Page 375 - Chemistry eBook

How to Write Half Reactions 3 (Reduction)
3. To find the half reaction of reduction:
a. Check which elements underwent reduction (the oxidation number decreased).
Mg0 +Cl20 Mg+2Cl2-1
Remember: GER left. In reduction, the oxidation number goes down. Cl2 started out as Cl20 and ended up as Cl2-1. Since Cl’s oxidation number went down, Cl underwent reduction.
b. Write out the half reaction of reduction:
Show just the reduction portion of the equation. Then, place the electrons (that were gained) on the left side of the equation (GER left).
Cl20 + 2e- Cl2-1. This is the half reaction for reduction.
We put 2 electrons, not one, on the left side because there are two Cl atoms: 1 x 2 = -2.
Don’t ignore the subscript. Apply it.
There are many Regents questions based on this topic.
UNIT 10: OXIDATION & REDUCTION © 374