1. Oxidation-reductions reactions are also called redox reactions.
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.
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) + 3O
2
(g)

2Fe
2
O
3
(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.
Fe
2
O
3
(s) + 3CO(g)

2Fe(s) + 3CO
2
(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
NO
3
spectator ions have been omitted.)
Cu(s) + 2Ag +

Cu 2+ + 2Ag(s) rev. 032612
This reaction can be expressed as two “halfreactions” where the loss and gain of electrons becomes evident
Oxidation half-reaction
Cu(s)

Cu 2+ + 2e- …. loss of electrons
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)

Cu 2+
2(Ag + + 1e-

+ 2e-
Ag(s))
---------------------------
Cu(s) + 2Ag +

Cu 2+ + 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 +

Cu 2+ + 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
Be sure to review the rules for assigning oxidation states to elements in compounds.

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.
Consider the reaction of iron burning in oxygen to make Fe
2
O
3
. Iron doesn’t actually lose three
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.
10. Despite the fact that most people are totally 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.
0 0 +3 -2
4Fe(s) + 3O
2
(g)

2Fe
2
O
3
(s) 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.
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, Na
3
BO
3
. a. 1 b. 2 c. 3 d. 4
2. Determine the oxidation number of chromium in
6. How many electrons could be gained or lost during the reduction of bismuth(III) ions? a. 2 electrons lost b. 3 electrons gained c. 4 electrons gained potassium dichromate, K
2
Cr
2
O
7
. a. 2 b. 3 c. 4 d. 6
3. What could be the possible result of the oxidation of nickel(II) ions? a. Ni b. Ni + c. Ni 2+ d. Ni 3+ d. 5 electrons lost
7. Determine the element oxidized in this reaction:
14H + + Cr
2
O
7
2 + 3C
2
O
4
2-

2Cr 3+ + 6CO
2
(g) + 7H
2
O a. H b. Cr c. C d. O
4. Which balanced equation is an oxidation half reaction? a. Fe

Fe 3+ + 3 e b. Fe

Fe 3- + 3 e c. Fe + 3 e d. Fe + 3 e -
-

Fe 3+

Fe 3-
5. In which reaction does reduction take place?
8. Determine the element reduced in this reaction:
6H + + 5SO
3
2 + 2MnO
4
-

5SO
4
2 + 2Mn 2+ + 3H
2
O(l) a. H b. S c. O d. Mn
9. Identify the element oxidized in this reaction: a. NaCl(aq) + AgNO
3
(aq)

AgCl(s) + NaNO
3
(aq) b. Na(s) + HOH(l)

NaOH(aq) + H
2
(g) c. NH
4
Cl(s)

NH
3
(g) + HCl(g) d. HCl(g) + H
2
O(l)

H
3
O + + Cl -
Zn(s) + 2HNO
3
(aq)

Zn(NO
3
)
2
(aq) + H
2
(g) a. Zn b. N a. C c. O
10. Identify the element reduced in this reaction:
Na
2
SO
4
(s) + 4C(s)

Na
2
S(g) + 4CO(g) b. Na c. O d. H 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.