11. Conservation of Mass,       Charge, and Energy   Previous PageNext Page
       Conservation of Electrons

There 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 method.

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 left:

This half-reaction is now balanced with respect to both number of atoms and overall charge, +2 on each side.

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