Balancing Redox Equations by the Ion-Electron Method or Half Cell Method
This method divides a reaction into two parts that are balanced separately. It provides a simple method for obtaining a net ionic equation for a redox reaction.
 
In this method the oxidation and reduction processes are divided into separate equations called half-reactions that are balanced separately. Then the half reactions are combined to give the overall net ionic reaction.
 
In many redox reaction in aqueous solution the H+ and OH- ions play an important role, as do the H2O molecules themselves. Because H+ and OH- can be consumed or produced by a redox reaction, and Because the products can change during a reaction if the solution changes from acidic to basic (or vice versa), redox reactions are generally carried out in solutions containing a substantial excess of either acid or base. Therefore, before you apply the ion-electron method, you have to know whether the reaction occurs in an acidic or basic solution.
 
In Acidic Solutions
Step 1 Divide the skeleton equation into half-reactions.
 
Step 2 Balance atoms other than H and O.
 
Step 3 Balance oxygen atoms by adding H2O to the side that needs O.
 
Step 4 Balance hydrogen by adding H+ to the side that needs H.
 
Step 5 Balance the charge by adding electrons.
 
Step 6 Make the electrons gained equal to the electrons lost and then add the two half-reactions.
 
Step 7 Cancel anything that is the same on both sides.
 
In Basic Solutions
Step 8 Add the same number of OH- as there are H+ to both sides of the equation.
 
Step 9 Combine OH- and H+ to form H2O
 
Step 10 Cancel any H2O that you can.
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