is another useful method for balancing redox equations: the half-reaction
method. In this method each substance that changes its oxidation
number is balanced separately in a half-reaction that includes electrons
explicitly. The two balanced half-reactions then are combined in
such a way as to cancel electrons from the final expression.
Balance the iron and permanganate reaction by the half-reaction
The unbalanced permanganate half-reaction is
Five electrons were needed because manganese goes from +7 to +2
oxidation state. Water molecules are added on the right to balance
the oxygen atoms in the permanganate, and protons are added to the
This half-reaction is now balanced with respect to both number of
atoms and overall charge, +2 on each side.