Balancing Redox Reactions
By assigning oxidation numbers to the reactants and products, one can determine how many moles
of each species are required for conservation of charge and mass, which is necessary to balance the
equation. In general, to balance a redox reaction, both the net charge and the number of atoms
must be equal on both sides of the equation. The most common method for balancing redox
equations is the half-reaction method, also known as the ion-electron method, in which the
equation is separated into two half-reactions—the oxidation part and the reduction part. Each half-
reaction is balanced separately, and they are then added to give a balanced overall reaction, in
which electrons do not appear explicitly by convention. Consider a redox reaction between KMnO 4
and HI in an acidic solution:
MnO 4 − + I− → I 2 + Mn2+
Step
1:
Separate the two half-reactions.
oxidation half-reaction: MnO 4 − → Mn2+
reduction half-reaction: I− → I 2
Step
2:
Balance the atoms of each half-reaction. First, balance all atoms except H and O. Next, in an
acidic solution, add H 2 O to balance the O atoms and then add H+ to balance the H atoms. (In
a basic solution, use OH− and H 2 O to balance the O’s and H’s.)
To balance the iodine atoms, place a coefficient of two before the I− ion.
2I− → I 2
For the permanganate half-reaction, Mn is already balanced. Next, balance the oxygens
by adding 4H 2 O to the right side.
MnO 4 − → Mn2+ + 4H 2 O