Unit 1: Chemistry Basics
1.42 Oxidation-Reduction Reactions
“Redox”
Textbook ch 4.4
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
Rules for Assigning Oxidation Numbers
Rules 1 & 2
1. The oxidation number elements in their
elemental form 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
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 O 4 (aq)   Na 2 S O 4 (aq)  H 2 O(l )
Trends in Oxidation and Reduction
Active metals:
Lose electrons easily
Are easily oxidized
Active nonmetals:
Gain electrons easily
Are easily reduced
Calculate the oxidation #s
Weirdo exceptions: Rules 3 & 4
3. The oxidation number of oxygen in
compounds is -2 (Note: Except in the peroxide ion in which
it has an oxidation number of −1.)
1
1
H2O2
4. The oxidation number of hydrogen in
compounds is +1 (Note: Except when bonded to a metal,
in which it has an oxidation number of -1)
2
1
CaH 2
Oxidation of a metal in Single
Replacement reactions
• Hydrochloric acid reacts with Zinc metal.
• Write oxidation #s
2HCl(aq) +
Zn(s) 
ZnCl2 + H2(g)
Note: As written Zinc replaces the hydrogen
ion in the reaction
WAIT! - How do you know the reaction WILL
occur???
• Will hydrochloric acid oxidize zinc metal?
HCl(aq) + Zn(s)  ??????
Sometimes chemicals don’t react.
Activity series
For single replacement
reactions we consult activity
series to predict if reaction
will occur.
We want to “see” if single
element can oxidized and
“replace” a like ion in the
compound.
Look at single element…is it
higher in the chart than a
like ion
(metal replace metals)
• I will give you a copy of the
activity series to use on test
• metals
–greater activity = easier to lose electrons
–i.e. easier to become a cation
• nonmetals
–greater activity = easier to gain electrons
–i.e. easier to become an anion
• Will an aqueous
solution
hydrochloric acid
oxidize zinc metal?
• If so, write the net
ionic equation
• Because Zn is
higher on activity
series than Hthe reaction will
occur. Zn will replace
H in the compound.
HCl(aq) + Zn(s) 
• calcium and lead
(II) nitrate
• copper and lithium
sulfate