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K+ + Cr2O72― + H+ + NO3― + C2H6O
K+ + NO3― + C2H4O + H2O + Cr3+ + NO3―
3. With spectators omitted, find two atoms that change oxidation number in the reaction.
***** ©2009 ChemReview.net v. k1 Page 393 Module 16: Half-Reaction Balancing Each +6 -2 +1 Cr2O72― +
Total -2 +1 -2 H+ + C2H6O +12 -14 -4 +6 -2 -1 +1 -2 +1 -2 +3 C 2 H 4 O + H 2 O + Cr 3+
-2 +4 -2 +3 The two atoms that change oxidation number are Cr and C.
See if you can solve from here by splitting then adding the two half-reactions.
4. Split the 4 particles containing those two atoms into two half-reactions, writing one
above the other.
Cr2O72― Cr3+ C2H6O C2H4O 5. Balance the half-reactions using the CA-WHe! method. *****
6 e─ + 14 H+ + Cr2O72─
C2H6O 2 Cr3+ + 7 H2O
C2H4O + 2 e─ + 2 H+ 6. Multiply the coefficients in each half-reaction to equalize the number of electrons in the
two half-reactions, then add the half-reactions.
6 e─ + 8 14 H+ + Cr2O72─
3C H O
26 2 Cr3+ + 7 H2O (1x) 3 C H O + 6 2 e─ + 6 2 H+
24 (3x) 8 H+ + Cr2O72─ + 3 C2H6O 2 Cr3+ + 7 H2O + 3 C2H4O Check: 26 H, 2 Cr, 10 O, 6 C atoms on both sides, +6 net charge on both sides.
7. Plug the trial coefficients from the total of the half-reactions into the original equation.
Then, using trial and error, finish balancing.
1 K2Cr2O7 + 8 HNO3 + 3 C2H6O 2 KNO3 + 3 C2H4O + 7 H2O + 2 Cr(NO3)3 8. Check: 2K, 2 Cr, 34 O, 26 H, 8 N, 6 C on both sides, neutral on both sides.
In balancing, try to avoid calculator use.
IF you find that you need a calculator to do the arithmetic of balancing and checking,
make flashcards of the math facts on which you are rusty. Practicing those flashcards
will give you the quick math recall that will help you to “keep your train of thought”
in upcoming more complex calculations. ©2009 ChemReview.net v. k1 Page 394 Module 16: Half-Reaction Balancing Practice:
1. First learn the steps above, then apply them to balance these redox reactions.
Cu(NO3)2 + NO + H2O Cu + HNO3 2. KMnO4 + HNO3 + H2O2 Mn(NO3)2 + O2 + KNO3 + H2O (Tip: H2O2 is hydrogen peroxide.)
3. MnO + PbO2 + HCl HMnO4 + PbCl2 + H2O (Tip: HMnO4 is an strong acid.) 4. In each reaction above, identify the reactant being oxidized.
If you need additional practice, work the examples and/or problems that have
supplied answers in any standard chemistry textbook. Balancing Redox: Which Method To Use?
We have used three methods of balancing equations: the trial and error method (in
Lesson 10B) that can be used on all equations, the oxidation number method for redox
equations (in Lesson 15C), and the half-reaction method for redox in this module.
Which method should you use?
• Trial and error is always a legitimate method of balancing. However, for
complex redox reactions, trial and error alone can be very time-consuming. Two additional methods can be used to balance redox equations.
• The oxidation-number method supplies four key trial coefficients. The remaining
coefficients must be determined by trial and error. • The half-reaction method often...
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- Spring '10