1. BALANCING REDOX REACTIONS
Oxidation Number Method

2. OXIDATION NUMBER METHOD
Redox reactions can be balanced using:
a) Half Reactions
b) Oxidation numbers
Goal: To balance the electrons that
are gained and lost
NOTE: When half-reactions are not provided, and a reaction
does not occur in either acidic or basic solution, then the
oxidation number method can be used to balance a redox
reaction

3. OXIDATION NUMBER METHOD
Example #1
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
Al(s) + MnO2(s)  Al2O3(s) + Mn(s)
0 +4 -2 +3 -2 0

4. OXIDATION NUMBER METHOD
Example #1
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
+3
Al(s) + MnO2(s)  Al2O3(s) + Mn(s)
0 +4 -2 +3 -2 0
-4

5. OXIDATION NUMBER METHOD
Example #1
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
+3
Al(s) + MnO2(s)  Al2O3(s) + Mn(s)
0 +4 -2 +3 -2 0
-4
Ratio = 4:3
Each Al atom loses 3, .: 4 Al loses 12
Each Mn atom gains 4, .: 3 Mn gains 12

6. OXIDATION NUMBER METHOD
Example #1
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
+3
4 Al(s) + 3 MnO2(s)  Al2O3(s) + Mn(s)
0 +4 -2 +3 -2 0
-4
Ratio = 4:3

7. OXIDATION NUMBER METHOD
Example #1
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
+3
4 Al(s) + 3 MnO2(s)  2Al2O3(s) + 3Mn(s)
0 +4 -2 +3 -2 0
-4
Ratio = 4:3

8. OXIDATION NUMBER METHOD
Example #1
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
4 Al(s) + 3 MnO2(s)  2Al2O3(s) + 3Mn(s)

9. OXIDATION NUMBER METHOD
Example #2
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
NH3(g) + O2(g)  NO2(g) + H2O(l)
-3 +1 0 +4 -2 +1 -2

10. OXIDATION NUMBER METHOD
Example #2
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
+7
NH3(g) + O2(g)  NO2(g) + H2O(l)
-3 +1 0 +4 -2 +1 -2
-2

11. OXIDATION NUMBER METHOD
Example #2
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
+7
NH3(g) + O2(g)  NO2(g) + H2O(l)
-3 +1 0 +4 -2 +1 -2
-2
Ratio = 4:7
Each N atom loses 7, .: 4 N loses 28
Each O atom gains 2  each O2 gains 4, .: 7 O2 gains 28

12. OXIDATION NUMBER METHOD
Example #2
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
+7
4 NH3(g) + 7 O2(g)  NO2(g) + H2O(l)
-3 +1 0 +4 -2 +1 -2
-2
Ratio = 4:7
Each N atom loses 7, .: 4 N loses 28
Each O atom gains 2  each O2 gains 4, .: 7 O2 gains 28

13. OXIDATION NUMBER METHOD
Example #2
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
+7
4 NH3(g) + 7 O2(g)  4 NO2(g) + 6H2O(l)
-3 +1 0 +4 -2 +1 -2
-2
Ratio = 4:7
Each N atom loses 7, .: 4 N loses 28
Each O atom gains 2  each O2 gains 4, .: 7 O2 gains 28

14. OXIDATION NUMBER METHOD
Example #2
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
4 NH3(g) + 7 O2(g)  4 NO2(g) + 6H2O(l)

15. OXIDATION NUMBER METHOD
Example #3: In acidic solution
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
Cu(s) + NO3-(aq)  NO2(g) + Cu2+(aq)
0 +5 -2 +4 -2 +2

16. OXIDATION NUMBER METHOD
Example #3: In acidic solution
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
+2
Cu(s) + NO3-(aq)  NO2(g) + Cu2+(aq)
0 +5 -2 +4 -2 +2
-1

17. OXIDATION NUMBER METHOD
Example #3: In acidic solution
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
+2
Cu(s) + NO3-(aq)  NO2(g) + Cu2+(aq)
0 +5 -2 +4 -2 +2
-1
Ratio = 1:2
Each Cu atom gains 2, .: 1 Cu gains 2
Each N atom loses 1, .: 2 N loses 2

18. OXIDATION NUMBER METHOD
Example #3: In acidic solution
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
+2
Cu(s) +2 NO3-(aq)  NO2(g) + Cu2+(aq)
0 +5 -2 +4 -2 +2
-1
Ratio = 1:2
Each Cu atom gains 2, .: 1 Cu gains 2
Each N atom loses 1, .: 2 N loses 2

19. OXIDATION NUMBER METHOD
Example #3: In acidic solution
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
+2
Cu(s) +2 NO3-(aq)  2 NO2(g) + Cu2+(aq)
0 +5 -2 +4 -2 +2
-1
Ratio = 1:2
Each Cu atom gains 2, .: 1 Cu gains 2
Each N atom loses 1, .: 2 N loses 2

20. OXIDATION NUMBER METHOD
Example #3: In acidic solution
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
4H+ + Cu(s) +2 NO3-(aq)  2 NO2(g) + Cu2+(aq) + 2H2O
1. Write down the reactant and product and balance atoms other than oxygen and hydrogen.
2. Add H2O molecules to balance the oxygen atoms.
3. Add H+ ions to balance the hydrogen atoms.
4. Add electrons to balance the charge on both sides of the equation.

21. OXIDATION NUMBER METHOD
Example #3: In acidic solution
1. Assign oxidation numbers to each element.
2. Identify the increase and decrease in oxidation numbers.
3. Determine the smallest whole-number ratio of the oxidized and reduced elements so that the total increase in oxidation
numbers equals the total decrease in oxidation numbers.
4. Use the determined ratio from step 3 to balance the oxidized and reduced reactants.
5. Balance the other elements/compounds by inspection.
6. Add H2O, H+, and OH- as needed to balance reactions in acidic or basic solution (recall the half-reaction method rules)
4H+ + Cu(s) +2 NO3-(aq)  2 NO2(g) + Cu2+(aq) + 2H2O