Balancing Redox Reactions
There are many methods to balance redox reactions, but it is best to pick a method and approach it systematically. Also very important here is that I am showing you the procedure to balance in acidic conditions which means you can add H2O and H+ as needed to fully balance the equations. Balancing in base requires extra steps and those are shown at the very bottom of the page.
The Half-Reaction Method
Here, the half reaction method will be presented. If another methods works better for you, then great. If not, learn this one and practice it. An important idea is that balancing Redox reactions is different in acidic conditions than it is in basic conditions. This is because the reaction involves either H+ or OH-, which will affect both the elements and the charge.
Acidic Conditions: Follow these steps to balance redox reaction in acidic solutions (H+ is present, not OH–).
- Identify the species for the oxidation and reduction and write them as two separate half reactions (they will not be balanced at this point).
- For each half-reaction, balance all elements except for hydrogen and oxygen.
- Next, balance the oxygens in the reaction by adding H2O.
- Now balance the hydrogens using H+ ions
- Now total the charge on each side of the reaction and balance the charge using electrons (e–).
- Make the electron count match in the reduction and the oxidation by multiplying one or both of the balanced half-reactions by whole numbers to equalize the number of the electrons.
- Now combine the two reactions to make your overall redox reaction. Cancel any possible species that are on both sides of the equation (H2O and/or H+). All the electrons will cancel, but note and remember the number that did cancel.
- Now do a final overall check by making sure that all the elements and charge are balanced on each side of the reaction.
The Oxidation Number Method
If you have properly learned how to assign oxidation numbers (previous section), then you can balance redox equations using the oxidation number method. Here, you do all the electron balancing on one line. You establish your two half reactions by looking for changes in oxidation numbers. You then use some arrows to show your half-reactions. Below is the modified procedure for balancing redox reactions using the oxidation number method.
- Assign oxidation numbers
- Draw an arrow connecting the reactant and product for the reduction and the oxidation (the half-reactions).
- Immediately balance for the element undergoing a change in oxidation number if need be.
- Take the difference in oxidation numbers and write the total number of electrons transferred over the arrow. Make sure you account for number of atoms that are changing on each side of the arrow.
- Scale each half-reaction so that the total electrons transferred match in each reaction. Remember to adjust both sides of your arrow.
- NOTE: You should now have the correct stoichiometry for the redox couples. You still need to balance out for the oxygens and hydrogens. All make a note of the total number of electrons transferred.
- Count oxygens on each side. Balance oxygens by adding water (H2O) to the appropriate side.
- Count hydrogens on each side. Balance hydrogens by adding protons (H+) to the appropriate side.
- You should be done now, but do the final check.
- Final Check: Calculate the sum of all charges on each side - the charge has to match.
Dr. McCord - Balancing a Redox Reaction
using the oxidation number method
Below is a video of Dr. McCord balancing a redox reaction using the oxidation number method in acid. The reaction is the classic permanganate/iron(II) reaction. In words the reaction is: permanganate ions reacts with iron(II) ions to make manganese(II) ions and iron(III) ions.
MnO4– + Fe2+ → Mn2+ + Fe3+
Balancing in Basic Conditions
If you are in basic conditions there is not going to be any substantial amount of H+ ions to react. All the oxygens and hydrogens must come from H2O and OH– (hydroxide). We will achieve this feat by balancing in acid first and then converting that answer into basic conditions by a simple neutralization of protons with hydroxide ions. The procedure is below.
Step 1: First, balance in acid and get your your answer. You can use either method described earlier - it doesn't matter which one, just get a balanced reaction "in acid". Once that is complete, Do these 5 additional steps (2-6) to "switch" from acidic conditions to basic conditions.
- Add the same number of moles of OH- to the side of the equation with H+ so that the acid is neutralized into water (H2O).
- Add the same number of OH– to the other side of the reaction.
- On the side with both H+ and OH–, cross both of them out and replace with the same number of moles of H2O. This is how we switch from acidic to basic conditions.
- Now check, and cancel out any waters that appear on both sides of the equation. H2O should only be on one side of the equation.
- Double check that all elements and charge are balanced.