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Transcript
CHAPTER 4

4.10 – Balancing Redox Reactions: The HalfReaction 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
Ch
apt
er
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 in an Acidic Solution
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
Example
Balance the following net ionic equation in acidic solution:
I1-(aq) + Cr2O72-(aq)
Cr3+(aq) + IO31-(aq)
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
Balancing Redox Reactions: The HalfReaction Method
Balance the following net ionic equation in basic solution:
MnO41-(aq) + Br1-(aq)
MnO2(s) + BrO31-(aq)
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)
Condition
◦ Fe(OH)2(s) + O2(g)  Fe(OH)3(s)
Basic
Basic condition