Download Oxidation & Reduction Ch. 20

Redox Rules to Learn
For Ionic and Covalent bonds


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
2(+1) + 2( ?) = 0
+2 -1
with OF2
where O is -1
where O is +2
? + 2(-1) = 0
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


The most electronegative element fluorine
ALWAYS has an oxidation number of -1 when
it is bonded to another element.
i.e.
HF

Assign oxidation
numbers to each
element
4.
NaH
5.
HCl
1.
Cl2
6.
K3PO4
2.
Cl-
7.
CaSO4
3.
MgCl2
8.
KF


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)

2. Increasing oxidation #
2. Decreasing oxidation #
3. Gaining oxygen
3. Losing oxygen
L E O goes G E R
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