Chemistry
Sensors:
Loggers:
Voltage
Any EASYSENSE
Logging time: SnapShot
05 Reduction potentials: Micro-voltaic cells
Read
An electrochemical cell uses an oxidation – reduction (redox) reaction to produce electrical energy.
Half-cells can be produced by placing a piece of metal into a solution containing a cation of the same
metal (e.g. copper (Cu) metal into a solution of copper sulphate (CuSO4 or Cu2+)).
In this practical, the half cell will be a small piece of metal placed onto a corresponding cation solution
absorbed onto a piece of filter paper. One filter paper can be used to make several half cells. To make
a complete cell two half cells are connected via a porous barrier or a salt bridge. The salt bridge in this
experiment is made from aqueous sodium nitrate (NaNO3). The red (+) lead of the Voltage sensor
makes contact with one metal and the black (–) lead with a second metal to complete the circuit and
make a complete cell. The voltage produced by the cell is measured by the sensor
By comparing the voltage values obtained for different combinations of half-cells, and by recording
which metal made contact with the red (+) and black (–) leads, you can establish the reduction potential
sequence for the unknown metals in this investigation.
For this activity, the Voltage sensor is used to measure the difference in potential (voltage) of the
micro-voltaic cells.
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•
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Ion – an electrically charged atom.
Anion - a negatively charged atom e.g. an atom with an excess of electrons e.g. Cl-, NO3-.
Cation – a positively charged atom e.g. an atom with a lack of electrons e.g. Mg2+, Na+.
Metals produce cations when they become ionised, non metals anions. E.g. salt (sodium chloride) will
form sodium+ and chloride- ions when in water. Any number before the – or + sign indicates the number
of charges required to bring the ion back to electrical neutral.
What you need
1.
An EASYSENSE logger
2. A Smart Q Voltage sensor.
3. Pipette.
4. Forceps.
5. Glass plate or large petri dish
6. 5 squares of metal foil (for example copper, iron, zinc, silver, and lead) these will be M1, M2
etc.
7. 5 solutions containing the same metal ions as the metal squares (e.g. metal sulphates,
chlorides). These will be M1+, M2+, etc.
8. Fine abrasive paper.
9. Large filter paper or paper towel.
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05 - 1(V2)
Preparing the half cells
Draw five small circles (in pencil) with connecting lines on a piece of circular filter paper, as shown in
the diagram.
Using a pair of scissors, cut wedges between the circles as shown to create a “star” shape.
Label the circles, on the arms of the star shape (in pencil), M1, M2, M3, M4, and M5. Place the filter
paper on top of the glass plate.
Apply sodium nitrate
solution here, etc.
Circles for cation solution
M1
M2
Cut and remove the
wedge shaped pieces
of filter paper
.
M5
M3
M4
Glass plate
Filter paper
Circles for cation solution
M1
Apply sodium nitrate
solution here, etc.
M2
M5
Filter paper
M3
M4
Cut and remove
the wedge shaped
pieces of filter paper
Glass plate
Drop sodium nitrate solution onto the centre of the star formed, you want to wet the paper to about
half way along the arms, no more.
•
Your teacher will supply you with 5 pieces of metal designated M1, M2, M3, M4, and M5.
•
Place 3 drops of each solution on the appropriate circle (M1+ on M1, etc.). Then place the piece of
metal on the wet spot with its respective cation.
•
The top side of the metal should be kept dry.
•
Add enough 1 mol dm-3 sodium nitrate (NaNO3) solution to make a continuous damp trail along a
line drawn between each circle and the centre of the filter paper.
•
You may have to dampen the filter paper with more NaNO3 during the experiment, take care not
to flood and short circuit the test arms.
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What you need to do
Part 1: Copper (M1) as the reference metal
•
Connect the Voltage sensor to an input of the logger.
•
From the EasySense software’s Home screen select SnapShot.
•
Use M1 (which should be copper) as the reference metal. You will be measuring the potential of
four cells by connecting M1 to M2, M1 to M3, M1 to M4 and M1 to M5.
