Electrochemistry Preliminary, Page 2

Step 3

  • Balance two half-reactions

  • From the previous page, we identify two half-reactions:

    Rules to follow in order to balance each equation:
    StepTo Do
    1Balance atoms other than H and O
    2Balance O by adding H2O
    3Balance H by adding H+
    4Balance charge by adding e-

    Using the above example, the half-reactions are balanced in the following way using the table above.

    Reduction RxnOxidation Rxn

    Reduction ReactionOxidation Reaction
  • Step 1 -- There is only Cl atom to adjust. The left-hand side contains one Cl. The right-hand contains one Cl, and therefore they are balanced.
  • Step 1 -- There is only one Cr atom on the both sides of the equation. Therefore, it is balanced in terms of atoms other than O and H.
  • Step 2 -- By adding one H2O to the right-hand side of the eqution make Oxygen atoms balanced.
  • Step 2 -- On the right-hand side, there are two Oxygen atoms more than those of the left-hand side. So, add two H2O to the left.
  • Step 3 -- Since two Hydrogens are introduced in Step 2, 2H+ must be added to the left-hand side of the equation.
  • Step 3 -- Since two H2O (4 H's) are added at Step 2, four H+ must be added to the right-hand side.
  • Step 4 -- Overall charge on the right-hand side is -1 due to the charge on the Cl- ion. The left-hand side has +1 before adding e-. In order to make the charge on the left-hand side to come out to be the same as the right is to add 2e- on the left.

  • Step 4 -- The charge on the left-hand side is -1. The righ-hand side is +2. Therefore, one should add three e- on the right-hand side to match the overall charges on the both side.
  • Hint for adding electrons: Since an electron, e-, has a minus charge, you should add to the side of the equation that is more positive!

    next page

    March, 2000, Nikita Matsunaga