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Balancing Redox Reactions by half-reactions

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					Balancing Redox Reactions

 The Half-reaction Method
   Balancing Redox Equations
• Many redox reactions occur in acidic or
  basic solution.
• H1+(aq) and OH1-(aq) ions, and H2O
  molecules often play an important role in
  these types of reactions
• The method of half-reactions can be used
  to balance a redox reaction given in the
  form of a net-ionic equation
    The Half-reaction Method
Eg1) Balance the following reaction which
 occurs in an acidic solution.


 I2(aq) + OCl1-(aq)  IO31-(aq) + Cl1-(aq)

STEP 1: Divide the skeleton equation into
half reactions
     I2  IO31-    (oxidation half)
    OCl1-  Cl1- (reduction half)
STEP 2: Balance atoms other than H
 &O
        I2  2IO31-

        OCl1-  Cl1-
STEP 3: Balance “O” atoms using H2O

         6H2O + I2  2IO31-

         OCl1-  Cl1- + H2O
STEP 4: Balance “H” atoms using H1+ ions

    6H2O + I2  2IO31- + 12H1+

    2H1+ + OCl1-  Cl1- + H2O
  STEP 5: Balance the charges on each side
   using electrons (THIS IS THE TRICKIEST
   STEP!)

   6H2O + I2  2IO31- + 12H1+ + 10e-

       2e- + 2H1+ + OCl1-  Cl1- + H2O


Note: Look at the total sum of charges on each
side of each half-reaction for this step!
STEP 6: Balance the # of electrons
 between the two half-reactions and add
 them together algebraically.

 1 x( 6H2O + I2  2IO31- + 12H1+ + 10e- )

5 x( 2e- + 2H1+ + OCl1-  Cl1- + H2O )
6H2O + I2  2IO31- + 12H1+ + 10e-

10e- + 10H1+ + 5OCl1-  5Cl1- + 5 H2O
1                       2
6H2O + I2  2IO31- + 12H1+ + 10e-

10e- + 10H1+ + 5OCl1-  5Cl1- + 5 H2O

    1H2O + I2  2IO31- + 2H1+
        5OCl1-  5Cl1-

H2O + I2 + 5OCl1-  2IO31- + 5Cl1- + 2H1+


       It’s balanced!
For basic solutions… 3 more steps!

STEP 1  6 are the same as acidic solutions,
 then…
STEP 7: Add the same # of OH1- to both sides to
 neutralize the H1+ ions
STEP 8: Combine OH1- and H1+ to form H2O
STEP 9: Cancel any H2O appearing on both sides
 of the equation.
Eg.2 Balance the following redox
  reaction if it occurs in a basic
              solution

MnO41- + C2O42-  MnO2 + CO32-
Homework!

				
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posted:11/1/2011
language:English
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