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Investigating the effect of the depth of the electrodes on the
voltage produced by a voltaic cell
Chemistry HL
Miika Karjomaa
000572-012
Mattlidens Gymnasium
May 2009
Miika Karjomaa
Candidate number: 000572-012
Introduction
A cell that produces electricity is referred to as a voltaic or a galvanic cell. For the cell to
produce electricity, it needs to be made of two half-cells, each consisting of an electrode
and an electrolyte solution, for example Cu(s) as the electrode and CuSO4(aq) as the
electrolyte solution. It is important that the positive ion in the electrolyte solution is the
same as the electrode, for example if Cu(s) is the electrode then the ion needs to be
Cu2+(aq) or Cu+(aq). The two half cells are connected by an electrical circuit and a salt
bridge, which completes the circuit; this means that without the salt bridge electricity
would not be produced.
Since electricity is produced by the movement of electrons, the two electrodes are named
based on the chemical changes that occur in them. The cathode is the electrode where
reduction occurs, and the anode is the electrode where oxidation occurs. Usually the nobler
electrode will be the cathode: this can be determined by looking at the standard reduction
θ
potentials, E . The electrode with the higher standard reduction potential will be the
electrode that is reduced.
The electrodes used in this experiment will be zinc and copper electrodes. Zinc has the
θ
θ
standard reduction potential, E , of -0.76V and copper has the E of +0.34V. This
means that copper is the nobler electrode of the two, meaning that it will be the cathode
and zinc will be the anode.
The reactions taking place in the electrodes are therefore the following.
2





Cu
aq
2
e

Cu
s
2





Zn
s
Zn
aq
2
e
The total cell reaction can be acquired by adding the two half-reactions together, which
gives
2

2







Cu
aq

Zn
s

Cu
s

Zn
aq
The voltaic cell can also be expressed in standard cell notation: by definition the cathode
half-cell is shown on the right and the anode half-cell is shown on the left.






