TEKS 10H: Understand and differentiate among acid-base reactions, precipitation reactions, and
oxidation-reduction reactions.
What are oxidation numbers?
• An oxidation number is a positive or negative
number assigned to an atom to indicate its degree
of oxidation or reduction.
• In general, a bonded atom’s oxidation number is
the charge that its ion would have.
• Oxidation numbers change during redox
reactions.
• You can use changes in oxidation number to
balance equations of complex redox reactions.
Rules for
Oxidation
Numbers
TEKS 10H: Understand
and Assigning
differentiate among
acid-base reactions,
precipitation reactions, and
oxidation-reduction reactions.
1. The oxidation number of a monatomic ion is equal in
magnitude and sign to its ionic charge. For example, the
oxidation number of (Br1–) is –1.
2. The oxidation number of hydrogen in a compound is +1,
except in metal hydrides, such as NaH, where it is –1.
3. The oxidation number of oxygen in a compound is –2, except
in peroxides—such as H2O2—where it is –1, and in compounds
with the more electronegative fluorine, where it is positive.
4. The oxidation number of an atom in uncombined (elemental)
form is 0.
5. For any neutral compound, the sum of the oxidation
numbers of the atoms in the compound must equal 0.
6. For a polyatomic ion, the sum of the oxidation numbers
must equal the ionic charge of the ion.
TEKS 10H: Understand and differentiate among acid-base reactions, precipitation reactions, and
oxidation-reduction reactions.
Sample Problem: Assigning
Oxidation Numbers to Atoms
What is the oxidation number of
each atom in SO2?
TEKS 10H: Understand and differentiate among acid-base reactions, precipitation reactions, and
oxidation-reduction reactions.
What are oxidation reduction reactions?
• Oxidation-reduction reactions involve the oxidation of a
reactant and the reduction of another reactant.
• Early chemists viewed oxidation as the gain of oxygen by an
element; reduction was viewed as the loss of oxygen by a
compound:
Oxidation: A gain of oxygen
Reduction: A loss of oxygen
• Modern chemists define oxidation-reduction reactions in
terms of a shift of electrons between reactants:
Oxidation: A gain of oxygen OR a loss of electrons
Reduction: A loss of oxygen OR a gain of electrons
• Oxidation-reduction reactions are also called redox reactions.
TEKS 10H: Understand and differentiate among acid-base reactions, precipitation reactions, and
oxidation-reduction reactions.
Magnesium metal can react with sulfur, forming
magnesium sulfide. In this reaction, magnesium
atoms lose two electrons and sulfur atoms gain
two electrons:
Magnesium is oxidized, and sulfur is reduced.
TEKS 10H: Understand and differentiate among acid-base reactions, precipitation reactions, and
oxidation-reduction reactions.
How do oxidation numbers change in chemical reactions?
In any redox reaction, the oxidation numbers of reacting
species change.
One reactant is oxidized and another is reduced.
+1 +5 — 2
0
+2 +5 — 2
0
2AgNO 3 (aq) + Cu( s) 
 Cu(NO 3 ) 2 (aq) + 2Ag ( s)
In this reaction, the oxidation number of Ag decreases from +1 to 0. It
gains an electron and is reduced to silver metal.
The oxidation number of copper increases from 0 to +2. It loses two
electrons and is oxidized.
• Oxidation is an increase in oxidation number.
• Reduction is a decrease in oxidation number.
TEKS 10H: Understand and differentiate among acid-base reactions, precipitation reactions, and
oxidation-reduction reactions.
A substance that undergoes oxidation loses electrons. It is
also the reducing agent. A substance that undergoes
reduction gains electrons. It is also the oxidizing agent.
Silver nitrate reacts with copper to form copper(II) nitrate and silver. The
equation may be written in ionic form:
2Ag++ 2NO3– + Cu  Cu2+ + 2NO3– + 2Ag
Copper is oxidized and silver is reduced.
Cu  Cu2+ + 2e– (loss of electrons)
2Ag++ 2e– 2Ag (gain of electrons)
Copper is the reducing agent
silver is the oxidizing agent.
TEKS 10H: Understand and differentiate among acid-base reactions, precipitation reactions, and
oxidation-reduction reactions.
Processes Leading to Oxidation and Reduction
Oxidation
Reduction
Complete loss of electrons (ionic reactions)
Complete gain of electrons (ionic reactions)
Shift of electrons away from an atom in a
covalent bond
Shift of electrons toward an atom in a
covalent bond
Gain of oxygen
Loss of oxygen
Loss of hydrogen by a covalent compound
Gain of hydrogen by a covalent compound
Increase in oxidation number
Decrease in oxidation number
TEKS 10H: Understand and differentiate among acid-base reactions, precipitation reactions, and
oxidation-reduction reactions.
Sample Problem
Identify what is oxidized, what is reducing, what is the reducing
agent, and what is the oxidizing agent for this reaction:
CuSO4 + Al -------> Cu + Al2(SO4)3
TEKS 10H: Understand and differentiate among acid-base reactions, precipitation reactions, and
oxidation-reduction reactions.
How do you balance
redox reactions?
One way to balance redox reactions is to use the oxidation-numberchange method. The following steps are used in this method:
• Write the skeleton equation for the reaction.
• Assign oxidation numbers to all the atoms in the
equation.
• Identify which atoms are oxidized and which are
reduced.
• Use a bracketed line to connect oxidized atoms that are
oxidized and another line to connect atoms that are
reduced. Write the oxidation-number change on the line.
TEKS 10H: Understand and differentiate among acid-base reactions, precipitation reactions, and
oxidation-reduction reactions.
• For example, the equation for the process used to
obtain metallic iron from iron ore in a blast furnace
would appear as follows:
• Make the total increase in oxidation number equal to
the total decrease by using coefficients.