Chemistry Reference
In-Depth Information
Remember that, as indicated in Rule #1, an atom that is in its free
elemental state has an oxidation number of 0. So, we will assign both the
carbon and oxygen on the reactant side of the equation oxidation numbers
of 0. As for the product side, we must recall Rule #6, which tells us that
when oxygen is found in a compound it shows an oxidation number of -2,
unless the compound happens to be a peroxide. Now, if each atom of oxy-
gen in the carbon dioxide has an oxidation number of -2, and there are two
atoms of oxygen, the total negative charge in the compound is -4. The car-
bon atom found in the carbon dioxide must have an oxidation number of
+4, if the one atom is able to cancel out both atoms of oxygen, leaving the
compound with a net charge of zero. Let's rewrite the equation, with the
oxidation numbers included:
C
0
(s)
+ O
2
0
(g)
C
4+
O
2
2-
(g)
carbon + oxygen yields carbon dioxide
Can you see that what is happening to the carbon fits into our modern
definition of oxidation? The carbon atom appears to lose four electrons, as
its oxidation number goes from 0 to +4. We say that carbon has been oxi-
dized. Because the oxidation has been caused by oxygen, in this case, we
call oxygen the oxidizing agent.
Did you notice that the oxidation number of oxygen has changed as
well? Instead of appearing to lose electrons, each oxygen atom appears to
gain two electrons, as its oxidation number goes from 0 to -2. Remember:
Gaining electrons makes the charge more negative. Because the oxidation
number of the oxygen goes down, we say that oxygen has been reduced. It
may seem strange that the substance that appears to gain electrons is said
to be “reduced,” but that is because it is gaining negative charges, which
reduces its oxidation number. The opposite of oxidation is reduction. Re-
duction is when a substance gains electrons or appears to gain electrons, as
its oxidation number is reduced algebraically. Because the carbon appears
to be the cause of the reduction, carbon would be considered the reducing
agent in this reaction.
If an atom is going to lose electrons, they must go somewhere. We say
that the charge must be conserved. There can be no oxidation without an
accompanying reduction; they go hand in hand in the same reaction. For
that reason, reactions of this type are called oxidation-reduction reactions,
or redox reactions for short.
Let's practice a couple examples of reactions where we are required to
assign oxidation numbers to the participants of the reaction and then iden-
tify the “players.”
