Transcript Balancing Redox Reactions: The Half

Chapter 4
4.10 – Balancing Redox Reactions: The
Half-Reaction Method
Oxidation numbers
Oxidation Number (State): A value which indicates whether an atom is neutral,
electron-rich, or electron-poor.
Rules for Assigning Oxidation Numbers
1.
2.
3.
4.
5.
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
1. Hydrogen can be either +1 or –
2. 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
Chapter 4/2
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)
3
Balancing Redox Reaction
using half equation
 Assign oxidation numbers to each atom from the given unbalanced equation
 Split the equation into half-reaction
 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
Copyright © 2008 Pearson Prentice Hall, Inc.
Chapter 4/4
Balancing Redox Reactions: The
Half-Reaction Method
 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
Copyright © 2008 Pearson Prentice Hall, Inc.
Chapter 4/5
Additional steps for balancing
equations in basic solution
 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
Example
Balance the following net ionic equation in acidic solution:
I1-(aq) + Cr2O72-(aq)
Cr3+(aq) + IO31-(aq)
Copyright © 2008 Pearson Prentice Hall, Inc.
Chapter 4/7
Balancing Redox Reactions: The
Half-Reaction Method
Balance the following net ionic equation in basic solution:
MnO41-(aq) + Br1-(aq)
MnO2(s) + BrO31-(aq)
Copyright © 2008 Pearson Prentice Hall, Inc.
Chapter 4/8
Example
 Balance the following net-ionic equation by the half-reaction method.
 Cu(s) + NO3-(aq)  Cu2+(aq) + NO2(g)
 I-(aq) + MnO4-(aq)  I2(aq) + MnO2(g)
Acidic condition
Basic Condition
 Fe(OH)2(s) + O2(g)  Fe(OH)3(s) Basic condition