•
When everything is ready, click the Start icon to begin collecting data.
•
Touch the red (+) lead of the Voltage sensor to M1 and the black (-) lead to the other metal
sample (for example, M2).
•
If the voltage drops to 0.00 V, reverse the leads, that is, switch the red (+) lead and the black
(-) lead.
•
Wait about 5 seconds. Click in the graph area to record the voltage.
•
Use Add Text to label which metal (M1, M2, M3, etc.) touched the red (+) and which metal (M1,
M2, M3, etc.) touched the (-).
•
Use the same procedure to measure the potential of the other three cells, continuing to use
M1 as the reference electrode.
Analysis
1.
You now need to arrange the five metals (including M1) into the results table from the lowest
reduction potential at the top (most negative) to the highest reduction potential at the bottom
(most positive). The reduction potential is the value that you measured and recorded.
2. M1, the reference metal, has an arbitrary value of 0.00 V.
3. If the other metal (e.g., M2, M3, or M4) was touched by the negative (black) lead of the
Voltage sensor, it will be placed above M1 in the chart (with a negative E value).
4. If the other metal was touched by the positive (red) lead of the Voltage sensor, it will be
placed below M1 in the chart (with a positive E value).
5. Record the numerical value of the reduction potential relative to M1 for each of the other
metals (M2 through M5).
Predictions
1.
Calculate a predicted potential difference for each of the remaining half-cell combinations
shown in Table 2a (M2/M3, M2/M4, M3/M5 and M3/M4, M3/M5 and M4/M5) using the
reduction potentials you just determined (in the results table).
2. Record the predicted cell potential differences in results table 2a.
3. Go on and confirm your predictions by testing the half cells and finish the experiment.
Part 2: Non-copper (M1) reference metals
• Measure the potential differences of the six remaining half-cell combinations using the same
procedure as in Part 1.
• If the NaNO3 salt bridge solution has dried, you may re-moisten it.
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• Record each measured potential in results table 2a.
• When you have finished recording select Stop.
Analysis
1.
Compare the measured half-cell reduction potentials with the predicted half-cell reduction
potentials in results table 2a.
2. Calculate the % difference between your prediction and the measured value for each of the
potentials you measured in Part 2.
3. Optional: Find a reduction potential chart in your textbook and identify the metals M2 through
M5. If your book has an oxidation potential chart, all the reactions will be reversed and the
signs will be switched on all the potentials. Reminder: H2 has a reduction potential of 0.00 V on
the textbook chart. Locate copper, M1, on the chart, and then determine possible identities of
the other metals using your experimental reduction potential sequence in Table 2a. You must
add the difference in potential between H2 and copper to all values in Table 2a.
Note: One of the metals has a 1+ oxidation state; the remainder of the metals have 2+
oxidation states.
Results table 1 (copy of recorded data)
Voltaic Cell
Measured Potential
(metals used)
(V)
M1 / M2
Metal Number of
(+) Lead
Metal Number of
(-) Lead
M1 / M3
M1 / M4
M1 / M5
Results table 1a
Metal (Mx)
Lowest (-) Reduction Potential, E°, (V)
Highest (+) Reduction Potential, E°, (V)
Results table 2a
Half-Cell
Combination
M2 / M3
Predicted Potential
(V)
Measured Potential
(V)
M2 / M4
M2 / M5
M3 / M4
M3 / M5
M4 / M5
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Percent Difference
(%)
Questions
1.
?
How well did your predictions match the measured values for the potential differences
measured using non-copper reference metals?
At the end of the practical
1.
Use forceps to remove each of the pieces of metal from the filter paper. Remember to
avoid getting the solutions on your hands.
2. Rinse each piece of metal with tap water, dry, and return it to the correct container.
3. Remove the filter paper from the glass plate using the forceps, and discard it as directed
by your teacher. Rinse the glass plate with tap water, making sure that your hands do not
come in contact with wet spots on the glass.
4. Clean and dry the leads of the Voltage sensor. (Do not wash them, instead, wipe the ends
with a slightly damp paper towel and then dry them.)
L3 Chemistry
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