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Chapter 19Electrochemistry

19 | 1

Balancing Redox Reactions

• Half-reaction method from Section 4.6 • Use with acidic or basic solutions• Focus on electron transfer • Represent H3O+(aq) as H+(aq) for simplicity

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Example

+2 +7 +3 +2

• Write the oxidation numbers of Fe and Mn

• Fe2+(aq) is oxidized from +2 to +3 in Fe3+(aq).

• Mn is reduced from +7 to +2 in Mn2+(aq).

• Given: The reaction occurs under acidic conditions.

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42+ 3+ 2+( ) ( ) (Fe + M )nO Fe + Mn ( )aq aq aq g- ®

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(aq)

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Half-Reaction Method in Acidic Solutions 1. Assign oxidation numbers 2. Identify what's being oxidized/reduced 3. Split into half-reactions4. Complete and balance each half-reaction

1. Balance all atoms except H and O.2. Balance O: add H2O to side of equation that needs O3. Balance H: add H+ to the side of equation that needs H.4. Balance the charge: add electrons

5. Finally, we combine the two half-reactions.1. Multiply each half-reaction by a factor to balance electrons.2. Add the two half-reactions3. Canceling species that occur on both sides 4. Reduce coefficients to the smallest whole numbers.5. Check that the equation is balanced

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Reduction half-reactionThe equation is balanced in Mn as it stands. Next, balance the O atoms.

Then, balance the H atoms.

Finally, balance the half-reaction in electric charge.

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- ® 2+4 2MnO ( ) Mn ( )+ 4H O( )aq aq l

- ®+ 2+4 2MnO ( )+ 8H ( ) Mn ( )+ 4H O( )aq aq aq l

- - ®+ 2+4 2MnO ( ) + 8H ( ) + 5e Mn ( )+ 4H O( )aq aq aq l

Reduction half-reaction ©2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

Example: Oxidation of iron(II) ion by permanganate ion in acidic solution.

Oxidation half-reaction-®2+ 3+( ) (Fe aq Fe aq + e)

Now multiply each half-reaction by a factor such that when the two half-reactions are added, the electrons will cancel.

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( )-®2+ 3+5 × Fe Fe + e

( )- - ®+ 2+4 21 × MnO + 8H + 5e Mn + 4H O

2 + 3+ 2+4 25Fe + MnO + 8H + 5e 5Fe + Mn + 4H O + 5e+ - - -®

( ) ( ) ( ) ( ) ( ) ( )2 + 3+ 2+4 25Fe + MnO + 8H 5Fe + Mn + 4H Oaq aq aq aq aq l+ - ®

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Zinc metal reacts with nitric acid, HNO3, to produce a number of products, depending on how dilute the acid solution is. In a concentrated solution, zinc reduces nitrate ion to ammonium ion; zinc is oxidized to zinc ion, Zn2+. Write the net ionic equation for this reaction.

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- ® 2+ +3 4Zn( ) + NO ( ) Zn ( ) + NH ( )s aq aq aq

0 + 5 + 2 – 3

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-® 2+Zn Zn + 2e (oxidation half-re( ) ( ) action)s aq

- ®+ +3 4 210H ( ) + NO ( ) NH ( ) + 3H O( )aq aq aq l- - ®+ +

3 4 210H ( ) + NO ( ) + 8e NH ( ) + 3H O( ) (reduction half-reaction)aq aq aq l

-

®

®

2+

+3 4

Zn( ) Zn ( )NO ( ) NH ( )

s aqaq aq

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( )2+ -4× Zn Zn +2e®- - ®+ +

3 4 210H + NO + 8e NH + 3H O

- - -®+ 2+ +3 4 24Zn + 10H + NO + 8e 4Zn + NH + 3H O + 8e

( ) ( ) ( )( ) ( ) ( )

®+ -3

2+ +4 2

4Zn + 10H + NO 4Zn + NH + 3H O l

s aq aqaq aq

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Additional steps for balancing oxidation–reduction equations in basic solution To balance a reaction in basic conditions, first follow the same procedure as for acidic solution.

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Then: 5. Note the number of H+ ions in the equation.

Add this number of OH− ions to both sides of the equation.

6. Simplify the equation by noting that H+ and OH− react to form H2O. Cancel water molecules that occur on both sides.

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Permanganate ion oxidizes sulfite ion in basic

solution according to the following:

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( ) ( ) ( ) ( )- - -®2 24 3 2 4MnO + SO MnO + SOaq aq s aq

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in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

After balancing the equation as if it were in acidic solution, we obtain the following:

Add 2 OH– to both sides of the equation.

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- - -®2 + 24 3 2 4 22MnO + 3SO +2H 2MnO + 3SO +H O

- - - - -+ ®2 + 24 3 2 4 22MnO + 3SO +2H 2OH 2MnO + 3SO +H O+ 2OH

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Replace 2H+ + 2OH– with 2H2O on the left side. Cancel one of these with the H2O on the right side.

( ) ( ) ( )( ) ( ) ( )

- -

- -

®24 3 2

22 4

2MnO + 3SO + H O 2MnO + 3SO + 2OH

aq aq l

s aq aq