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Balancing Redox Reactions

Balancing Half-Reactions

  1. Balance the atoms being oxidized or reduced.
  2. Balance oxidation numbers by adding electrons.
  3. Balance charge by adding H+ (acidic solutions) or OH- (basic solutions).
  4. Balance H atoms by adding H2O.

Balancing Redox Reactions

  1. Write the half reactions.
  2. Balance each half reaction.
  3. Combine the half reactions to eliminate the electrons from the overall reaction.

Example

Fe2+(aq) + MnO4-(aq) ---> Fe3+(aq) + Mn2+(aq) (acidic solution)

1. Write the half reactions:

Fe2+(aq) ---> Fe3+(aq) (oxidation)

MnO4-(aq) ---> Mn2+(aq) (reduction)

2. Balance each half reaction:

Fe2+(aq) ---> Fe3+(aq) + e-

MnO4-(aq) + 8H+(aq) + 5e- ---> Mn2+(aq) + 4H2O

3. Combine the half reactions to give the overall reaction:

                    5Fe2+(aq) ---> 5Fe3+(aq) + 5e-
MnO4-(aq) + 8H+(aq) + 5e- ---> Mn2+(aq) + 4H2O
------------------------------------------------------------
5Fe2+(aq) + MnO4-(aq) + 8H+(aq) ---> 5Fe3+(aq) + Mn2+(aq) + 4H2O

Try balancing the lead-acid battery reaction:

Pb(s) + PbO2 (s) + SO42-(aq) ---> PbSO4(s)


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