How to Balance Redox in Acidic Solutions
Oxidation-reduction (redox) reactions are essential in the field of chemistry, and many experiences in everyday life relate directly to them. The transfer of electrons is at the heart of this subject; it does not refer to transferring oxygen atoms, despite this common misconception. You cannot have a reduction without an oxidation, and vice versa. A common way to remember which process is which is by using the acronym LEO GER (Loss of Electrons, Oxidation; Gain of Electrons, Reduction). These two go hand in hand and provide the underlying chemistry of many processes, including batteries, combustion, metallurgy, corrosion and respiration.
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Instructions
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1
Write the unbalanced net ionic equation, that is, the equation you wish to balance without having done anything to it yet.
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2
Decide which atoms are oxidized and which are reduced, and write the two unbalanced half-reactions. Determining which species has been oxidized and which reduced can be tricky, so pay close attention. Once you separate the net ionic equation into an oxidation piece and a reduction piece, ensure that you have included every atom that was in the original reaction.
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3
Balance both half-reactions for all atoms except O and H. This should be the easy part, because it works just like the normal balancing of reactions.
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Balance each half-reaction for O by adding water to the side with less O, and balance for H by adding H+ to the side with less H. This is where the acidic solution that your reaction is in plays a role. It supplies the H+ to the reactions. If you were balancing in basic solution, this is the step that would be different.
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Balance each half-reaction for charge by adding electrons to the side with greater positive charge (remember that electrons are negatively charged), and then multiply by suitable factors to make the electron count the same in both half-reactions. It is essential that you multiply by factors to ensure that the electron count is the same in each half-reaction. You will need these to cancel out when you add them together later. Don't forget to multiply the entire reaction (both reactants and products) by the factor.
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Add the two balanced half-reactions together, and cancel electrons and other species that appear on both sides of the equation. Picture the reaction arrow like an equals sign: If you have the same thing on each side, you can cancel them out. You may not be able to completely cancel a particular species, but you can often cancel a fraction of that species.
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Check your answer by making sure the equation is balanced both for atoms and for charges. That is, make sure you have the same number of atoms of each type on each side. Also, the charges should add together on each side to the same number. For example, +6 on the reactant side should means you have a +6 on the product side.
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