Voltaic Cells

 

Objectives

• Create small-scale voltaic cells and measure standard reduction potential
• Investigate optimal conditions to create concentration cells
• Use the Nernst equation to calculate cell potential at nonstandard conditions (concentration cell)

Safety

Waste Disposal:  All chemicals used must go in the proper waste container for disposal. 

Materials and Equipment 

Voltmeters with alligator clips, Well Plate, Metal electrodes (copper, aluminum, magnesium, zinc), 0.1 M metal ion solutions (CuSO₄, Mg(NO₃)₂, Zn(NO₃)₂,), disposable pipets (one for each metal solution) , String (2-3 cm in length, or longer), 1 M KNO₃, 10 mL graduated pipette and pipette pump, Thermometer to get room temperature.

Background

Useful Equations for Today’s Experiment

For a Concentration Cell,

 

 

To find Standard Electrode (Half-Cell) Potentials, use Appendix L in Openstax Chemistry 2e Textbook

 

Procedure

Take good notes and do your calculations in your lab notebook as you complete this experiment Then fill in the summary sheet.

Part 1 (Work in Partners):

1.      Using a well-plate, you will set up 3 voltaic cells each containing two half-cells:

a.      Cell A:  First Half-cell contains Copper (Cu) and Second Half-cell contains Cu

b.      Cell B: First Half-cell contains Cu and Second Half-cell contains Zinc (Zn)

c.      Cell B: First Half-cell contains Cu and Second Half-cell contains Magnesium (Mg)

First draw a diagram on your summary sheet to label which well you add each metal solution to, including both the metal and concentration in your label.  The two half-cells for each voltaic cell MUST be in adjacent wells.

2.      Next, prepare your voltaic cells by adding a few drops of the appropriate metal solution into the wells as indicated in your diagram.  Each well should be about ¾ full.

3.      Use the short string soaked in 1 M KNO₃ as a salt bridge for each of your voltaic cells.  You’ll need to make sure the string is saturated with KNO₃ solution and is long enough to connect each of the half-cells but not too long that it gets in the way.  Use your tweezers to help place each side of the string in each of your half-cells.  You’ll want to use a new piece of string for each voltaic cell to avoid contamination.

4.      Prepare your voltmeter by changing the setting to read voltage (V).  Use the alligator clips on the voltmeter to secure a strip of metal to use for your electrode. For the first voltaic cell, you will need a copper electrode and another copper electrode.  Always use an electrode of the same metal that is in the solution in each half-cell.  (e.g. in a half-cell containing the solution with Cu²⁺ ions, use a solid metal Cu electrode). Switch out electrodes as needed as you measure the potential of each voltaic cell.  Clean off each electrode with a damp paper towel between measurements to avoid contamination.

5.      Taking care to NOT TOUCH the string salt bridge with each electrode, insert the corresponding electrode into the half-cells of the first voltaic cell and quickly read and record the potential in the units of volts.  If the reading is negative, this just means the anode and cathode are switched on the voltmeter.  Leaving the electrodes in the alligator clips, just unplug the wires at the voltmeter and switch them to get a positive value.

6.      Fill in the table on your summary sheet with your measured potential of each voltaic cell.  Use the table of standard cell potential values and the equations given in the Background section to finish filling in this table before moving on to Part 2.

 

Part 2 (Continue to work in Partners):

Now you will investigate concentration cells.  Depending on what bench you and your partner are working at, you’ll look at a different variable of voltaic cells to test the optimal conditions to get the most accurate potentials for your concentration cells.  You will ONLY INVESTIGATE ONE OF THESE VARIABLES.

For all benches, you will need to prepare one or two dilutions (I and II) using the metal ion solutions provided in Part 1 and the graduated glass pipette.  Prepare a total of 10 mL for each dilutions.  Use the dilution equation C₁V₁ = C₂V₂.

Variable #1:  Concentration

If you are at the “Shower” Bench (bench closest to safety shower), you will use copper ion solutions of different concentrations to investigate how concentration difference affects the cell potential.

            Dilution I: 0.01 M CuSO₄

            Dilution II: 0.001 M CuSO₄

With the original 0.1 M CuSO₄ and your two new dilutions, prepare TWO concentration cells in your well plate.  Measure the potential of each of these concentration cells.  Comment on how the potential is different (or the same) in your two concentration cells and explain why. 

 

Variable #2:  Metal Ion Solution in Half-Cell

If you are at the “Middle” Bench (the bench in the middle of the lab), you will make dilutions of zinc and magnesium to investigate if there is a difference in measured cell potential with different metal ion solutions and electrodes.

Dilution I: 0.01 M Zn(NO₃)₂

            Dilution II: 0.01 M Mg(NO₃)₂

With the original 0.1 M Zn(NO₃)₂ and Mg(NO₃)₂ solutions, prepare TWO concentration cells (one cell containing both concentrations of Zn solutions, and the other cell containing both concentrations of Mg solutions).  Measure the potential of each of these concentration cells.  Make observations on how the potential is different with the different metals.  Compare to the standard reduction potential of each metal and explain why.

 

Variable #3:  Size of Half-Cell container.

If you are at the “Fire Extinguisher” Bench (bench closest to fire extinguisher), you will use different size containers for your half-cells and compare their measured potential over time.

You will test the effect of concentration of solution used in the half-cell

Prepare ONE dilution

Dilution I: 0.01 M Zn(NO₃)₂

With the original 0.1 M Zn(NO₃)₂ solution and your dilution prepare a concentration cell in your well plate and a second concentration cell in the provided small beakers.  Use the longer pieces of string to connect the concentration cell using the beakers.  Make initial measurements of the cell potential in each of your concentration cells.  Then wait 10-15 minutes and take an additional measurement.  Note if there are any differences and explain why.

OPTIONAL Part 3

If you still have time, use the Nernst equation to calculate the predicted cell potentials for each of your concentration cells.  Note any differences from your measured values.