The Chemical Reaction, Part 5 - Balancing a Redox Equation in an Acidic Medium

One of the more challenging chemistry problems is balancing the equation of a Redox Reaction occurring in an acidic solution.  The challenge comes from the fact that the aqueous acidic medium takes part in the reaction.  Because water can be oxidized and reduced, it is introduced into the balanced equation.  A particular reaction will favor water oxidation or reduction, depending on the activities of the "other" reactants compared to that of water.  Activity is a measure of a reacting species' ability to provide electrons.  In other words, activity measures the ability of a reactant to reduce another reactant; which also means that activity measures the ability of a reactant to undergo oxidation itself.

Water participates in a redox reaction when its activity (arbitrarily assigned a value of "0") is between the activities of the two primary reactants.

The following graphic image illustrates the process of balancing a redox equation for a reaction taking place in an acidic solution.

The reader should note that the order of certain actions are important.  For instance, a mass balance of the element undergoing oxidation or reduction needs to be done before an electron number is assigned for that half reaction.

That's all for now.  As always, thank you for reading!

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