Balancing Redox

Reactions

Half Reaction Method1. Write the formula equation if it is not given.

Then write the ionic equation.Formula eq: H2S + HNO3 H2SO4 + NO2 + H2O

Ionic eq: H2S + H+ + NO3- 2H+ + SO4

2- + NO2 + H2O

2. Assign oxidation numbers. Delete substances containing only elements that do not change oxidation state. H2S + NO3

- SO42- + NO2

3. Write the half reaction of oxidation H2S SO4

2-

4. Balance the atoms. Add H2O to balance for extra oxygens, then add H+ to balance for hydrogens H2S + 4 H2O SO4

2- + 10H+

-In basic solutions add OH- to cancel out the positive charge from the H+

5. Balance the charge by adding e-

-Look at the change in oxidation state

H2S + 4 H2O SO42- + 10H+ + 8e-

6. Repeat steps 1-5 for the half-reaction for reduction

NO3- NO2

NO3- + 2H+ + e- NO2 + H2O

7. Conserve the charge by adding coefficients in front of the half-reactions so that they cancel out1(H2S + 4 H2O SO4

2- + 10H+ + 8e-)8(NO3

- + 2H+ + e- NO2 + H2O) .

8. Combine the half-reactions, and cancel out anything common to both sides8NO3

- + H2S + 6H+ 8NO2 + 4H2O + SO42-

9. Combine ions to form the compounds shown in the original formula equation.

8HNO3- + H2S 8NO2 + 4H2O + SO4

2- + 2H+

10. Check to ensure that all other ions balance out

8HNO3- + H2S 8NO2 + 4H2O + H2SO4

Balancing Redox Full Reactions in Acid Solution

MnO4- + C2O4

2- → MnO2 + CO32-

MnO4- → MnO2

C2O42- → CO3

2-

2MnO4

- + 3C2O42- + 6H2O + 8H+ → 2MnO2 + 6CO3

2- + 12H+ + 4H2O

2MnO4- + 3C2O4

2- + 2H2O → 2MnO2 + 6CO32- + 4H+

1. Separate into half reactions2. Balance each half reaction3. Add the two half reactions4. Simplify5. Check

+ 4H+ + 3e- + 2H2O2(

3( + 2H2O + 2e-+ 4H+ )2

)

2 4

Balancing Redox Full Reactions in Basic Solution

2MnO4- + 3C2O4

2- + 2H2O → 2MnO2 + 6CO32- + 4H+

1. Balance in acid solution2. Add OH- to neutralize the H+

4OH-4OH-

4H2O2

Balancing Redox Full Reactions in Basic Solution

2MnO4- + 3C2O4

2- + → 2MnO2 + 6CO32- +

-12 -12

4OH- 2H2O

Balancing Redox Full Reactions in Acid Solution

Fe + O2 → H2O + Fe(OH)3

Fe + → + Fe(OH)3

O2 → H20

4Fe + 12H2O + 3O2 +12H+ → 12H+ + 4Fe(OH)3 + 6H2O

4Fe + 6H2O + 3O2 → 4Fe(OH)3

3H2O 3H+ + 3e -)

+ H2O)+ 4H+4e- +3(

4(

6

Balance the redox reactionAs → H2AsO4

- + AsH3 (alkaline)You must separate into two half reactions!

3As + 12H2O + 15H+ + 5As → 3H2AsO4- + 18H+ + 5AsH3

8As + 12H2O → 3H2AsO4- + 3H+ + 5AsH3

3OH- 3OH-

3OH- + 8As + 9H2O → 3H2AsO4- + 5AsH3

As

As

H2AsO4-

AsH3

+ 4H2O + 6H+ + 5e-

3H+ + + 3e-

3( )

5( )

→

→3