South Pasadena  Chemistry
Name ___________________________________
Period ___ Date ___/___/___
12  Electrochemistry
OXIDATION-REDUCTION
Some chemicals gain or lose electrons when they react. This change is part of rust and corrosion, batteries, and
fuel cells. We want to recognize when this change is taking place and assign the names Oxidation and
Reduction to these changes. Watch for the following changes which act as clues.
Oxidation:
 The charge of an atom increases.
 An atom loses electrons. Ex: A  A+ + e
Reduction:
 The charge of an atom decreases (is reduced).
 At atom gains electrons. Ex: B + e  B
(electrons are on the right side or arrow)
(electrons are on the left side of arrow)
We have seen this idea before. We used the mnemonic: “LeO the lion says GeR”.
LeO reminds us that ________________________________________________
GeR reminds us that ________________________________________________
Recognizing RedOx:
For each of the following reactions, indicate whether the underlined element is undergoing [O]xidation,
[R]eduction, or [N]either oxidation nor reduction.
Note: Think of HCl as H+ and Cl. Think of ZnCl2 as Zn2+ and 2Cl, etc.
____
____
1.
2.
Na  Na+ + e
F2 + 2e  2F
____
____
8.
9.
Fe2+  Fe3+ + e
Pb + Cl2  PbCl2
____
____
____
____
____
3.
4.
5.
6.
7.
HCl  H+ + Cl
Zn + 2HCl  H2 + ZnCl2
Zn + 2HCl  H2 + ZnCl2
Zn + 2HCl  H2 + ZnCl2
AgNO3 + NaCl  AgCl + NaNO3
____
____
____
____
____
10.
11.
12.
13.
14.
Pb + Cl2  PbCl2
H2 + Cl2  2HCl
H2 + Cl2  2HCl
Na+ + e Na
2F F2 + 2e
Half-Reactions:
Consider question 9 above. One chemical is oxidized and one is reduced. If one chemical gains electrons, there
must be a chemical that loses electrons. We call these oxidation-reduction reactions or “redox” for short.
Which equations represent redox reactions? _____ _____ _____ _____ _____ _____ _____
Now, consider the equations in questions 1 & 2. These are called half-reactions. They focus on only one of
the two changes (oxidation or reduction). A whole redox equation can be written as two separate half-reactions.
Which equations above would be called half-reactions? __1__ __2__ _____ _____ _____
Example: Pb + Cl2  PbCl2
turns into
Pb  Pb2+ + 2e
Rewrite H2 + Cl2  2HCl as two half-reactions.
and
Cl2 + 2e  2Cl
______________________
______________________
When we combine two half-reactions, we get a complete redox equation. Notice that the electrons must cancel.
Example:
2Cl  Cl2 + 2e
Br2 + 2e 2Br 







Br2 + 2Cl  Cl2 + 2Br
Practice Questions:
1.
Write half-reactions for the following:
a) silver metal turns into silver ion
___________________________________
b) sulfur atom turns into sulfide ion
___________________________________
c) calcium ion turns into calcium metal
___________________________________
Combine these two half-reactions into a complete redox equation. (Make sure the e’s cancel.)
Na  Na+ + e
and
F2 + 2e  2F
2.
Group Lab Activity
1.
Turn your desks to form groups of four.
2.
Send one partner to get goggles for everyone in the group.
3.
Send one partner to get a tray with:
 test tube rack
 test tube with solid CuCl2
 stirring rod
 test tube holder
 bottle of distilled water
 paper towel
What color is the CuCl2? _________
Add a few drops of water to the solid. What color does it change into? __________
Add water until the tube is 1/3 full. Stir the solution. What color is the solution? __________
When the CuCl2(s) dissolves, the two ions, ______ and ______ are formed.
The Instructor will hand out a piece of aluminum foil. What is its chemical symbol? ______
[Note: is it neutral or an ion?]
4.
5.
6.
7.
8.
Roll the foil loosely so it will fit into the test tube. Add the foil to the solution and make at least three
observations. Use the stirring rod to push the foil into the solution.



9.
Use the following to write two half-reactions to describe the changes:
2+
3+
Cu°
Cu
Al°
Al

e
Oxidation Half-Reaction:
Reduction Half-Reaction:
Overall Redox Reaction:
10.
Clean up the tube by dumping the contents into the waste container.
Place the test tube into the container of soapy water.
Return the tray to the counter. Replace the test tube and paper towel. Return the goggles.