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Allen, Donna L. Bowen High School
One form of energy may be transformed into another. How can
electricity be changed into chemical energy? How can chemical energy
be transformed into electricity?
Strips of copper, zinc, lead, magnesium
0.5 M solutions of soluble salts of copper, zinc, lead and magnesium
(acetates or chlorides)
0.05 M copper (II) sulfate
plastic cups (preferably clear)
index cards (support strips)
voltmeters (milliammeters, galvanometers as available)
I. Changing electricity into chemical energy
A. electrolysis of water
B. electrolysis of saturated sodium chloride solution
II. Changing Chemical energy into electricity
A. A zinc strip is put into a 0.5 M solution of copper(II) sulfate
and allowed to stand for at least two hours. Another cup of
the same solution without the zinc strip should be prepared at
the same time for comparison. Record observations. Write
equation. A reaction has occurred spontaneously with the
liberation of a small amount of heat energy. Could that
energy be harnessed in the form of electricity?
B. Stick strips of copper and zinc into a lemon and connect to a
meter. Is electricity being generated?
C. Moisten a small piece of paper towel in your mouth and put it
between a penny and a dime. Touch the penny to one lead to
the meter and the other lead to the dime. Is electricity
being generated? This is a Voltaic cell.
D. In order to cause electrons to pass through an external
circuit instead of reacting directly with the liberation of
heat energy, the two electrodes must be separated. However the
electrons must have a pathway or the circuit will not be
complete. This can be accomplished with a semipermeable
membrane or a salt bridge. The salt bridge can be a U shaped
glass tube filled with a conducting solution, but we will use a
strip of porous paper which has been dampened with a saturated
solution of sodium or potassium chloride.
Fill one cup with the solution of zinc acetate and mount a strip of
zinc metal through a slit in an index card in this solution. Fill
another cup with a solution of copper acetate and mount a strip of
copper metal in this cup. Stand the two cups side by side and hang a
damp strip of paper over the sides so that one end is in the zinc
acetate solution and the other end is in the copper(II) acetate
solution. Now connect the two electrodes by copper wires to the meter.
Does the meter give evidence that an electric current is generated?
Record whatever quantity and units the meter shows.
Repeat, using all the possible combinations of the four metals. Note
which metal is at the positive electrode. Does current flow if the
metals are reversed. Twelve observations.
- +Cu Pb Mg Zn
Can we rank the metals in order activity? Arrange on a line.
III. Standard Reduction Potentials - The potential difference across a
Voltaic cell is easily measured, but it is impossible to measure an
individual electrode potential. Therefore the hydrogen electrode which
consists of a platinum electrode immersed in a 1.0 M solution of
hydrogen ions is assigned a value of zero and all other half cells are
measured with respect to this. Here are some standard reduction
Cu ---> Cu+2 + 2e -0.337 v
Pb ---> Pb+2 + 2e 0.126 v
Mg ---> Mg+2 + 2e 2.370 v
Zn ---> Zn+2 + 2e 0.763 v
Oxidation potential equals reduction potential with the sign changed.
The predicted potential difference is the algebraic sum of the
oxidation and reduction potentials.
Example: Zn + Cu = 0.763 -(-0.337) = +1.10 v
The observed voltage is always less than the predicted voltage.
Calculate the predicted voltage for each of these cells, having first
written the equation and identified the element which is oxidized. How
do these values compare with differences taken from the number line
IV. Discuss drawings of:
A. Flash light dry cell
B. Alkaline dry cell
C. Automobile battery