Oxidation-Reduction Reactions Chemistry Worksheet Page 8
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Chapter 6
Oxidation-Reduction Reactions
Sample Study
Tip-off You are asked to determine the oxidation number of an atom, or you need to
assign oxidation numbers to atoms to determine whether a reaction is a redox reaction,
Sheet 6.1
and if it is, to identify which element is oxidized, which is reduced, what the oxidizing
Assignment
agent is, and what the reducing agent is.
of Oxidation
General Steps
Numbers
Use the following guidelines to assign oxidation numbers to as many atoms as
you can. (Table 6.2 provides a summary of these guidelines with examples.)
O
3
bjeCtive
The oxidation number for each atom in a pure element is zero.
The oxidation number of a monatomic ion is equal to its charge.
When fluorine atoms are combined with atoms of other elements, their
oxidation number is ‒1.
When oxygen atoms are combined with atoms of other elements, their
oxidation number is ‒2, except in peroxides, such as hydrogen peroxide,
H
O
, where their oxidation number is ‒1. (There are other exceptions that
2
2
you will not see in this text.)
The oxidation number for each hydrogen atom in a molecular compound or
a polyatomic ion is +1.
If a compound’s formula contains one element for which you cannot assign
an oxidation number using the guidelines listed above, calculate the oxidation
number according to the following rules.
The sum of the oxidation numbers for the atoms in an uncharged formula is
equal to zero.
The sum of the oxidation numbers for the atoms in a polyatomic ion is equal
to the overall charge on the ion.
Example See Example 6.1.
Table 6.2
Guidelines for Assigning Oxidation Numbers
Oxidation
Examples
Exceptions
number
Pure element
0
The oxidation number
none
for each atom in
Zn, H
, and S
is zero.
2
8
is +2.
Monatomic ions
charge on
Cd in CdCl
none
2
ion
Cl in CdCl
is ‒1.
2
H in LiH is ‒1.
Fluorine in the
‒1
F in AlF
is ‒1.
none
3
combined form
F in CF
is ‒1.
4
Oxygen in the
‒2
O in ZnO is ‒2.
O is ‒1 in peroxides,
combined form
such as H
O
O in H
O is ‒2.
2
2
2
+1
O is +1.
Hydrogen in the
H in H
H is ‒1 when combined
2
combined form
with a metal.
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