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Redox Equations
Chemistry Materials from Ray Tedder

You need to know how to write a net ionic equation before attempting to balance redox (oxidation-reduction) equations.  If you don't remember how to write a net ionic equation, now is the time to review.  You should also be able to recognize the different between a full equation and a net ionic equation. 

Steps for balancing redox equations

using the half-reaction method.

 

1.      If the equation is already in net ionic form, go to step 2.  If not, write the skeleton net ionic equation for the reaction (and that means that you should leave out the spectator ions).  In some cases, it will not be possible to balance the net ionic equation, but you must still write the soluble salts, strong acids, and strong bases in ionic form.

 

2.      Determine the oxidation number of all atomic species in the net ionic equation.  Its the change in oxidation number of these atomic species that determines the oxidation (increase in oxidation number) or reduction (reduction in oxidation number).

 

3.      Write the oxidation and reduction half-reactions from the net ionic equation.

a.      Note: Its the change in the oxidation number that determines whether an oxidation or reduction half takes place, not the change in charge number on a poly-atomic ion.

b.      An oxidation half-reaction includes electrons.  You must determine which side of the half-reaction has more electrons and add that number of electrons to the other side of the equation so that there are an equal number of electrons on both sides.

 

4.      Balance the atoms and charges in each half reaction.

a.      If one side of the half-reaction equation has more oxygen atoms than the other, add as many waters as are needed to balance the half-reaction for oxygen.

b.      If one side of the half-reaction equation has more hydrogen atoms than the other, add as many hydrogen ion as are needed to balance the half-reaction for oxygen.

c.      If the reaction is taking place in an acid solution, skip this step and go to d.  If this reaction is taking place in a basic solution, then add an equal number of hydroxide ions to both sides of the half reaction to neutralize the hydrogen ions.

d.      Check to see that both sides of each half-reaction equation have the same number of each atom on both sides of the arrow. 

e.      Check to see that both sides of each half reaction have the same number of charges when the charges on molecules and ions are added together.

 

5.      Multiply the half reactions by the numbers that is necessary so that the same number of electrons is lost in the oxidation half reaction as is gained in the reduction half reaction.

 

6.      Add the balanced half-reactions together and cancel those things that are exactly the same on both sides of the equation. 

a.      If you started with an equation that included spectator ions, you may replace those spectator ions now (if that is needed).

 

 

Balancing redox equations Example 1:

 

Balance the following equation, using half-reactions for the redox part of the equation.

redoxex1.jpg
Step 1A: Write a full ionic equation:
redox1step1.jpg
Step 1B: Cancel spectator ions to write a skeleton net ionic equation:
redox1step1b.jpg
Step 2: Determine the oxidation number of all the atomic particles in the equation:
redox1step2.jpg
Step 3: Determine the oxidation and reduction half reactions and write them:
redox1step3.jpg
Step 4: Balance the electrons, atoms, and charges in each half reaction:
redox1step4.jpg
Step 5: Multiply the half reactions an equal number of electrons on both sides of the arrow:
redox1step5.jpg
Step 6A: Add the half-reactions together.
redox1step6a.jpg
Step 6B: Cancel those things that are exactly the same on both sides.
redox1step6b.jpg

You can check your work one more time by checking to make sure that the equation is both mass balanced and charge balanced.

If you find errors on this page or you have questions, please contact:
Ray Tedder <TedderAR@spartanburg6.k12.sc.us>.