Electrochemistry

Allen, Donna L.                          Bowen High School
                                         933-7200
                           

Objectives:

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?  

Apparatus needed: 

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)
insulated wires                         
one lemon

Recommended strategy:

I. Changing electricity into chemical energy      
     A. electrolysis of water
     B. electrolysis of saturated sodium chloride solution
     C. electroplating
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
          Cu      XX
          Pb              XX
          Mg                      XX
          Zn                              XX
     Write equations: 
     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 
potentials:                                  
     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 
above?  

IV. Discuss drawings of: 
     A. Flash light dry cell
     B. Alkaline dry cell
     C. Automobile battery