Oxidation Numbers & Redox Reactions
The oxidation number of an element indicates the number of electrons lost, gained, or shared as a result of chemical
bonding. The change in the oxidation state of a species lets you know if it has undergone oxidation or reduction.
Oxidation can be defined as "an increase in oxidation number".
In other words, if a species starts out at one oxidation state and ends up at a higher oxidation state it has undergone
oxidation.
Conversely,
Reduction can be defined as "a decrease in oxidation number".
Any species whose oxidation number is lowered during the course of a reaction has undergone reduction.
Example:



Na + Cl2 -----> 2NaCl
The Na starts out with an oxidation number of zero (0) and ends up having an oxidation number of
1+. It has been oxidized from a sodium atom to a positive sodium ion.
The Cl2 also starts out with an oxidation number of zero (0), but it ends up with an oxidation number
of 1-. It, therefore, has beenreduced from chlorine atoms to negative chloride ions.
The substance bringing about the oxidation of the sodium atoms is the chlorine, thus the chlorine is called
an oxidizing agent. In other words, the oxidizing agent is being reduced (undergoing reduction).
The substance bringing about the reduction of the chlorine is the sodium, thus the sodium is called a reducing
agent. Or in other words, the reducing agent is being oxidized (undergoing oxidation).
Oxidation is ALWAYS accompanied by reduction. Reactions in which oxidation and reduction are occurring are usually
called Redox reactions.
Rules for Assigning Oxidation Numbers
There are several rules for assigning the oxidation number to an element. Learning these rules will simplify the task of
determining the oxidation state of an element, and thus, whether it has undergone oxidation or reduction.
1. The oxidation number of an atom in the elemental state is zero.
Example: Cl2 and Al both are 0
2. The oxidation number of a monatomic ion is equal to its charge.
Example: In the compound NaCl, the sodium has an oxidation number of 1+ and the
chlorine is 1-.
3. The algebraic sum of the oxidation numbers in the formula of a compound is zero.
Example: the oxidation numbers in the NaCl above add up to 0
4. The oxidation number of hydrogen in a compound is 1+, except when hydrogen forms compounds
called hydrides with active metals, and then it is 1-.
Examples: H is 1+ in H2O, but 1- in NaH (sodium hydride).
5. The oxidation number of oxygen in a compound is 2-, except in peroxides when it is 1-, and when
combined with fluorine. Then it is 2+.
Example: In H2O the oxygen is 2-, in H2O2 it is 1-.
6. The algebraic sum of the oxidation numbers in the formula for a polyatomic ion is equal to the charge
on that ion.
Example: in the sulfate ion, SO42-, the oxidation numbers of the sulfur and the oxygens add
up to 2-. The oxygens are 2- each, and the sulfur is 6+.
Application Problems:
1. What is the oxidation number of chromium in
Na2CrO4
Cr2O722. Given the unbalanced equation below:
Cr2O3(s) + Al(s) ----> Cr(s) + Al2O3(s)
a. identify the oxidation state of each element
b. identify the oxidizing agent
c. identify the reducing agent
Practice Problem 1:
Determine the oxidation number of each element in the following compounds:
(a) BaO2
(b) (NH4)2MoO4
(c)Na3Co(NO2)6
(d) CS2
Summary
Oxidizing Agents
Reducing Agents
Oxidation State
Decreases
Increases
# of Electrons
Gained
Lost
Substance is...
Reduced
Oxidized
By looking at each element's oxidation state on the reactant side of a chemical equation in comparison to the same element's oxidation
state on the product side, one can determine if the element is being reduced or oxidized. Thus, one is able to conclude
the oxidizing andreducing agents of a chemical reaction.
Practice Problems
1. Identify the oxidizing agent and the reducing agent in the following redox reaction.
MnO2(s) + 4 H+(aq) + 2 Cl-(aq) → Mn2+(aq) + 2 H2O(l) + Cl2(g)
2. In the reaction, 2 NO2(g) + 7 H2(g) → 2 NH3(g) + 4 H2O(g), is hydrogen an oxidizing agent or a reducing agent? Explain.
3. a) An element that is oxidized is a(n) __________ agent.
b) An element that is reduced is a(n) __________ agent.
4. What is the reducing agent in the following redox reaction: 5 SO32- + 2 MnO4- + 6 H+ → 5 SO42- + 2 Mn2+ + 3 H2O?
5. What is the oxidizing agent in the following redox reaction: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)?
Now that you know about oxidizing and reducing agents, test yourself with some of these problems.
