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. This is because the 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.
In Basic Solutions – do steps 1à 5 and then continue with 6à8.
Step 6 Add the same number of OH- as there are H+ to both sides of the equation.
Step 7 Combine OH- and H+ to form H2O
Step 8 Cancel any H2O that you can.
Review the sample problems for balancing half reactions in acidic and basic solutions on Pg. 671 (acidic solutions) and Pg. 672 (basic solutions).
Complete practice problems Pg. 673 #6, 7
Balancing Equations by the Half-Reaction Method:
Recall: A net ionic equation can be broken down into two half-reactions (1 oxidation, 1 reduction)
If you take 2 half-reactions and add them together, balance the number of electrons on both sides (remember, we can’t create or destroy electrons), and cancel the electrons out we get a net ionic equation.
To balance a net ionic equation using half reactions, we must combine the half-reactions so that,
# of electrons lost through oxidation = # of electrons gained through reduction
How can we do this?
Use the least common multiple (LCM) of the number of electrons for the oxidation and reduction half reactions.
Eg. For the reaction of aluminum nitrate and magnesium metal.
Oxidation half-reaction
Mg à Mg2+ + 2e-
Reduction half-reaction
Al3+ + 3e- à Al
The multiples for 2 are (2, 4, 6, 8, 10) ie. 1x2 = 2, 2x2 = 4, 3x2=6, 4x2=8, 5x2=10
The multiples for 3 are (3, 6, 9, 12)
The LCM for 2 and 3 is 6 (The smallest multiple that they both share)
We multiply each half reaction by the LCM to balance the number of electrons in the 2 half-reactions.
In the example above:
Oxidation half-reaction(x3)
3Mg à 3Mg2+ + 6e-
Reduction half-reaction (x2)
2Al3+ + 6e- à 2Al
Now we can add the half-reactions together:
3Mg + 2Al3+ + 6e- à 3Mg2+ + 6e- + 2Al
Cancel the electrons from both sides to get the balanced net ionic equation.
3Mg + 2Al3+ à 3Mg2+ + 2Al
Add in the states and spectator ions to get the balanced chemical equation.
3Mg(s) + 2Al(NO3)3(aq) à 3Mg(NO3)2(aq) + 2Al(s)
Steps for Balancing Chemical Equations by the Half-Reaction Method
1. Write an unbalanced net ionic equation (if not provided)
2. Divide the unbalanced net ionic equation into oxidation and reduction reactions (you may need to determine oxidation numbers for all elements to figure out what is oxidized and what is reduced.
3. Balance each half-reaction separately.
4. Find the LCM.
5. Add coefficients to so that each half-reaction has a number of electrons = LCM.
6. Add the balanced half-reactions.
7. Remove electrons from both sides.
8. Remove identical molecules or ions present on both sides.
9. To get the balanced chemical equation, add spectator ions.
10. Include the state for each reactant and product.
Putting it all together!!!!
Balancing Redox reactions in Acidic and Basic solutions.
· Use the rules for balancing half-reactions in acidic or basic solutions to obtain balanced half reactions.
· Find the net ionic equation and balanced chemical equation using LCM’s