Balancing redox equations
There are two methods that we’ll use to balance redox reaction equations. The first is called the oxidation number method:
- Assign oxidation numbers to all atoms in the equation.
- Identify which atoms change their oxidation number.
- Draw an arrow and indicate how many electrons were gained or lost by writing the number in brackets next to the arrow.
- If the atom has a subscript on one or both of the atoms, write the least common multiple of those subscripts after the brackets.
- 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.
- 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.
- Balance the oxygen atoms using H2O(l).
- Balance the hydrogen atoms using H+(aq).
- 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:
- Divide the equation into oxidation and reduction half-reactions.
- 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.
- Calculate the sum of the charges (not oxidation numbers) on both sides, and then add electrons to one side to balance the charge.
- Multiply both half-reactions so that electrons lost equal electrons gained.
- Add the half-reactions together, cancelling out anything that appears on both sides of the net equation.
- 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.