Oxidation and Reduction
Chapter 20
Day 1
INTRO TO REDOX
Definition
 A reaction in which electrons are transferred from one
substance to another
 Oxidation cannot occur without reduction
 Oxidation:
1. Losing of electrons
2. Increasing the oxidation charge (0 to +1 or -2 to -1)
 Reduction
1. Gaining of electrons
2. Reducing the oxidation charge (-1 to -2 or 1 to 0)
Oxidation Numbers
 Number of charges the atom would have in a molecule if
electrons were transferred completely
 Rule #1
Oxidation number of a free element is zero
Example: Mg, H2, Na, P4, S8, K
 Rule #2
Oxidation number of an ion is the charge of the ion
Example: Mg2+, O2−, Al3+, Br−
 Rule #3
Oxidation number for H is +1 when bonded to a nonmetal
*Exception: when bonded to a metal it is −1
Example: HCl, HF, LiH, H2O, NaH, MgH2
 Rule #4
Oxygen’s oxidation number is −2
*Two Exceptions: peroxides and fluorine
Example: H2O2
OF2
Practice
1. HNO3
5. NH3
2. NO3−
6. K3PO4
3. MgCl2
7. CaSO4
4. CO32−
8. SiCl4
Whiteboard Practice
1. Give the oxidation # of the nitrogen atom:
a. N2
c. N2O4
e. NO2−
b. NO
d. N2O5
f. NH4+
2. Determine the oxidation # of the sulfur atom
a. H2S
c. H2SO4
e. HS− g. SO2
b. S
d. S22−
f. SO2
3. Indicate the oxidation # of phosphorus in each:
a. HPO3
c. H3PO3
e. H4P2O7
b. H3PO2
d. H3PO4
f. H5P3O10
Day 2
½ REACTIONS
Redox Reaction
 2 Mg + O2
2 MgO
Oxidation ½ Reaction
 When an electron is lost (electrons appear as products)
 X0
X+1 + e−
 Oxidation # goes up
Reduction ½ Reaction
 When an electron is gained (electrons appear as reactants)
 e− + Y+1
Y0
 Oxidation # goes down
Ways to Remember
L E O goes G E R
OIL
RIG
O L X
S E I
E C D
T A
R T
O I
N O
S N
X S O
S
I
I
D
N
A
G
T
I
O
N
E S A
D
I
U
N
C
I
T
N
I
G
O
N
A
I
N
L
E
C
T
R
O
N
S
E
D
U
C
T
I
O
N
Examples:
 Ex1
Na0 
 Ex2
Fe+3 
Fe+2
 Ex3
N+1 
N+2
 Ex4
Mg+2
Mg0
Na+1
Day 3
REDOX REACTIONS
Example 1:
HCl + Zn  ZnCl2 + H2
Example 2:
Mg + N2  Mg3N2
Example 3:
H2S + Cl2  HCl + S
Example 4:
Fe + O2  Fe2O3
Classwork
Assign oxidation numbers for each element. Then, for the following
balanced redox reactions answer the following questions:
1) Fe(aq) + H2O2(aq)  Fe+2(aq) + 2 OH-1(aq)
a. Write all the oxidation numbers (identify what changes)
b. Write the ½ reactions on the lines
_______________________
________________________
(oxidation or reduction)
(oxidation or reduction)
c. Add the electrons to each ½ reaction
d. Identify each ½ reaction as oxidation or reduction
2) Zn(s) + HCl(aq)  H2(g) + ZnCl2(aq)
3) SbCl5 + 2 KI  SbCl3 + I2 + 2 KCl