Oxidation Numbers:

In order to recognize which reactant is oxidized and which is reduced it is
important to be able to follow the movement of the electrons.

Oxidation numbers allow us to do this. An oxidation number is the real or
apparent charge of an ion or an atom when we consider the bonds to be ionic.

Now learn the rules so that you are able to assign the oxidation number to
any element!!!
Oxidation Numbers:

The charge on a monatomic ion
is the oxidation number.

Elements always have an
oxidation number of zero.

Fe3+ is +3

K
-K is 0

Br1- is -1

N2
-N is 0

Hydrogen is always +1 unless it
is in a metal hydride and then it
is -1.

Oxygen is always -2 unless it is
in a peroxide and then it is -1.

HNO3
- H is +1

HNO3- O is -2

CaH2
- H is -1

H2O2 - O is -1
Determining Oxidation
Numbers

The sum of oxidation numbers
in a neutral compound must be
zero.

The sum of oxidation numbers
in a polyatomic ions must equal
the charge on the ion.

NO31-


K2CrO4

O is -2

K is +1

Cr must be +6
Cr2O3

O is -2

O is -2

N is +5

Cr must be +3

Note that elements can have more than one
oxidation number.
Examples:

Determine the oxidation number
of sulfur in the following
molecules:

H2S

SO2

SO32-

H is +1 because the molecule is
neutral, S must be -2

O is -2, there are two O in the molecule
therefore S must be +4

O is -2, there are three O in the ion
therefore S must be +4 to give an
overall charge of 2-
Use of Oxidation Numbers:

In a redox reaction, one element loses electrons,
while another gains them. Oxidation numbers allow
you to keep track of these electron transfers and also
allow you to determine whether a reaction is a redox
reaction or not!

The following three reactions take place when coal
with impurities of sulfur is burned. The result is the
production of acid rain!

For each of these reactions determine the oxidation
numbers of each element and then determine if the
reaction is a redox reaction or not.

1.

S+
 ___
O2 
SO2
____
____ ,_____

Redox? _______

2.

2SO2 +
 ___ ,____

O2
____
Redox? ________

2SO3
____ ,____

3.

SO3 +
H2O

__, __
__ ,__

Redox? _____

1. 0 0

2. +4, -2

3. +6, -2 +1 , -2 +1, +6, -2 No change in the oxidation numbers of the
elements therefore reaction number three is not a REDOX reaction.
+4 ,-2
0

H2SO4
__, __, __
Sulfur is oxidized, oxygen reducedREDOX
+6, -2 Sulfur is oxidized, oxygen is reduced  REDOX.
Reducing – Oxidizing Agents

Mg(s) +
 0
S(s) 
0
MgS(s)
+2,-2 (oxidation numbers)

Reducing Agent
 The substance that loses electrons.
 Mg is oxidized in the previous reaction but it is the REDUCING AGENT.
 Oxidation number increases.

Oxidizing Agent

The substance that gains electrons.

S is reduced in the previous reaction but it is the OXIDIZING AGENT.

Oxidation number decreases.

Lead contamination in drinking water can result in brain and nervous
system damage. If tap water is slightly acidic it can dissolve the lead solder
in pipes by the following reaction:

Pb(s) +

In this case the hydrogen ions from the acidic tap water oxidize the lead,
while the lead reduces the hydrogen ions.
2H+(aq) 
Pb2+(aq) + H2(g)
Reactivity of Metals

Metals (M) vary in their reactivity with acids (HA).

M(s) + 2HA(aq)  H2(g) + MA(aq)

The metal is oxidized, the hydrogen is reduced, the metal dissolves and hydrogen gas is produced.

The metal is considered to be the reducing agent.

Identify the most reactive and least reactive metal by examining the following picture;

Complex ions or neutral compounds may also be oxidizing and reducing
agents. Consider the reaction that occurs when nitrogen dioxide gas bubbles
in a solution containing silver ions.

Ag+(aq) + NO2(g) + H2O(l)  Ag(s) + NO3-(aq) +2H+(aq)

+1

Silver is reduced and nitrogen in the nitrogen dioxide is oxidized.

The oxidation numbers of oxygen and hydrogen do not change.

NO2 is the reducing agent. Silver ions are the oxidizing agent.
+4,-2
+1, -2
0
+5, -2
+1

Use the oxidation numbers to identify the oxidizing and reducing agents in the
following reactions:

A)
CH4 + 2 O2  CO2 + 2H2O

B)
Pb + H2SO4 

C)
2Mg + CO2  C + 2MgO

Oxidizing Agent
Reducing Agent

A)
O2
CH4

B)
H2SO4
Pb

C)
CO2
Mg
H2 + PbSO4
Chapter 20.2-Read pages
639  643

Page 641 #9 & # 10

Page 643 #13  #16

http://www.youtube.com/watch
?v=EHe8-AFMsMA

Worksheet Redox -2