1. CHAPTER 4
4.9 – Balancing Redox Reactions: The Half- Reaction Method

2. Chapter 4: Reactions in Aqueous Solution
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Oxidation numbers
Oxidation Number (State): A value which indicates whether an atom is neutral, electron-rich, or electron-poor.
Rules for Assigning Oxidation Numbers
An atom in its elemental state has an oxidation number of 0.
A monatomic ion has an oxidation number identical to its charge
An atom in a polyatomic ion or in a molecular compound usually has the same oxidation number it would have if it were a monatomic ion
Hydrogen can be either +1 or –
Oxygen usually has an oxidation number of -2
Halogens usually have an oxidation number of -1
The sum of the oxidation numbers is 0 for a neutral compound and is equal to the net charge for a polyatomic ion
Copyright © 2008 Pearson Prentice Hall, Inc.

3. Writing the half Oxidation and Reduction Reactions
Write the separate half oxidation and reduction reactions for the following equation. 2Cs(s) + F2(g) 2CsF(s)

4. Balancing Redox Reaction using half equation in an Acidic Solution
Chapter 4: Reactions in Aqueous Solution
Balancing Redox Reaction using half equation in an Acidic Solution
11/12/2018
1. Assign oxidation numbers to each atom from the given unbalanced equation
2. Split the equation into half-reaction
3. Complete and balance each half reaction
Balance all atoms except O and H
Balance oxygen atoms by adding H2O to one side of the equation
Balance hydrogen atoms by adding H+ ions to one side of the equation
Balance the number of electrons being transferred
4. Combine the half-reaction to obtained the final balanced equation
The electrons must be cancelled
Simplify the equation by reducing coefficients and canceling repeated species
After you’re done, double check your balanced equation
The half-reaction method focuses on the individual half-reactions which is important for electrochemistry.
Copyright © 2008 Pearson Prentice Hall, Inc.

5. Chapter 4: Reactions in Aqueous Solution
11/12/2018
Example
Balance the following net ionic equation in acidic solution:
Cr3+(aq) + IO31-(aq)
I1-(aq) + Cr2O72-(aq)
Copyright © 2008 Pearson Prentice Hall, Inc.

6. Additional steps for balancing equations in basic solution
Chapter 4: Reactions in Aqueous Solution
11/12/2018
Additional steps for balancing equations in basic solution
Follow all steps from the acidic solution ( 1 4)
Add the desired number of –OH react with H+ ions in the reaction; add to both side of the reaction. This is a neutralization step
Simplify the equation by noting that H+ combines with –OH to give H2O
Cancel any repeating H2O and –OH ions and reduce reaction to the lowest coefficients
The half-reaction method focuses on the individual half-reactions which is important for electrochemistry.
Copyright © 2008 Pearson Prentice Hall, Inc.

7. Balancing Redox Reactions: The Half-Reaction Method
Chapter 4: Reactions in Aqueous Solution
11/12/2018
Balancing Redox Reactions: The Half-Reaction Method
Balance the following net ionic equation in basic solution:
MnO2(s) + BrO31-(aq)
MnO41-(aq) + Br1-(aq)
Copyright © 2008 Pearson Prentice Hall, Inc.

8. Example
Balance the following net-ionic equation by the half-reaction method.
Cu(s) + NO3-(aq)  Cu2+(aq) + NO2(g) Acidic condition
I-(aq) + MnO4-(aq)  I2(aq) + MnO2(g) Basic Condition