11_Measuring Electrochemical Cells 2009.pdf

8/12/2019 11_Measuring Electrochemical Cells 2009.pdf

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Electrochemical Cells

Determining Relative Reduction Cell Potentials

In lab there are five half-reactions. By measuring the voltage between a variety of half-cells you are to place the half-cell reductions in order of decreasing half-cell potential.The following cells can be made and measured in the 24-well plate.

Ag++ e!!Ag(s)

Zn2+

+ 2e!!Zn(s)Fe3++ e!!Fe2+(s)Cu

2++ 2e!!Cu(s)

2I2+2e!!I!(aq)

Create the five half-cells above in the well plate. Fill five wells half-way with a different

ion solution. In these five solutions place the appropriate metal or graphite electrode.For the Fe

3+and I!solutions use a graphite electrode. The solutions in the well plate

should not go down the drain. When it is time to clean up your cells, empty the solutionsinto the metal waste beaker in the hood.

Sign Convention of the Volt Meter

The volt meter (truly a multi-meter since it also measures current and impedance) that

you have should be turned on by setting the center dial to the 2V DC position. (Allelectrochemical cells generate a continuous flow of electrons, while the voltage from the

wall provides an oscillating current flow, AC). The volt meter will read the potential

difference between two half-cells. The potential will read positive when the red lead (+)is attached to the cathode (reduction half-cell) and will read a negative voltage when thered lead is attached to the cell anode (oxidation half-cell). Naturally, switching the leads

will change the sign of the voltage on the meter. The sign of the voltage is useful fordetermining which cell is at the higher reduction potential.

Filter Paper Salt Bridges

You will need multiple salt bridges. It is important that each combination of cells youtest have its own salt bridge. With a pencil, label each salt bridge indicating which end is

placed in which solution. After labeling each salt bridge, place the collection on a watchglass and drip saturated KNO3(aq) solution over the strips so that each is completely

dampened.

Measuring Cell Voltages

Select a pair of half-cells to measure the potential difference between. Pick one on your


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own or begin with zinc and iodine cell combination. Connect one of the leads from thevolt meter to the zinc electrode and place it in the zinc ion solution. Attach the other volt

meter lead to the graphite lead and place the graphite in the iodine solution. Find the saltbridge that you have labeled Zn and I2. Place the salt bridge into each half cell. Record

the voltage of the half cell, and identify the sign of the voltage. Be sure that from the

sign of the voltage you can identify which cell is being oxidized and which cell is beingreduced. Does Zn oxidize I2or does Zn reduce I2?

For each cell you measure, write the cell notation in your lab notebook. For the case ofthe zinc /iodine cell you should write:

Zn(s)"Zn2+(1M)""I2(aq 1M),I!(1M) "C(graphite)

The electrodes are identified in the cell notation at the beginning (anode) and at the end(cathode). Notice that the reduction of I2to I

!all takes place in the aqueous phase so

there is not a phase separation bar, ", between reactants and products.

From this measurement you should conclude that 2I!+ 2e!!I2(aq) is a greater half-cellvoltage than Zn2++ 2e!!Zn(s). If you determine a half-cell voltage lower than zinc,you do not need to measure this half-cell against I2because if it is lower than Zn it is alsolower than the I2reduction potential.

Continue measuring cell potential differences for different half cell combinations, writing

the cell notation and measured voltage until you can order the five half-cell reactionsabove from largest to smallest reduction potential.

Concentration Cell

Choose a copper unknown solution to investigate. Construct the followingelectrochemical cell and record the voltage observed:

Cu(s)"Cu2+(???M)""Cu (1 M)"Cu(s)

From the sign of the observed voltage in your concentration cell, is the 1M Cu2+

solution

the anode or the cathode?

Use the Nernst equation to calculate the Unknown concentration:

where R= 8.314 J/mol K; n= # of electrons transferredF = 96, 500 J/Vmol


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Electrolytic Cell

At the front of the room your TA has an electrolytic cell. This is a non-spontaneousredox reaction which when attached to a battery of greater voltage can be made to run

spontaneously.

The non-spontaneous reaction in the electrolytic cell is probably pretty complex becausewe are using a very large potential, but one redox reaction occurring is:

H2O " 2e

+ 2H++ #O2 Ecell= !1.23V

2H2O +2e" H2+ 2OH

Ecell= !0.83V

Using the above reaction answer in your notebook as you run this cell with your teaching

assistant:

1. At which electrode (anode or cathode) will hydrogen gas be produced?

2. If phenolphthalein is added to the cell, which electrode (anode or cathode) will turnpink?

Cells in Series

What is the voltage of a car battery? 12 volts. How does the battery get that large of avoltage? The battery is really six 2 volt batteries wired in series. In this experiment

prepare a battery set in series.

At the front of the room there are two Daniels cells Zn(s) + Cu

2+

"

Zn

2+

+ Cu(s).Measure and record the cell voltage for each cell.

Now connect the two cells in series as is shown in the figure below.

Measure and record the voltage of the cells in series. Examine the 9-volt battery at the

front of the room. Estimate the voltage of a single cell in the 9-volt battery.

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11_Measuring Electrochemical Cells 2009.pdf