4/20/14
Oxidation-Reduction
(Redox) Reactions
Redox Reactions
Chapter 18
+
O2
→
“redox” reactions: rxns in which electrons are
transferred from one species to another
l  oxidation & reduction always occur
simultaneously
l  we use OXIDATION NUMBERS to keep
track of electron transfers
l 
Rules for Assigning
Oxidation Numbers:
Rules for Assigning
Oxidation Numbers:
1) the ox. state of any free (uncombined)
element is zero.
2) The ox. state of an element in a simple ion is
the charge of the ion.
Mg2+ → oxidation of Mg is +2
l 
Ex: Na, S, O2, H2, Cl2, O3
Rules for Assigning
Oxidation Numbers:
l 
Rules for Assigning
Oxidation Numbers:
3) the ox. # for hydrogen is +1
4) the ox. # of fluorine is always –1.
(unless combined with a metal, then it has an ox. # of –1)
Ex: NaOH (H bonded to O) v. NaH (H bonded to Na)
H = +1
H = -1
1
4/20/14
Rules for Assigning
Oxidation Numbers:
Rules for Assigning
Oxidation Numbers:
5) the ox. # of oxygen is usually –2.
6) in any neutral compound, the sum of the
oxidation #’s = zero.
Why USUALLY? Not -2 when it’s in a
peroxide, such as hydrogen peroxide:
H2O2
Rules for Assigning
Oxidation Numbers:
Rules for Assigning
Oxidation Numbers:
7) in a polyatomic ion, the sum of the oxidation
#’s = the overall charge of the ion.
**use these rules to assign oxidation #’s;
assign known #’s first, then fill in the #’s
for the remaining elements:
Examples: Assign oxidation #’s to
each element:
Examples: Assign oxidation #’s to
each element:
a)
NaNO3
b)
SO32+4 -2
Na = +1
O = -2
Therefore, N = +5
O = -2, therefore S must = +4 to balance the
charges and have an overall charge of 2-
2
4/20/14
Examples: Assign oxidation #’s to
each element:
Examples: Assign oxidation #’s to
each element:
Do on your own:
c)
HCO3-
d) 
H3PO4
e) 
Cr2O72-
f) 
K2Sn(OH)6
+1 +4 -2
Definitions
To help you remember…
OIL RIG
Oxidation: the process of losing electrons
(ox # increases)
l  Reduction: the process of gaining electrons
(ox # decreases)
l  Oxidizing agents: species that cause
oxidation (they are reduced)
l  Reducing agents: species that cause
reduction (they are oxidized)
l 
OR…
LEO GER
l 
l 
Losing Electrons → Oxidation
Gaining Electrons → Reduction
l 
l 
Oxidation Is Loss
Reduction Is Gain
Are all rxns REDOX rxns?
You must determine this…
l 
a reaction is “redox” if a change in oxidation
# happens; if no change in oxidation #
occurs, the reaction is nonredox.
3
4/20/14
Examples
Examples
MgCO3 is an ionic
compound, so what is
Mg’s charge in an ionic
compound?
Which oxidation numbers do we already
know?
MgCO3 è MgO
+2
+4 -2
+2
The carbonate ion CO3
is the other ion, let’s
figure out C because we
already know O.
-2
+
CO2
+4 -2
Zn + CuSO4 è ZnSO4 + Cu
0, free
element
2-
Is this a redox or nonredox
reaction?
NONREDOX (no change in
oxidation numbers)
Examples
NaCl + AgNO3 è AgCl + NaNO3
+2 +6 -2
+2
Break down
this ionic
compound
into its ions
Cu and SO42-
+6
-2
0
Is this a redox or
nonredox reaction?
REDOX reaction
So, Cu must be Cu2+
O = -2 in SO42-, so S must
be…?
Examples
CO2 + H2O è C6H12O6 + O2
+4 -2
+1 -2
0
+1
-2
0
What happened to Carbon? It went from +4 oxidation #
to 0.
Redox or nonredox?
Was Carbon oxidized or reduced?
REDUCED
OIL RIG (oxidation is losing electrons so
oxidation number increases, where as
reduction is gaining electrons so oxidation
number decreases)
4