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