Balancing Redox Equations in Acidic Solution

To balance oxidation-reduction equations that occur in solution.
It is one thing to balance an oxidation-reduction (redox) reaction when you are given the full unbalanced equation, and quite another when you are given only certain parts of it. In this problem, you will be given some information about an oxidation-reduction reaction and you will have to come up with the full balanced equation.

For example, say you know that SO2 is oxidized to SO42− and Br2 is reduced to Br−in acidic solution. From this information you can determine the half-reactions to be as follows:



And since there is a gain or loss of two electrons in each of these half-reactions, you might be inclined to write the full equation like this:


But even though the electrons are balanced, oxygen is not balanced. What has been missed? 

The secret to this problem lies in the fact that the reaction occurs in acidic solution. This means that H2O is present as well as an excess of H+ ions. Therefore you should add H+ and H2O to the equation where appropriate. For example,


needs two more O atoms on the left. Therefore you must add 2H2O to the left side of the equation to get


Now you need four more H atoms on the right, so you should add 4H+ to the right side of the equation to get


Now the equation is properly balanced.

Note that, in basic solution, H2O and OH−are used to balance the equation instead of H2O and H+.

In acidic solution, MnO4− is reduced to Mn2+ , whereas Fe2+ is oxidized to Fe3+

Given the partial equation

? MnO4− + ? Fe2+ → ? Mn2+ + ? Fe3+

what must the coefficients be so that the electrons are balanced?



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Posted on September 25, 2013, in #question and tagged , , , , , , , , , , . Bookmark the permalink. 5 Comments.

  1. Given the equation
    how many H2O molecules should be added to the right side of the equation to balance the oxygen atoms?


  2. Given the equation
    how many H+ ions should be added to the left side to balance the hydrogen atoms?



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