ELECTROCHEMICAL CELLS

James Miller
                          

Objectives:
      To show that some chemical changes can produce electrical energy.    
      To determine experimentally the order in which some common metals will lose 
electrons (oxidation potentials). Materials: 200 mL of 1 M CuSO4, ZnSO4, FeSO4, 
MgSO4, Pb(C2H3O2)2, Na2SO4, SnCl2, U - tube, voltmeter (projection or 
demonstration type), 9 100 mL beakers, thermometer, 2 alligator clips, pieces of 
Cu, Fe, Mg, Pb, Sn, Zn, flashlight, extra flashlight cell and hacksaw (optional), 
glass wool, NiSO4, Ni. 

Strategies: 
      Begin the lesson by turning on a flashlight. All students are familiar with 
its operation. Ask how the flashlight is able to produce light. Most of the class 
will say it depends on the " batteries ". Continue by asking what is in the cells 
that produces the light. Perhaps some of the students will know. If not, open a 
cell with a hacksaw. Students will begin to get the idea that the materials in the 
cell must produce the electricity. 
     Divide 100 mL of CuSO4 into each of two beakers. Measure and record the 
temperature of the solution in each beaker. Repeat with 100 mL of ZnSO4 in 2 
additional beakers. Have students note the appearance of a small piece of Zn and 
Cu. Add a piece of Zn to one beaker of CuSO4 and add a piece of Cu to one of the 
beakers containing ZnSO4. Set the four beakers aside and continue. 
     Assemble an electrochemical cell using CuSO4 in one beaker and ZnSO4 in 
another. Use a U - tube filled with Na2SO4 (ends plugged with glass wool) as a 
bridge between the beakers. Most general chemistry texts have a diagram of this 
setup. Connect a piece of Cu to the positive terminal of the voltmeter. Attach a 
piece of Zn to the negative terminal. Put the Cu into the beaker of CuSO4 and the 
Zn into the beaker of ZnSO4. Measure and record the voltage produced. Construct 
additional cells substituting each of the following for the Zn - ZnSO4: Fe - 
FeSO4, Pb - Pb(C2H3O2)2, Mg - MgSO4, Sn - SnCl2, Ni - NiSO4. Record the voltages 
for each cell. The voltages may not be the same as those given on a table of 
standard oxidation potentials; however, the order will be. 
     Observe the contents of the four beakers set aside earlier. The Cu will be 
unchanged indicating no reaction. The Zn will have a red coating which may be 
recognized as Cu. The temperature of the ZnSO4 and CuSO4 solutions and the Cu in 
ZnSO4 will be the same, but the temperature of the Zn in CuSO4 will be measurably 
higher. The temperature change along with the color change indicate a chemical 
reaction has occurred. Electrons were transferred from the Zn to the Cu+2. The 
voltages obtained from each cell indicate the ease with which the metals will give 
up electrons. The higher the voltage, the more likely the metal will lose its 
electrons.