Oxidation-Reduction Reactions
“Redox”
LEO SAYS GER
Oxidation and Reduction (Redox)
 Electrons are transferred
 Spontaneous redox rxns can transfer
energy
 Electrons (electricity)
 Heat
 Non-spontaneous redox rxns can be
made to happen with electricity
Oxidation Reduction Reactions
(Redox)
0
1
0
1
2 Na  Cl 2  2 Na Cl
Each sodium atom loses one electron:
0
1

Na  Na  e
Each chlorine atom gains one electron:
0

1
Cl  e  Cl
LEO says GER :
Lose Electrons = Oxidation
1
0
Na  Na  e

Sodium is oxidized
Gain Electrons = Reduction
0

1
Cl  e  Cl
Chlorine is reduced
Not All Reactions are Redox Reactions
Reactions in which there has been no change
in oxidation number are not redox rxns.
Examples:
1 5 2
1
1
1
1
1 5 2
Ag N O3 (aq)  Na Cl (aq)  Ag Cl (s)  Na N O3 (aq)
1 2 1
1
6 2
1
6 2
1
2
2 Na O H (aq)  H 2 S O4 (aq)   Na 2 S O4 (aq)  H 2 O(l )
Rules for Assigning Oxidation Numbers
Rules 1 & 2
1. The oxidation number of any uncombined
element is zero
2. The oxidation number of a monatomic ion
equals its charge
0
0
1
1
2 Na  Cl 2  2 Na Cl
Rules for Assigning Oxidation Numbers
Rules 3 & 4
3. The oxidation number of oxygen in
compounds is -2
4. The oxidation number of hydrogen in
compounds is +1
1
2
H2O
Rules for Assigning Oxidation Number
Rule 5
5. The sum of the oxidation numbers
in the formula of a compound is 0
1
2
H2O
2(+1) + (-2) = 0
H
O
2
2 1
Ca(O H ) 2
(+2) + 2(-2) + 2(+1) = 0
Ca
O
H
Rules for Assigning Oxidation Numbers
Rule 6
6. The sum of the oxidation numbers in the
formula of a polyatomic ion is equal to
its charge
? 2
N O3

? 2
S O4
2
X + 3(-2) = -1
N
O
X + 4(-2) = -2
S
O
 X = +5
 X = +6
The Oxidation Number Rules SIMPLIFIED
1. The sum of the oxidation numbers in
ANYTHING is equal to its charge
2. Hydrogen in compounds is +1
3. Oxygen in compounds is -2
Reducing Agents and Oxidizing Agents
 The substance reduced is the oxidizing agent
 The substance oxidized is the reducing agent
0
1

Na  Na  e
Sodium is oxidized – it is the reducing agent
0
1

Cl  e  Cl
Chlorine is reduced – it is the oxidizing agent
Trends in Oxidation and Reduction
Active metals:
Lose electrons easily
Are easily oxidized
Are strong reducing agents
Active nonmetals:
Gain electrons easily
Are easily reduced
Are strong oxidizing agents
Redox Reaction Prediction
#1
Important Oxidizers
Formed in reaction
MnO4- (acid solution)
MnO4- (basic solution)
MnO2 (acid solution)
Cr2O72- (acid)
CrO42HNO3, concentrated
HNO3, dilute
H2SO4, hot conc
Metallic Ions
Free Halogens
HClO4
Na2O2
H2O2
Mn(II)
MnO2
Mn(II)
Cr(III)
Cr(III)
NO2
NO
SO2
Metallous Ions
Halide ions
ClOHO2
Redox Reaction Prediction
#2
Important Reducers
Formed in reaction
Halide Ions
Free Metals
Metalous Ions
Nitrite Ions
Sulfite Ions
Free Halogens (dil, basic sol)
Free Halogens (conc, basic sol)
C2O42-
Halogens
Metal Ions
Metallic ions
Nitrate Ions
SO42Hypohalite ions
Halate ions
CO2