Balancing Redox Reactions
Oxidation Number Method
There are two methods used to balance redox
reactions. First we will look at the oxidation
number method.
1) Assign oxidation numbers to all atoms in
the equation.
Cu + HNO3  Cu(NO3)2 + NO2 + H2O
2) Identify the atoms oxidized and reduced.
Cu + HNO3  Cu(NO3)2 + NO2 + H2O
3) Balance electrons lost and gained using
appropriate coefficients.
Cu + HNO3  Cu(NO3)2 + NO2 + H2O
4) Make the change in oxidation numbers equal
in magnitude by adjusting coefficients in the
equation.
Cu + HNO3  Cu(NO3)2 + NO2 + H2O
5) If necessary, use the conventional method to
balance the remainder of the equation.
Cu + HNO3  Cu(NO3)2 + NO2 + H2O
Try this one yourself:
K2Cr2O7 + H2O + S → SO2 + KOH + Cr2O3
Balancing Redox Reactions
Oxidation Number Method
Balance the following aqueous oxidation
reduction reaction that occurs in an acidic
solution.
MnO4– + I–→ MnO2 + I2
Step 1: Assign oxidation numbers to all the
atoms in the reaction. Write the number
above the appropriate atoms and show
electrons lost and gained.
Step 2: Determine the change in oxidation
number and what is being oxidized and
reduced.
Step 3: Make the change in oxidation numbers
equal in magnitude by adjusting
coefficients in the equation.
Step 4: Add up the ion charges and balance
with H+ since the reaction occurs in an
acidic solution.
Step 5: Water is now added to the opposite side
to balance the H and O atoms.
Try: ClO3- + Cl-  Cl2 + ClO2
Balancing Redox Reactions
Balance the following aqueous oxidation
reduction reaction that occurs in a basic
solution.
MnO4– + C2O42– → CO2 + MnO2
For basic solutions, instead of balancing with
H+(aq), use OH-(aq). Then add water to the
opposite side to balance the O and H atoms.
Try: Zn + NO3-  ZnO22- + NH3