Arkansas State University Department of Chemistry and Physics

3.   MnO₄^-   +    I^-   -------->   MnO₂     +    I₂   (basic solution)

Step 1. Break into half-reactions:

  MnO₄^-    ------->    MnO₂
          I^-     ------->     I₂

Step 2. Balance atoms other than H and O

  MnO₄^-    ------->    MnO₂
      2  I^-     ------->     I₂

Step 3. Balance O by adding H₂O

  MnO₄^-    ------->   MnO₂   +   2 H₂O
      2  I^-     ------->     I₂

Step 4. Balance H by adding H⁺

 4 H⁺ +  MnO₄^-    ------->    MnO₂    +   2 H₂O
                  2  I^-     ------->     I₂

Step 5. Balance charge by adding electron(s)

3 e^- +  4 H⁺ +  MnO₄^-    ------->   MnO₂       + 2 H₂O
                               2  I^-     ------->     I₂   +    2 e^-

Step 6. Electrons lost = electrons gained

 (3 e^- +  4 H⁺ +  MnO₄^-    ------->    MnO₂       + 2 H₂O) x 2
 (                             2  I^-     ------->     I₂   +    2 e^- ) x 3

Gives:
 6 e^- + 8 H⁺ + 2 MnO₄^-    ------->   2 MnO₂       + 4 H₂O
          6 I^-      ------->     3  I₂   +   6 e^-

Step 7. Cancel like terms and add half reactions

  8 H⁺ + 2 MnO₄^-   +   6 I^-       ------->   2 MnO₂       + 4 H₂O +  3 I₂

Step 8. Since it is a basic solution, add OH^-  for each H⁺ (add OH^- to both sides of eqn)

8 OH^-   +    8 H⁺  + 2 MnO₄^-   +   6 I^-       ------->   2 MnO₂    + 4 H₂O +  3  I₂   +   8 OH^-

9. Combine H⁺ with OH^- to form H₂O

8 OH^-   +    8 H⁺  + 2 MnO₄^-   +   6 I^-       ------->   2 MnO₂   +  4 H₂O +   3 I₂   +   8 OH^-
Gives:
      8 H₂O   +    2 MnO₄^-   +   6 I^-       ------->   2 MnO₂   +   4 H₂O +  3 I₂   +   8 OH^-

10. Cancel like terms:

   4 H₂O   +    2 MnO₄^-   +   6 I^-       ------->   2 MnO₂    +  3  I₂   +   8 OH^-
 

Back to worksheet