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LAB 28
DEMO: ELECTROCHEMICAL CELLS
BACKGROUND:
PART 3
You learned in lab 27 metal ions are reduced by other
Ag(s) / Ag+(aq) and Cu(s) / Cu+2(aq)
metals. That was a qualitative lab, you only
Record the voltmeter reading from the demonstration
determined which ion was a better thief of electrons,
when the salt bridge is in place.
not how much better. This lab will measure how good
a thief a metal ion is. You also had a spontaneous
If the needle is deflected in the wrong direction what
reaction occur because you placed the metal directly
does that mean?
into the metallic ion solution. In this lab you will keep
the two half-reactions separate from each other and
PART 4
will measure the flow of electrons through a wire and
effect of changing concentration on voltage
ultimately through a voltmeter.
1. Cell will be set up as in part 1 except a porous cup
Now you know which ion is the best thief in each set
will be used instead of the salt bridge.
up, you can just refer back to your results in lab 27.
Knowing which metallic ion steals electrons in each
2. Inside the porous cup will be the zinc strip and
set up will allow you to “see” which way electrons
Zn(NO3)2 solution. The copper strip and Cu(NO3)2
flow through the wire. The needle on the voltmeter
solution will be in the beaker.
will also point in that direction. You also know
reduction is occurring in that cell. The metal strip in
The porous cup allows ions to move through it but the
this cell will be called the cathode. The metal strip in
water molecules cannot.
the other cell will have oxidation occurring at it and
will be called the anode. For any electrochemical cell 3. Connect the leads as before and note the reading. If
to work, a complete circuit is needed. To complete
it is higher than part 1 it is because there is less
this circuit a salt bridge will be used. Your instructor
resistance through the cup than the bridge and
will discuss how it works and what its made of. You
therefore ions can move more freely and more
must remember that electrons do not swim, only ions
electrons flow, thus increasing the voltage. Note the
do.
voltage.
PROCEDURE:
Build the following set-up:
v
Zn
4. Now the instructor adds 2 M Na2S slowly to the
beaker, note the voltage. They will continue to add
the sodium sulfide until all change is noted.
salt bridge
Cu
Questions and Calculations:
1. Why won’t a cell operate without the salt bridge?
2. What is the purpose of the salt bridge or porous
cup?
Zn(NO3)2
Cu(NO3)2
PART 1
Zn(s) / Zn+2(aq) and Cu(s) / Cu+2(aq)
Record the voltmeter reading from the demonstration
when:
A. no salt bridge in place
B. salt bridge in place
PART 2
Pb(s) / Pb+2(aq) and Cu(s) / Cu+2(aq)
Record the voltmeter reading from the demonstration
when the salt bridge is in place.
3. A. Write the half-reactions occurring in each cell
in part 1.
B. Write the overall reaction.
C. What is the flow of electrons through the wire?
D. What is the direction of movement of ions
through the salt bridge?
E. Using the table of Eo values in your packet,
determine the expected value. Explain why
experimental values are different than table
values.
4. Write the overall reaction for step 4 and explain the
results obtained using Le Chatlier’s principle.
LAB 28
DEMO: ELECTROCHEMICAL CELLS
5. Sketch a cell set up similar to the ones used in steps
1-3 using the zinc half-cell in one and Br2(l) for the
other. Since bromine is a liquid an inert electrode
must be made of a conducting metal, use platinum.
A. show the direction of electron flow through
the wire.
B. write the equation for each half-reaction
occurring at each electrode and the overall
reaction.
C. predict the voltage of the cell. (Eo)
D. It is known that heat is produced when the
reaction occurs. Would the voltage of the cell
increase or decrease if the temperature was
increased? Explain using Le Chatlier’s
principle.