Unformatted text preview: egative charges on both sides. Balanced.
2c. 2 e─ + 2 H2O + 1 C2H4O 1 C2H6O + 2 OH─ ***** ©2009 ChemReview.net v. k1 Page 386 Module 16: HalfReaction Balancing Lesson 16B: Balancing By Adding HalfReactions
Steps in Adding HalfReactions
Two balanced halfreactions can be added to give a balanced redox equation.
The key principle in adding halfreactions is that the electrons lost must equal the electrons
gained: the number of electrons in one halfreaction must equal the number of electrons on
the other side of the arrow in the other halfreaction.
The steps to adding two halfreactions are
1. Balance each halfreaction. (The two balanced halfreactions that result must have their
e─ terms on opposite sides; check your work if they do not.)
2. Multiply each halfreaction by a number (based on a lowest common denominator) to get
the same number of electrons in both halfreactions.
3. Add the two halfreactions. All terms on the left side of the two arrows are added
together on the left side of the arrow in the final, total reaction. All terms on the right
are added on the right. Cancel “like terms” on both sides. The like number of electrons
on each side must cancel.
4. Check to make sure that the resulting trial redox equation is balanced for atoms and
charge. If not, modify the final coefficients by trial and error.
Use those steps to balance and add these two halfreactions.
Q. Add: P b2 + Pb
Ag+ Ag *****
Answer
1. Balance each halfreaction separately for atoms and charge.
If possible, write the reactions so that the arrows line up, one below the other, to
emphasize that we separately add the two columns separated by the arrows.
Pb
e─ + Ag+ Pb2+ + 2 e─
Ag 2. Multiply each halfreaction by a lowest common denominator (LCD) to get the same
number of electrons in both halfreactions.
Multiply all terms in the bottom equation by 2. This keeps the halfreaction balanced,
but makes the electron count equal but on opposite sides in the two equations..
Pb
─ + 2 Ag+
2e Pb2+ + 2 e─
2 Ag (2x) 3. Add the two halfreactions by adding terms that are on the same side of the arrows,
then cancel like terms on both sides. The electrons must cancel. ©2009 ChemReview.net v. k1 Page 387 Module 16: HalfReaction Balancing Pb
─ + 2 Ag+
2e Pb2+ + 2 e─ Pb + 2 Ag+ Add: Pb2+ + 2 Ag 2 Ag
That’s the total redox reaction. 4. Check: 1 Pb, 2 Ag atoms on both sides; +2 charge on both sides. Balanced. Summary: To add two halfreactions, the electron count must be the same in both halfreactions, and the electrons must be on opposite sides.
*****
Flashcards
Oneway cards (with notch) Back Side  Answers To balance a redox halfreaction, the steps are CAWHe! In balancing a redox halfreaction, O atoms are
balanced by adding Water In balancing a redox halfreaction, H atoms are
balanced by adding H+ In balancing a redox halfreaction, charge is
balanced by adding e─ To add two redox halfreactions
the electrons must be Equal in number and on opposite sides To balance a half...
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This note was uploaded on 10/28/2010 for the course CHAP 12 taught by Professor Lebec during the Spring '10 term at Marlboro.
 Spring '10
 Lebec

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