Balancing half-reactions in basic solution

Balancing half-reactions in acidic solution

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Reminder: a half-reaction MUST be balanced both for atoms and charge in order to be correct. It is VERY easy to balance for atoms only, forgetting to check the charge.


Here is the half-reaction to be considered:

PbO2 ---> PbO

It is to be balanced in basic solution.

Here is a second half-reaction:

MnO4¯ ---> MnO2 [basic soln]

As I go through the steps below using the first half-reaction, try and balance the second half-reaction as you go from step to step. The answer will appear at the end of the file. Before looking at the balancing technique, the fact that it is in basic solution can be signaled to you in several different ways:

1) It is explicitly said in the problem.
2) A base (usually a strong base) is included as one of the reactants.
3) An OH¯ is written just above the reaction arrow.
4) A term like "base" or "basic soln" is written after the half-reaction. Usually in parentheses or square brackets.

Here's the last point before teaching the technique. There are three other chemical species available in a basic solution besides the ones shown above. They are:


The water is present because the reaction is taking place in solution, the hydroxide ion is available because it is in basic solution and electrons are available because that's what is transfered in redox reactions. All three will be used in getting the final answer.

Step One to Four: Balance the half-reaction AS IF it were in acid solution. I hope you got that. The half-reaction is actually in basic solution, but we are going to start out as if it were in acid solution. Here are the 4 acid steps:

1) Balance the atom being reduced/oxidized.
2) Balance the oxygens (using H2O).
3) Balance the hydrogens (using H+).
4) Balance the charge.

When you do that to the above half-reaction, you get:

2e¯ + 2H+ + PbO2 ---> PbO + H2O

Step Five: Convert all H+ to H2O. Do this by adding OH¯ ions to both sides. The side with the H+ will determine how many hydroxide to add. In our case, the left side has 2 hydrogen ions, while the right side has none, so:

2e¯ + 2H2O + PbO2 ---> PbO + H2O + 2OH¯

Notice that, when the two hydroxide ions on the left were added, they immediately reacted with the hydrogen ion present. The reaction is:

H+ + OH¯ ---> H2O

Step Six: Remove any duplicate molecules or ions. In our example, there are two water molecules on the left and one on the right. This means one water molecule may be removed from each side, giving:

2e¯ + H2O + PbO2 ---> PbO + 2OH¯

The half-reaction is now correctly balanced.

By the way, notice the 2OH¯. Be careful to read that as two hydroxide ions (2 OH¯) and NOT twenty hydride ions (2O H¯). People have been known to do that.

How did you do with the other one? If you didn't do it, go back and try it, then click for the answer.

Or you could examine another example (in basic solution), then click for the permanganate answer.

NH3 ---> N2H4

Here are the steps:

1) 2NH3 ---> N2H4 + 2H+ + 2e¯ (balanced as if in acid solution; there were no oxygens to balance.)
2) 2OH¯ + 2NH3 ---> N2H4 + 2H2O + 2e¯ (add two hydroxides to each side; this is the correct answer, there are no duplicates to strike out.)

One more comment: sometimes, the "fake acid" method can be skipped. For example, this half-reaction:

Fe ---> Fe(OH)3

might show up. Balancing it directly in basic seems fairly easy:

Fe + 3OH¯ ---> Fe(OH)3 + 3e¯

And yet another comment: there is an old-school method of balancing in basic solution, one that the ChemTeam learned in high school, lo these many years ago. Even though the "fake acid" method is much easier, there are still teachers that insist on the old-school method (which uses hydroxide directly and does not rely on hydrogen ion), but they are increasingly rare. There is an individual on Yahoo Answers who answers balancing questions using the old-school method.

10 More Examples: Balance each half-reaction, the reaction being in basic solution.

1) NiO2 ---> Ni(OH)2

2) BrO4¯ ---> Br¯

3) SbO3¯ ---> SbO2¯

4) Cu2O ---> Cu

5) S2O32¯ ---> SO32¯

6) Tl+ ---> Tl2O3

7) Al ---> AlO2¯

8) Sn ---> HSnO2¯

9) CrO42¯ ---> Cr(OH)3

10) HfO(OH)2 ---> Hf

Answers to Ten More Examples

Balancing half-reactions in acidic solution

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