Voltaic Cells

Introduction

A voltmeter is used to rank metals from the most easily reduced to the most difficult to reduce.

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1. Using the first row of a 24well plate, half fill the wells with appropriate solutions along with the metal as listed below. Any order may be used.
   • Ag metal+0.1 M Ag NO₃
   • Cu metal+0.1 M Cu(NO₃)₂
   • Fe nail+0.1 M FeSO₄
   • Zn metal+0.1 M Zn(NO₃)₂
   • Mg ribbon+0.1 M Mg(NO₃)₂
   • Pb metal+0.1 M Pb (NO₃)₂
2. Make a salt bridge from filter paper or chromatography paper. Using scissors, cut the paper into a shape so that it will form a continuous bridge for 6 wells. The shape appears as:

   

3. To make the salt bridge complete, place the cut out filter paper into a beaker containing a saturated solution of KNO₃. Remove the salt bridge and place it into wells A1 to A6.
4. Place the black lead on any one of the 6 metals. Then check the red (+) lead against each of the other 5 metal. Record results in the table.
5. Repeat the procedure for the other 5 metals by checking each metal to a black (-) lead, and the corresponding red lead possibilities. Record your results on a table.
6. Positive values indicate the metal attached to the black lead is reduced. Examine the data table and write a table of reduction half reactions in order of reduced easiest to reduced with most difficulty.
7. Use a standard E° table of half reactions to check the order of your metals. Hand in a report.

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Wear safety goggles and aprons in the lab at all times. Use caution in dealing with chemicals. Do not ingest chemicals and avoid contact with the skin. Be careful in dealing with electrical equipment as misuse can lead to electrical shock and burns.

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Values in this matrix are expected to be related by symmetry around the matrix diagonal from the top left to the bottom right. For example, when the black lead is on magnesium and the red lead on iron, the same value of the reading should be found but with opposite sign from when the colored leads are switched. In this case, the result is 0.8 volt and -1.1 volt.

These cells happen to have been arranged from the least active (silver) to the most active (magnesium).

One possible way to present this experiment is to let the students choose their own order for data collection, and then use the data to decide upon the order.

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• This experiment can be done without Ag metal and AgNO₃ solution. The experiment can be performed with any of the four metals and corresponding solutions.
• An unknown can be worked into the scheme. For example, delete Pb / Pb(NO₃)₂ from the table. Then, after the data has been collected by the student for the remaining pairs, set up a Pb metal / Pb(NO₃)₂ cell with salt bridge that can be tested to find its position in the reduction series of half reactions.
• The FeSO₄ solution should be freshly prepared.
• Use a high impedance voltmeter.

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• Ag (wire, rod, or foil)
• Cu (wire, rod, or foil)
• Fe nail
• Zn (wire, rod, or foil)
• Mg ribbon
• Pb (wire, rod, or foil)

• 0.1 M FeSO₄ (dissolve 2.8 g FeSO₄•7H₂O in enough distilled water to make 100 mL of solution)
• 0.1 M Zn(NO₃)₂ (dissolve 3.0 g Zn(NO₃)₂•6H₂O in enough distilled water to make 100 mL of solution)
• 0.1 M Cu(NO₃)₂ (dissolve 2.3 g Cu(NO₃)₂•2.5H₂O in enough distilled water to make 100 mL of solution)
• 0.1 M Mg(NO₃)₂ (dissolve 2.6 g Mg(NO₃)₂•6H₂O in enough distilled water to make 100 mL of solution)
• 0.1 M Pb(NO₃)₂ (dissolve 3.3 g Pb(NO₃)₂ in enough distilled water to make 100 mL of solution)
• 0.1 M AgNO₃ (dissolve 1.7 g AgNO₃ in enough distilled water to make 100 mL of solution)

• saturated KNO₃
• scissors
• filter paper
• voltmeter

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The original version of this lesson was developed by:

Robert Curtright
Lincoln Northeast HS
Lincoln, Nebraska

Edward Kinzer
Lakeview HS
Columbus, Nebraska 68601

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