Created by Michelle Davis Science Department Chair Lithia Springs High School, GA

In this method: Write separate equations for the reduction and oxidation processes Balance each half equation Add them together for the overall balanced equation MnO 4 − ( aq ) + C 2 O 4 2− ( aq )  Mn 2+ ( aq ) + CO 2( aq ) We will use the following equation to illustrate this method: But first some basic rules! This rxn occurs in acidic solution!

1.Assign oxidation numbers to determine what is oxidized and what is reduced. 2.Write the oxidation and reduction half-reactions. 3.Balance each half-reaction. a.Balance elements other than H and O. b.Balance O by adding H 2 O. c.Balance H by adding H +. d.Balance charge by adding electrons. 4.Multiply the half-reactions by integers so that the electrons gained and lost are the same. 5.Add the half-reactions, subtracting things that appear on both sides. 6.Make sure the equation is balanced according to mass. 7.Make sure the equation is balanced according to charge.

MnO 4 − + C 2 O 4 2-  Mn 2+ + CO Since the manganese goes from +7 to +2, it is reduced. Since the carbon goes from +3 to +4, it is oxidized.

C 2 O 4 2−  CO 2 To balance the carbon, we add a coefficient of 2: C 2 O 4 2−  2 CO 2 The oxygen is now balanced as well. To balance the charge, we must add 2 electrons to the right side. C 2 O 4 2−  2 CO e −

MnO 4 −  Mn 2+ The manganese is balanced; to balance the oxygen, we must add 4 waters to the right side. MnO 4 −  Mn H 2 O Remember, we can do this because the rxn occurs in aqueous solution and the water solvent may be reacting in the process. To balance the hydrogen, we add 8 H + to the left side. 8 H + + MnO 4 −  Mn H 2 O Remember, we can do this because the rxn is occurring in acidic solution which means H+ is present!

8 H + + MnO 4 −  Mn H 2 O To balance the charge, we add 5 e − to the left side. 5 e − + 8 H + + MnO 4 −  Mn H 2 O Now, we are ready to combine the two half-reactions!

C 2 O 4 2−  2 CO e − 5 e − + 8 H + + MnO 4 −  Mn H 2 O To attain the same number of electrons on each side, we will multiply the first reaction by 5 and the second by 2. 5 C 2 O 4 2−  10 CO e − 10 e − + 16 H MnO 4 −  2 Mn H 2 O Go ahead and make cancellations beginning with electrons.

5 C 2 O 4 2−  10 CO 2 16 H MnO 4 −  2 Mn H 2 O When we add these together, we get: 16 H MnO 4 − + 5 C 2 O 4 2−  2 Mn H 2 O + 10 CO 2

If a reaction occurs in basic solution, one can balance it as if it occurred in acid. Once the equation is balanced, add OH − to each side to “neutralize” the H + in the equation and create water in its place. If this produces water on both sides, you might have to subtract water from each side.

Brown, T.L., Bursten, B.E., LeMay, Jr., H. E. (2006). Chemistry: The central science. Upper Saddle River, New Jersey: Pearson Education, Inc. Bookstaver, J.D. (2006). Electrochemistry. In Instructor Resource Center for Chemistry: The central science (disk 2, chap.20, lecture presentation). Upper Saddle River, New Jersey: Pearson Education, Inc.