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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: HalfReaction 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 halfreactions.
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4. Split the 4 particles containing those two atoms into two halfreactions, writing one
above the other.
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Cr2O72― Cr3+ C2H6O C2H4O 5. Balance the halfreactions using the CAWHe! method. *****
6 e─ + 14 H+ + Cr2O72─
C2H6O 2 Cr3+ + 7 H2O
C2H4O + 2 e─ + 2 H+ 6. Multiply the coefficients in each halfreaction to equalize the number of electrons in the
two halfreactions, then add the halfreactions.
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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 halfreactions into the original equation.
Then, using trial and error, finish balancing.
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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: HalfReaction 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 halfreaction 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 timeconsuming. Two additional methods can be used to balance redox equations.
• The oxidationnumber method supplies four key trial coefficients. The remaining
coefficients must be determined by trial and error. • The halfreaction method often...
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 Spring '10
 Lebec

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