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UNIT 4: Oxidation and
Reduction
Chemistry 2 Honors
Northwestern High School
Dr. J. Venables
Determining Oxidation
Numbers
• The oxidation numbers of all atoms in a molecule or ion
add up to the total charge (0 for a molecule, the ion’s
charge for an ion)
• Elements by themselves are the charge shown
• O is almost always -2 (exception H2O2)
• F is almost always -1
• Group 1 elements are +1
• Group 2 elements are +2
Determining Oxidation
Numbers
• Examples: Determine the oxidation numbers of each
atom in the following:
1. NaC2H3O2
2. Na2SO4
3. C2O424. SO32-
Oxidation-Reduction
Reactions
• Zn added to HCl yields the spontaneous reaction
Zn(s) + 2H+(aq)  Zn2+(aq) + H2(g).
• The oxidation number of Zn has increased from 0 to 2+.
• The oxidation number of H has reduced from 1+ to 0.
• Zn is oxidized to Zn2+ while H+ is reduced to H2.
• H+ causes Zn to be oxidized and is the oxidizing agent.
• Zn causes H+ to be reduced and is the reducing agent.
• Note that the reducing agent is oxidized and the oxidizing
agent is reduced.
Balancing OxidationReduction Reactions
• Law of conservation of mass: the amount of each element
present at the beginning of the reaction must be present at
the end.
• Conservation of charge: electrons are not lost in a
chemical reaction.
Half Reactions
• Half-reactions are a convenient way of separating
oxidation and reduction reactions.
Half Reactions
• The half-reactions for
Sn2+(aq) + 2Fe3+(aq)  Sn4+(aq) + 2Fe2+(aq)
are
Sn2+(aq)  Sn4+(aq) +2e2Fe3+(aq) + 2e-  2Fe2+(aq)
• Oxidation: electrons are products.
• Reduction: electrons are reactants.
Loss of
Gain of
Electrons is
Electrons is
Oxidation
Reduction
Balancing Equations by the Method of Half Reactions
1. Write down the two half reactions.
2. Balance each half reaction:
a. First with elements other than H and O.
b. Then balance O by adding water.
c. Then balance H by adding H+.
d. If it is in basic solution, remove H+ by adding OHe. Finish by balancing charge by adding electrons.
Multiply each half reaction to make the number of
electrons equal.
4. Add the reactions and simplify.
5. Check!
3.
Example:
MnO4- + C2O42- → Mn2+ + CO2
Examples – Balance the following oxidation-reduction
reactions:
1. Cr (s) + NO3- (aq)  Cr3+ (aq) + NO (g) (acidic)
2. Al (s) + MnO4- (aq)  Al3+ (aq) + Mn2+ (aq) (acidic)
3. PO33- (aq) + MnO4- (aq)  PO43- (aq) + MnO2 (s)
(basic)
4. H2CO (aq) + Ag+ (aq) 
HCO3- (aq) + Ag (s) (basic)
Examples – Balance the following oxidation-reduction
reactions:
1. Cr + NO3- + 4H+  Cr3+ + NO + 2H2O
2. 5Al + 3MnO4- + 24H+  5Al3+ + 3Mn2+ + 12H2O
3. 3PO33- + 2MnO4- + H2O  3PO43- + 2MnO2 + 2OH4.
H2CO + 4Ag+ + 5OH-  HCO3- + 3 H2O + 4Ag