6. Determine the oxidizing and reducing agent of the following chemical equation for aerobic respiration.
C6H12O6 (s) + 6 O2 (g) --> 6 CO2 (g) + 6 H2O (l)
7.Is A the oxidizing or reducing agent? Is B the oxidizing or reducing agent? Which one is reduced and which one is oxidized?
A
B
Loses
electrons
Gains
electrons
8. In a redox reaction, there must be
A) an oxidizing agent and no reducing agent
B) a reducing agent and no oxidizing agent
C) a reducing agent and an oxidizing agent
D) no reducing or oxidizing agent
9. Which of the following will be a strong reducing agent, which of the following will be a strong oxidizing agent?
NO3-, NO, N2H4, NH3
More practice on identifying oxidizing and reducing agents can be found here.
Sample Exercise 20.1 Identifying Oxidizing and Reducing Agents
Identify the oxidizing and reducing agents in the oxidation-reduction reaction
2 H2O(l) + Al(s) + MnO4
–
(aq) → Al(OH)4
–
(aq) + MnO2
(s)
Answer: Al(s) is the reducing agent; MnO4
–
(aq) is the oxidizing agent.
Oxidation Reduction Practice Problems
Identify what is oxidized and what is reduced.
Identify the oxidizing agent and the reducing agent.
CH4
+
2 O2
2 H2O
+
CO2
+
3 CO2
carbon (CH4) is oxidized
oxygen is reduced
The oxidizing agent is oxygen.
The reducing agent is methane (CH4).
2 Al2O3
+
3C
aluminum is reduced
4 Al
carbon is oxidized
The oxidizing agent is Al2O3.
The reducing agent is carbon.
4 Fe
+
3 O2
2 Fe2O3
Iron (Fe) is oxidized.
Oxygen is reduced.
The oxidizing agent is oxygen.
The reducing agent is zinc.
2 Zn
+
O2
2 ZnO
Zinc is oxidized.
Oxygen is reduced.
The oxidizing agent is oxygen.
The reducing agent is zinc.
FeCl2
+
Zn
ZnCl2
Iron (Fe) is reduced.
Zinc is oxidized.
The reducing agent is zinc.
The oxidizing agent is FeCl2.
2 Cu
+
O2
Copper is oxidized.
Oxygen is reduced
The reducing agent is copper.
The oxidizing agent is oxygen.
2 CuO
+
Fe
Solutions
1. Cl- is the oxidizing agent because it gains one electron (starting with an oxidation state of -1 in Cl- and increasing to 0 in Cl2).
Remember that gaining electrons means it is "reduced". MnO2 is the reducing agent because it loses two electrons (starting with an
oxidation state of +4 in MnO2 and decreasing to +2 in Mn2+). Keep in mind that losing electrons means it is "oxidized".
2. In this reaction, hydrogen loses one electron. Hydrogen is oxidized, thus making it the reducing agent.
3. a) An element that is oxidized is a reducing agent because the element loses electrons.
b) An element that is reduced is an oxidizing agent because the element gains electrons.
4. SO32- is the reducing agent because it lost two electrons, sulfur goes from an oxidation state of +4 in SO32- to an oxidation state of +6
in SO42-.
5. Cu2+ (aq) is the oxidizing agent because it gains two electrons, going from an oxidation state of +2 in Cu 2+(aq) to an oxidation state of
0 in Cu(s).
6.The oxidizing agent is oxygen. The reducing agent is glucose.
The oxygen is being reduced, so it is an oxidizing agent. The glucose is being oxidized, so it is a reducingagent.
7. When A loses electrons, it is being oxidized, thus is a reducing agent.
8. C. In a redox reaction, there is always an oxidizing and reducing agent.
When B gains electron, it is being reduced, thus is an oxidizing agent.
A is oxidized. B is reduced.
9. NO3- is most likely to be a strong oxidizing agent. NH3 is most likely to be a strong reducing agent. You determine the likelihood for
oxidation or reduction by comparing the oxidation numbers of nitrogen. Since NO 3- has the highest oxidation number of +5, compared to
the other molecules, it will most likely be the oxidizing agent. Since NH3 has an oxidation state of -3, it has the lowest oxidation
state and will most likely be the reducing agent.