Transcript Oxidation & Reduction Ch. 20

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Oxidation Reduction Reaction (Redox):
A reaction in which electrons are
transferred from one substance to another.
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Oxidation cannot occur without reduction
Definition of Oxidized:
1. Losing of electrons (LEO)
2. Increasing the ox. number, ie: 0 to +1
-2 to -1
3. Gaining oxygen
Definition of Reduced:
1. Gaining of electrons (GER)
2. Reducing the ox. number, ie: -1 to -2
1 to 0
3. Losing oxygen
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2Mg + O2  2MgO
Mg atom transfers two electrons to each
oxygen atom.
As a result 2 Mg atoms become Mg2+ and two
oxygen atoms become O2Mg loses two electrons
O gains two electrons
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2Mg + O2  2MgO
Mg atom transfers two electrons to each
oxygen atom.
As a result 2 Mg atoms become Mg2+ and two
oxygen atoms become O2Mg loses two electrons
O gains two electrons
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Can oxidation occur without reduction?
What are the three definitions of oxidation?
What are the three definitions of reduction?
What is a way to remember oxidation/reduction in
terms of transfer of electrons?
Does the oxidation number increase or decrease
when there is oxidation?
Does the oxidation number increase or decrease
when there is reduction?
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When an electron is lost (An electron is found
on the product side) there is oxidation
◦ X0  X+1 + e-
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When an electron is gained (An electron is
found on the reactant side) there is reduction
◦ e- +Y+1  Y0
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Ex5
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Ex6
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Ex7
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Ex8
start
finish
Na0Na+1 +1e- (LEO)oxidized
1e-+ Fe+3Fe+2
(GER)reduced
+1
+2
+1e
N N
2e-+ Mg+2Mg0
(LEO)oxidized
(GER)reduced
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Whatever is oxidized is called the reducing
agent
Whatever is reduced is called the oxidizing
agent
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Ex5
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Ex6
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Ex7
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Ex8
start
finish
Na0Na+1 +1e- oxidized
reducing agent
1e-+ Fe+3Fe+2
reduced
oxidizing agent
+1
+2
+1e
N N
oxidized
reducing agent
2e-+ Mg+2Mg0
reduced
oxidizing agent
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What is a reducing agent?
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What is a oxidizing agent?
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Are metals generally oxidized or reduced?
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Are nonmetals generally oxidized or reduced?
Redox Rules to Learn
For Ionic and Covalent bonds
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Definition: Electonegativity is the atom’s
ability to attract electrons.
Fluorine is the most electronegative element
Decrease EN
electrons get
further away
from protons
Increase EN
More protons to
attract electrons
Oxidation number of an uncombined atom = zero
(no compounds or ions)
i.e.
ie.
0
0
0
Mg, H2, Na
0
0
0
0
0
0 0
All H2 O2 N2 Cl2 Br2 I2 F2 ’s ox. # are zero
Oxidation number of
a monatomic ion = charge of the ion
i.e.
+2
Mg2+ ,
-2
O2-
,
+3
Al3+
-1
, Br-
The sum of the oxidation numbers
of a neutral compound = 0
i.e.
+1 -2
H2O
2(+1) - 2 = 0
+4 -2
CO2
4 + 2(-2) = 0
+2 -2
CO
=0
+1 -1
NaCl
=0
The oxidation number for H is +1
Exception: when bonded to a less
electronegative element (metal),
then it is -1.
i.e.
i.e.
+1 -1
+1 -1
+2 -1
LiH, NaH, MgH2
+1 -1
+1 -1
HF, HCl
Exception
Oxygen’s oxidation number is -2, in a compound
Two exceptions: peroxides and fluorine
+1
? = -1
in H2O2
where O is -1
2(+1) + 2( ?) = 0
+2 -1
with OF2
? + 2(-1) = 0
where O is +2
fluorine (more electronegative)
Oxidation number of
polyatomic compound = zero
polyatomic ion = the charge of the ion
i.e. compounds:
-3 +1
NH3
-3 + 3(+1) = 0
SiCl4
K3PO4
i.e. ions:
+1
NH4+
CO32-3+ 4(1) = +1
-3
SO42-
NO3-
Oxidation numbers for metals in:
Group 1A metal compound
2A metal compound
3A Aluminum compound
+1 -1
i.e. NaCl
+2 +6 -2
MgSO4
+3
AlPO4
+2 + ? + 4(-2)
=0
=0
=0
+1
+2
+3
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The most electronegative element fluorine
ALWAYS has an oxidation number of -1 when
it is bonded to another element.
i.e.
HF
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Assign oxidation
numbers to each
element
4.
NaH
5.
HCl
1.
Cl2
6.
K3PO4
2.
Cl-
7.
CaSO4
3.
MgCl2
8.
KF
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If oxidation number goes up it is oxidized.
If oxidation number goes down it is reduced.
Balance the reactions,
write the oxidation numbers for each atom
and write the oxidizing and reducing agents.
Loss of electron oxidation (LEO)
Reducing agent
Ex1
+1 -1
0
+2 -1
0
2 HCl + Zn  ZnCl2 + H2
+2 + 2(-1) =0
Gain of electron reduction (GER)
Oxidizing agent
Ex2
Mg + N2  Mg3N2
Ex3
H2S + Cl2  HCl + S
Ex4
Fe
+ O2 
Fe2O3
There are three definitions of oxidation and
reduction:
 Oxidation
Reduction
1. Losing of electrons (LEO) 1. Gaining of electrons (GER)
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2. Increasing oxidation #
2. Decreasing oxidation #
3. Gaining oxygen
3. Losing oxygen
L E O goes G E R
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