Steps for balancing redox equations
using the half-reaction method.
1. If the equation is already in net ionic form, go to step 2.
If not, write the skeleton net ionic equation for the reaction (and that means that you should leave out the spectator
ions). In some cases, it will not be possible to balance the net ionic equation,
but you must still write the soluble salts, strong acids, and strong bases in ionic form.
2. Determine the oxidation number of all atomic species in the net ionic equation. Its the change in oxidation number of these atomic species that determines the oxidation (increase in oxidation
number) or reduction (reduction in oxidation number).
3. Write the oxidation and reduction half-reactions from the net ionic equation.
a. Note: Its the change in the oxidation number that determines whether an oxidation or reduction
half takes place, not the change in charge number on a poly-atomic ion.
b. An oxidation half-reaction includes electrons. You
must determine which side of the half-reaction has more electrons and add that number of electrons to the other side of the
equation so that there are an equal number of electrons on both sides.
4. Balance the atoms and charges in each half reaction.
a. If one side of the half-reaction equation has more oxygen atoms than the other, add as many waters
as are needed to balance the half-reaction for oxygen.
b. If one side of the half-reaction equation has more hydrogen atoms than the other, add as many hydrogen
ion as are needed to balance the half-reaction for oxygen.
c. If the reaction is taking place in an acid solution, skip this step and go to d. If this reaction is taking place in a basic solution, then add an equal number of hydroxide ions to both
sides of the half reaction to neutralize the hydrogen ions.
d. Check to see that both sides of each half-reaction equation have the same number of each atom on
both sides of the arrow.
e. Check to see that both sides of each half reaction have the same number of charges when the charges
on molecules and ions are added together.
5. Multiply the half reactions by the numbers that is necessary so that the same number of electrons
is lost in the oxidation half reaction as is gained in the reduction half reaction.
6. Add the balanced half-reactions together and cancel those things that are exactly the same on both
sides of the equation.
a. If you started with an equation that included spectator ions, you may replace those spectator ions
now (if that is needed).
Balancing redox equations Example 1:
Balance the following equation, using half-reactions
for the redox part of the equation.