Zn
s
ZnCl
aq
CuSO
aq
Cu
s
2
4
The single vertical lines represent a phase boundary, showing the change from the solid
electrode to the aqueous solution. The double vertical line shows the salt bridge, which
connects the two half cells together. It is also important that the two electrodes are on the
opposite sides of the standard cell notation, since it is the aqueous solutions that are
connected together by the salt bridge, not the electrodes.
Research question
How does the depth of the electrodes in the solution affect the voltage produced by the
voltaic cell?
Hypothesis
I believe the deeper the electrodes are in their respective aqueous solutions, the higher the
voltage produced will be. This is because if the reactive surface area of the electrodes is
increased, oxidation and reduction should be able to occur at a faster rate at their respective
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Miika Karjomaa
Candidate number: 000572-012
electrodes. However, the limiting factor of the salt bridge needs to be taken into account,
since it will remain constant and might thus not allow the electron flow to be as high as it
could be, when considering just the electrodes. Even taking this into account, the voltage
should increase.
Variables
Controlled
Temperature of the system will be assumed to remain constant at room temperature. If
necessary, the temperature of the solutions can be adjusted by using a water bath.
Concentration of electrolyte solutions will be kept constant by using solutions of the
same concentration for all the measurements.
The electrodes used will be the same in all the measurements, therefore the size, shape,
thickness and metal of the electrodes are controlled.
Length and width of the salt bridge will be kept constant in all the samples by measuring
the length and width of the filter paper.
Wetness of the salt bridge, since filter paper is used, the salt bridge has the risk of drying
a little. To control this a new salt bridge will be used for each sample.
The surface of the electrodes will need to be sanded between each measurement to ensure
that the electrodes have the same amount of molecules to be oxidised and reduced
respectively in all samples.
Independent
The depth of electrodes will be changed by regular intervals by adjusting the positions of
the clamps (see Figure 1). This essentially changes the reactive surface area of the
electrodes.
Dependent
The amount of electricity produced will be observed by looking at the volt meter and
noted in Volts.
Chemicals & Equipment
Table 1 shows the chemicals and equipment required in the experiment.
Equipment
Chemicals
Stand(s)
CuSO4 aqueous solution
Beaker(s) (100 cm3)
ZnCl3 aqueous solution
Filter paper
Zn electrode
Clamp with rubber coating
Cu electrode
Clamp
Volt meter
Wire(s)
Metal rod(s)
Paper stacks/Petri dishes
Measuring tape / ruler ±0.05 cm
The use of the equipment in Table 1 is partly shown in Figure 1, which shows the
apparatus used in the experiment.
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Miika Karjomaa
Candidate number: 000572-012
Apparatus
Zn(s) electrode
Cu(s) electrode
V
metal rod
clamps
salt bridge
stand
beaker
ZnCl3(aq)
CuSO4(aq)
Figure 1
When constructing the apparatus and while performing the experiment, attention should be
paid to the salt bridge, as it should not touch either of the electrodes at any point of time.
Furthermore, ensure that the electrodes do not come in contact with the beakers: special
attention needs to be paid when doing sample 5 with a depth of 5 cm.
Method
1. Pour 100 cm3 of ZnCl2 solution and CuSO4 solution to their respective 100 cm3
beakers.
2. Attach the electrodes into their respective stands as shown in Figure 1, use the
clamp with the rubber coating to hold the electrode so that as little of the electricity
produced as possible is absorbed by the apparatus.
3. Polish the electrodes with sand paper before each measurement to ensure that the
conditions remain the same.
4. Attach the wires into the electrodes and connect them into to the voltmeter.
5. Prepare the method of changing the depth of the electrodes. Usable pieces of
equipment include stackable 1 cm tall petri dishes and 1 cm stacks of paper
(NOTE: Remember to take into account that paper shrinks if pressure is applied,
only use paper if time is an issue and no other method is available).
6. Prepare the salt bridge by soaking a filter paper in KNO3 solution. Make sure that
the salt bridges used in consecutive samples are of the same length and width.
7. Use a measuring tape or a ruler to make the depths of the electrodes as described by
the appropriate sample in Table 2.
Sample
Depth of Zn(s) electrode
Depth of Cu(s) electrode
1
2
3
4
5
(cm±0.05 cm)
1
2
3
4
5
(cm±0.05 cm)
1
2
3
4
5
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Miika Karjomaa
Candidate number: 000572-012
8. Place the salt bridge between the two half-cells.
9. Note the voltage shown by the voltmeter NOTE: If the voltmeter shows a negative
value, simply switch the places of the wires attached to it.
10. Repeat Steps 2-9 for all the samples.
Safety & Environment
Copper sulphate and zinc chloride should be handled with care and contact with skin
should be avoided. If the solutions do come in contact with skin or eyes, rinse immediately
with plenty of water. Furthermore, copper sulphate is very toxic to the aquatic life, and
therefore should especially not be disposed of in the sink.
Data Collection
In this section the collected raw data will be presented and explained and the observations
made will be noted.
Table 3 shows the raw data collected in this experiment
Sample
Depth of Cu electrode Depth of Zn electrode Voltage (V±0.001V)
1
2
3
4
5
(cm±0.05 cm)
1
2
3
4
5
(cm±0.05 cm)
1
2
3
4
5
1.034
1.028
1.023
1.020
1.015
Observations
Although a new salt bridge was used for all the samples, the blue CuSO4 solution could
still be seen to move across the salt bridge. Furthermore, after the experiment the colour of
the CuSO4 had changed slightly. Furthermore, once the electrodes were taken out of the
electrolyte solutions, their colour had changed slightly. This suggests that the properties of
their surfaces might have changed.
Data Processing
In this section the raw data will be processed and shown in a graph, furthermore the
