Redox Rxns: Part II: Balancing Redox Equations Chapters 5 Sec 5'1

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Redox RxnsPart II Balancing Redox
EquationsChapters 5 Sec 5.1 5.2of Brady
Senese 5th edition

• Dr. C. Yau
• Fall 2009

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Redox Rxns
Review Redox Rxns involve a transfer of
electrons. Oxidation is INCREASE in oxidation
number, loss of electrons Reduction is DECREASE
in oxidation number, gain of electrons
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Redox Rxns
AgNO3 (aq) Cu (s) Cu(NO3)2
Ag Ag
Ag
Cu Cu2 What is oxidized?
What is reduced? Which is the oxidizing
agent? Which is the reducing agent?
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Balancing Redox Under Acidic Conditions

• 1. Verify that rxn is redox.
• Divide skeleton eqns into half-reactions.
• Balance atoms other than H and O.
• Balance O by adding H2O to each side of eqn.
• Balance H by adding H to side that needs H.
• Balance charge by adding electrons.
• Make the of electrons gained equal to number
  lost
• Add two half-reactions and cancel anything that
  is the same on both sides of eqn.

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Balancing Redox Under Acidic Conditions
Example Cr2O72- Fe2 Cr3
Fe3
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Example 5.6 p.185 Balance the following eqn. The
rxn occurs in an acidic solution. MnO4- H2SO3
SO42- Mn2
Do Pract Exer 8, 10, 11
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Balancing Redox Under BASIC Conditions
Follow the same steps as for acidic conditions
but add on the following steps at the end 9.
Add to BOTH sides, the same number of OH- as
there are H (Under basic condition, there cannot
be any H). 10. Combine H and OH- to form
H2O. 11. Cancel any H2O that you can.
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Example of redox under basic conditions
This is a half-rxn balanced under acidic
conditions. What do we do with it if the rxn was
under basic conditions? 2H2O SO2
SO42- 4H 2 e-
MnO4- C2O42- MnO2 CO32- H2O2
ClO2 ClO2- O2 S8 NO3-
SO2 NO
Do Pract Exer 12, 13 p.187