Oxidation and Reduction
Introduction to Oxidation and
Reduction I
 Oxidation-Reduction
(redox)
reactions occur when one or
more electrons are transferred
from one substance to another
 Batteries
Oxidation and Reduction
The
substance that loses
electrons is said to be
oxidized
The substance that gains
electrons is said to be
reduced
Oxidation and Reduction
 OIL RIG
 Oxidation Is the Loss of
electrons
 Reduction Is the Gain of
electrons
Oxidation and Reduction
LEO
says GER
 Loss
of Electrons is Oxidation
 Gain Electrons is Reduction
Oxidation and Reduction
 In
order to determine which
substance is oxidized and
which substance is reduced,
you must calculate the
oxidation state of each
element, in each compound
in the reaction
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Predicting When Redox Occurs
 Oxidation
and reduction always
occur together as a pair
 Redox is occurring when there is
a change in the oxidation states of
at least 2 elements in the reaction
Predicting When Redox Occurs
 Redox
can occur in:
 Synthesis
 Decomposition
 Single
replacement
 Combustion
 Redox
can never occur in
double replacement reactions!
Rules for Oxidation States
The usual oxidation states for
common groups are:







Group 1:
Group 2:
Group 3:
Group 15:
Group 16:
Group 17:
+1
+2
+3
-3
-2
-1
Rules for Oxidation States
Fe(s)
Fe0
O
in O2(g)
O0
Rules for Oxidation States
oxidation state of any
atom in it’s free, naturally
occurring state is zero
The
Rules for Oxidation States
Na(s)
doesn’t
occur free in nature!
It’s too reactive 
Na
in NaCl(s)
Na+1
2
Rules for Oxidation States
 The most common oxidation state
is listed at the top ; the least
common at the bottom:
Predicting When Redox Occurs
 CH4
+ O2 → CO2 + H2O
 C-4H4+1
+ O20 → C-4O2-2 + H2+1O-2
Predicting When Redox Occurs
 An
easy way to spot redox is to
look for a substance that is
alone on one side of the
equation, but is in a compound
on the other side
Predicting When Redox Occurs
 It’s
best to use the number line
when determining the change in
oxidation state:
→ Oxidation →
-5 -4 -3 -2 -1 0 +1 +2 +3 +4 +5
← Reduction ←
 What
are the oxidation states of the
atoms in nitric acid?
HNO3
 Answer:
H+1

N+5

O-2

 (1)(+1) + (1)(+5) + (3)(-2) = 0
 Notice that the sum of the charges
equals the charge on the compound!
 What
are the oxidation states of
the atoms in the dichromate ion?
 Answer: (Cr2O7)-2
Cr+6

O-2

 (2)(+6) + (7)(-2) = -2
 Notice that the sum of the charges
equals the charge on the
polyatomic ion!
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