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

Chapter 4: Reactions in Aqueous Solution 11/12/2018 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.

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)

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.

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.

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.

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.

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