Balancing Redox Reactions

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Balancing Redox Reactions

• The Half-Cell Method

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• The Half-Cell Method

• The method is slightly different for reactions
  that occur in acidic from those that occur in
  basic solution. An extra step is added to the
  process.

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Redox Reactions in Basic Solution

• Water and hydroxide ion are available to the
  reaction because the reaction is occurring in an
  acidic, aqueous solution, but they are not
  initially shown.
• The initial steps are identical to balancing a
  redox reaction in acidic solution. When all those
  steps have been completed, additional steps
  replace the hydrogen ion, present in acidic
  solution, with the hydroxide ion, present in
  basic solutions.

Example NO21- Al ? NH3 AlO21-
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The Half-Cell Method (in acidic or basic
solution)
Step 1 Divide the reaction into an oxidation half
and a reduction half.
Example NO21- Al ? NH3 AlO21- is divided
into these two half-reactions NO21- ? NH3
(reduction N changes from 3 to -3) Al ?
AlO21- (oxidation Al changes from 0 to 3)
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The Half-Cell Method (in acidic or basic
solution)
Step 2 For each half-reaction, balance by
inspection all elements except oxygen and
hydrogen.
Example Both N and Al are already balanced
NO21- ? NH3 Al ? AlO21-
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The Half-Cell Method (in acidic or basic
solution)
Step 3 For each half-reaction, balance oxygen by
adding water to the side deficient in oxygen
atoms. Water is available because the reaction
occurs in aqueous solution.
Example In the N half, the products side is
deficient In the Al half, the reactants side is
deficient NO21- ? NH3 2 H2O 2 H2O Al ?
AlO21-
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The Half-Cell Method (in acidic or basic
solution)
Step 4 For each half-reaction, balance hydrogen
by adding hydrogen ion to the side deficient in
hydrogen atoms.
Example In the N half, the reactants side is
deficient In the Al half, the products side is
deficient 7 H1 NO21- ? NH3 2 H2O 2 H2O
Al ? AlO21- 4 H1
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The Half-Cell Method (in acidic or basic
solution)
Step 5 For each half-reaction, balance the
charges by adding electrons to the side with the
more positive (or less negative) net charge.
Example In the N half, the reactants side has a
net charge of 6, the products side, 0 In the Al
half, reactants side has a net charge of 0, the
products side, 3 6 e- 7 H1 NO21- ? NH3
2 H2O 2 H2O Al ? AlO21- 4 H1 3 e-
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The Half-Cell Method (in acidic or basic
solution)
Step 6 Find the lowest common multiple of the
number of electrons in the two half-reactions.
Balance the electrons by multiplying each
half-reaction by the appropriate factor.
Example The lowest common multiple of 6 and 3 is
6 1(6 e- 7 H1 NO21- ? NH3 2 H2O) 6 e-
7 H1 NO21- ? NH3 2 H2O 2(2 H2O Al ?
AlO21- 4 H1 3 e-) 4 H2O 2 Al ? 2 AlO21-
8 H1 6 e-
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The Half-Cell Method (in acidic or basic
solution)
Step 7 Add the two equations back together,
canceling species that are duplicated on both
sides of the reaction.
Example 6 e- 7 H1
NO21- ? NH3 2 H2O
4 H2O 2 Al ? 2 AlO21- 8 H1 6 e- 6 e-
7 H1 NO21- 4 H2O 2 Al ? NH3 2 H2O 2
AlO21- 8 H1 6 e- All the electrons cancel,
1 H remains on the products side, and 2 H2Os
remain on the reactants side NO21- 2 H2O 2
Al ? NH3 2 AlO21- H1
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The Half-Cell Method (in acidic or basic
solution)
Step 8 Check to see if the equation is balanced.
NO21- 2 H2O 2 Al ? NH3 2 AlO21- H1
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The Half-Cell Method (in basic solution)
Step 9 To the side of the equation that has H1
ions, add enough OH1- ions to neutralize all the
H1, forming water (H1 OH1- ? H2O). Add an
equal number of OH1- ions to the other side of
the equation to keep it balanced. Eliminate
duplicate water molecules.
Step 9 To the side of the equation that has H1
ions, add enough OH1- ions to neutralize all the
H1, forming water (H1 OH1- ? H2O). Add an
equal number of OH1- ions to the other side of
the equation to keep it balanced. Eliminate
duplicate water molecules.
Example OH1- NO21- 2 H2O 2 Al ? NH3 2
AlO21- H1 OH1- OH1- NO21- 2 H2O
2 Al ? NH3 2 AlO21- H2O OH1-
NO21- H2O 2 Al ? NH3 2 AlO21-
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The Half-Cell Method (in acidic or basic
solution)
Step 10 Check to see if the equation is still
balanced.
OH1- NO21- H2O 2 Al ? NH3 2 AlO21-
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Steps for Balancing Redox Reactions in Basic
Solution using the Half-Cell Method
1. Divide the reaction into two halves.
2. Balance, by inspection, all elements except O
and H.
3. Balance O by adding H2O.
4. Balance H by adding H1.
5. Balance charges by adding electrons.
6. Multiply each equation by appropriate factors
to make the number of electrons lost in the
oxidation half equal the number gained in the
reduction half.
7. Add the two halves together, canceling
duplicate species.
8. Check to see if all elements and the charges
balance.
9. Add OH1- ions to neutralize all the H1,
forming water. Add an equal number of OH1- ions
to the other side. Eliminate duplicate water
molecules.
10. Check to see if all elements and the charges
still balance.