Things to Know About Oxidation-Reduction Reactions
rev. 032612
1. Oxidation-reductions reactions are also called
redox reactions.
This reaction can be expressed as two “halfreactions” where the loss and gain of electrons
becomes evident
2. Redox reactions always involve a change in the
oxidation number of at least two elements. For a
redox reaction you must be able to identify the
element oxidized and the element reduced.
Oxidation half-reaction
Cu(s)  Cu2+ + 2e- …. loss of electrons
3. Oxidation never occurs without reduction.
Reduction never occurs without oxidation. Oxidation
and reduction occur simultaneously.
4. All single replacement reactions and combustion
reactions are redox. Some synthesis and
decomposition reactions are redox. No double
replacement reactions are redox. There are many
additional reactions which don’t fit any of the simple
reaction patters that are redox reactions.
5. The term “oxidation” originally referred to the
combination of an element with oxygen. When thin
strands of iron metal rapidly combine with oxygen
(combustion) we get iron(III) oxide as the product.
4Fe(s) + 3O2(g)  2Fe2O3(s)
(Combustion is defined as “rapid oxidation”)
6. The term “reduction” originally referred to the
formation of a metal from the ore. The refining of
iron involves the reaction of iron(III) oxide and
carbon monoxide to form elemental iron and carbon
dioxide gas.
Fe2O3(s) + 3CO(g)  2Fe(s) + 3CO2(g)
7. Oxidation and reduction are sometimes defined in
terms of a “loss” or “gain” of electrons, but this is
only true when actual ions are involved.
Oxidation involves a “loss of electrons.”
Reduction involves a “gain of electrons.”
When copper metal is placed in a beaker of silver
ions in the form of soluble silver nitrate, then silver
ions literally gain an electron to become silver metal
and copper metal literally loses two electrons to
become copper(II) ions. This redox reaction can be
represented by the following net ionic equation. (The
NO3- spectator ions have been omitted.)
Cu(s) + 2Ag+  Cu2+ + 2Ag(s)
Reduction half-reaction
Ag+ + 1e-  Ag(s) …. gain of electrons
The number of electrons lost by the copper must
equal the number of electrons gained by the silver.
No electrons can be unaccounted for. Therefore, by
doubling the reduction half-reaction and adding the
two half-reactions together, we get the “whole”
reaction. Notice that the number of electrons lost
and gained are the same, and cancel out and
disappear from the “whole” reaction, which is our net
ionic equation.
Cu(s)  Cu2+ + 2e2(Ag+ + 1e-  Ag(s))
--------------------------Cu(s) + 2Ag+  Cu2+ + 2Ag(s)
8. OILRIG is a pneumonic for remembering the
definition of oxidation and reduction in terms of
electron loss and gain.
Oxidation Is Loss, Reduction Is Gain
9. A more practical way of defining oxidation and
reduction is in terms of changes in the oxidation
states of two or more elements.
Oxidation is the increase in oxidation number.
Reduction is the decrease in oxidation number.
Consider the reaction of copper and silver ions.
Oxidation numbers have been assigned to each
element and ion.
0
+1
+2
0
Cu(s) + 2Ag+  Cu2+ + 2Ag(s)
The copper went from zero to +2, an increase in
oxidation number. An increase in oxidation number
is defined as oxidation. Copper was oxidized. The
oxidation number of silver went from +1 to zero, a
decrease in oxidation number. A decrease in
oxidation number is defined as reduction.
Some reactions don’t really involve gaining or losing
electrons. Any “gain” or “loss” is really only on
paper since in reality there is no actual transfer of
electrons.
The oxidation number of iron went from zero to +3,
an increase. Oxidation is defined as an increase in
oxidation number. Therefore, iron was oxidized.
The oxidation number of oxygen went from zero to –
2, a decrease. Reduction is defined as a decrease in
oxidation number. Therefore, oxygen was reduced.
Consider the reaction of iron burning in oxygen to
make Fe2O3. Iron doesn’t actually lose three
electrons as it combines with oxygen. Nor does
oxygen actually gain any electrons. In reality
electrons will be shared between the two as iron(III)
oxide is formed. But, the reaction is a redox reaction
because there have been changes in the oxidation
states of iron and oxygen.
10. Despite the fact that most people are totally
unaware, redox reactions are everywhere in your
daily life. Your muscles contract and your heart
beats because of redox reactions. The very energy
needed to power your brain comes from redox
reactions. All of those things in which “batters are
included” operate because of the redox reactions in
the batteries. Cars rust and metals corrode because of
redox reactions. Even those fancy glasses that get
dark in the sun and light indoors do so because of
redox reactions.
0
0
+3 -2
4Fe(s) + 3O2(g)  2Fe2O3(s)
How do the things in these pictures relate to redox reactions? Try to do better than “one-word” answers.
Sample Redox Questions
1. Determine the oxidation number of boron in sodium
borate, Na3BO3.
a. 1
b. 2
c. 3
6. How many electrons could be gained or lost during
the reduction of bismuth(III) ions?
d. 4
a.
b.
c.
d.
2. Determine the oxidation number of chromium in
potassium dichromate, K2Cr2O7.
a. 2
b. 3
c. 4
d. 6
7.
2 electrons lost
3 electrons gained
4 electrons gained
5 electrons lost
Determine the element oxidized in this reaction:
14H+ + Cr2O72- + 3C2O42-  2Cr3+ + 6CO2(g) + 7H2O
3. What could be the possible result of the oxidation of
nickel(II) ions?
a. H
a. Ni
b. Ni+
c. Ni2+
8.
4. Which balanced equation is an oxidation half
reaction?
a.
b.
c.
d.
5.
Fe
Fe
Fe
Fe
b. Cr
c. C
d. O
d. Ni3+
 Fe3+ + 3 e Fe3- + 3 e+ 3 e-  Fe3+
+ 3 e-  Fe3-
Determine the element reduced in this reaction:
6H+ + 5SO32- + 2MnO4-  5SO42- + 2Mn2+ + 3H2O(l)
a. H
9.
b. S
c. O
d. Mn
Identify the element oxidized in this reaction:
Zn(s) + 2HNO3(aq)  Zn(NO3)2(aq) + H2(g)
In which reaction does reduction take place?
a. Zn
b. N
c. O
d. H
a. NaCl(aq) + AgNO3(aq)  AgCl(s) + NaNO3(aq)
10. Identify the element reduced in this reaction:
b. Na(s) + HOH(l)  NaOH(aq) + H2(g)
c. NH4Cl(s)  NH3(g) + HCl(g)
d. HCl(g) + H2O(l)  H3O + Cl
+
Na2SO4(s) + 4C(s)  Na2S(g) + 4CO(g)
a. C
b. Na
c. O
d. S
-
When aluminum wire is placed into a solution containing silver ions, the aluminum begins to dissolve and silver metal
begins to form on the aluminum wire.
11. Write the balanced reduction half-reaction as an ionic equation.
12. Write the balanced oxidation half-reaction as an ionic equation.
13. Write the complete balanced net ionic equation for this reaction. Redox reactions must be “charge balanced” as well as
“mass balanced”, which means that in addition to having the same number of each kind of element on each side, the number of
electrons lost by the element that is oxidized is equal to the number of electrons gained by the element that is reduced.