Balancing redox equations

There are two methods that we’ll use to balance redox reaction equations. The first is called the oxidation number method:

  1. Assign oxidation numbers to all atoms in the equation.
  2. Identify which atoms change their oxidation number.
  3. Draw an arrow and indicate how many electrons were gained or lost by writing the number in brackets next to the arrow.
  4. If the atom has a subscript on one or both of the atoms, write the least common multiple of those subscripts after the brackets.
  5. Find the least common multiple of the products on the two arrows. Add multipliers to the front of the brackets so that the products are equal.
  6. Add coefficients—just the number left of the brackets if it already has a subscript, or the product of the left number and the right number if it doesn’t have a subscript.
  7. Balance the oxygen atoms using H2O(l).
  8. Balance the hydrogen atoms using H+(aq).
  9. Basic solutions: Add OH to both sides equal in number to the H+ ions. Combine H+ and OH to make H2O, and cancel out anything that appears on both sides of the equation.

The second method is the half-cell, half-reaction, or ion-electron method:

  1. Divide the equation into oxidation and reduction half-reactions.
  2. Balance each half-reaction for number of atoms: first balance atoms other than H and O, then add H2O to balance O, and finally add H+ to balance H.
  3. Calculate the sum of the charges (not oxidation numbers) on both sides, and then add electrons to one side to balance the charge.
  4. Multiply both half-reactions so that electrons lost equal electrons gained.
  5. Add the half-reactions together, cancelling out anything that appears on both sides of the net equation.
  6. Refer to step 9 in the oxidation number method for basic solutions.

You won’t learn either of these methods just by reading this. Balancing redox equations isn’t that difficult, but it requires practice.