrite
OH1-
hydroxide
ClO21-
chlorite
ClO
chlorate
1-
Exercise
For each of the following reactions find
the element oxidized and the element
reduced
HNO3 +
+1+5-2
•
•
•
I2
 HIO3 +
0
+1+5-2
NO2
+4-2
N is reduced from +5 to +4. It is reduced.
I is increased from 0 to +5 It is oxidized
The hydrogen and oxygen remain unchanged.
Agents
• all redox reactions have one element oxidized
and one element reduced
• the compound that supplies the electrons (is
oxidized) is the reducing agent
• the compound that gains the electrons (is
reduced) is the oxidizing agent
Exercise
For each of the following reactions find
the element oxidized, element reduced,
and the agents.
HNO3 +
+1+5-2
•
0
 HIO3 +
NO2
+1+5-2
+4-2
N is reduced from +5 to +4.
–
•
I is the oxidizing agent that caused this
I is oxidized from 0 to +5.
–
•
I2
N is the reducing agent that caused this
The hydrogen and oxygen remain unchanged.