uncertainties related to this experiment will be calculated.
Table 4 shows the change in the voltage between the samples, meaning that the difference
between the voltage between the different depths is showed.
Sample
Depth of electrodes
Voltage (V±0.001V) Change in voltage
between the samples
(cm±0.05 cm)
(V±0.001V)
1
1
1.034
0
2
2
1.028
0.006
3
3
1.023
0.005
4
4
1.020
0.003
5
5
1.015
0.05
5
Miika Karjomaa
Candidate number: 000572-012
The data in Table 4 was plotted in graph, which can be seen in Figure 2, which also
contains the uncertainties shown in Table 4 in the form of error bars.
The effect of the depth of Cu and Zn electrodes on voltage
1.04
Voltage
(V)
1.035
1.03
1.025
1.02
1.015
1.01
1
1.5
2
2.5
Figure 2
3
3.5
4
4.5
5
Depth of electrodes (cm)
As can be seen in Figure 2, the trend suggested by the raw data collected is fairly clear.
The deeper the depth of both of the electrodes in their respective electrolyte solutions, the
lower the voltage produced. This trend is rather hard to explain, since the theory would
suggest that the trend would be the exact opposite. On the other hand, it could be
considered that the change in voltage is quite minimal, and could be due to uncertainties
and systematic errors in the experiment. This would mean that it would be possible to
argue that the voltage should not be affected by the depth of the electrodes, and that the
negative slope of the diagram is simply caused by the electrodes being used for a
prolonged period of time.
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Miika Karjomaa
Candidate number: 000572-012
Conclusion
The data collected from the experiment suggests that my hypothesis was wrong, since
Figure 2 shows that the depth of the electrodes correlated negatively to the voltage, the
larger the value of the depth the lower the voltage. Although this is what my experimental
data suggests, I still believe that my hypothesis is correct or at least not this much off. A
third possibility that exists besides the depth of the electrode increasing or decreasing the
voltage of the cell is that the depth would have no effect at all on the voltage. This would
make sense had it only been one electrode that’s depth was increased, as then the other one
would limit the voltage. In this case, both electrodes depth was changes and thus the
voltage should have increased.
Evaluation
In this experiment there are many factors that can be improved, which concern mostly the
chemicals and equipment that were used. If the method was followed to the letter, no major
issues should arise. However, at the time this experiment was conducted, the electrolyte
solutions the school had available had already been used before for an unknown number of
times. This means that the ion concentration and balance in the solution might have already
been disturbed, which would affect the oxidation and reduction taking place, thus affecting
results by increasing the error percentage. As the extent to which the previous use of the
electrolyte solutions affected the experiment could not be calculated, they add a large
uncertainty to the experiment.
In addition, the electrodes used in this experiment might have added an uncertainty which
cannot be included in the calculations as well. Due to time restrictions mostly caused by
the extensive preparations necessary, it was not possible for the electrodes to be sanded in
this experiment. This means that one controlled variable was in fact not controlled, as the
surface of the electrodes definitely changed during the course of the experiment. If the
reactive surface area and thus the available ions on the electrons was not the same for all
the samples, then the later samples will produce a lower voltage, as the previous samples
have already consumed some of the ions in the electrodes. It could be argued that as the
depth of the electrodes was increased, the fresh surface should compensate and produce
somewhat reliable results, however, the variable still remains uncontrolled. Admittedly, the
actual experiment does not necessarily take long to perform, and thus it could be claimed
that the electrodes did not spend that enough time in the electrolyte solutions to be
completely ruined. In the case of the electrodes it should also be remembered that they
have been used previously as well.
Furthermore, taking into account the time available for the experiment, the method with
which the height of the electrodes is changed becomes important. The most effective
method would be to have stackable equipment which would be 1 cm thick, and these could
then be piled under the beakers: this way there would be no need to touch the clamps. This
would save time and eliminate the necessity of constantly measuring the depth of the
electrodes, since the depth could be calculated by simply looking at the number of blocks
used. In reality these blocks could be petri dishes or something similar. Adjusting the
height of the electrodes using the clamps attached to the stand is possible, but this would
require large amounts of time, and should therefore be avoided.
Another factor that needs to be considered is the amount of electricity absorbed by the
apparatus. As the electrodes were held by clamps with plastic coating, the loss should be
quite minimal. However, it is still there and needs to be mentioned. This loss could be
reduced by using smaller clamps still with plastic coating, as then the surface area of the
clamp touching the electrode would be reduced and thus the loss of electricity will not be
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Miika Karjomaa
Candidate number: 000572-012
as great. The clamps cannot be removed completely because the electrodes do need to be
held firmly in order to ensure that the depth remains as the wanted depth throughout the
experiment. However, depending on the type of electrode and the way it is attached to the
wire, this connection alone might be enough to support the electrode and consequently
nothing except the electrolyte solution would have to touch the electrodes. The issue with
this method is simply that the connection might not be stable enough.
Moreover, the differences in the voltages could be made more significant by choosing
electrodes that have a higher difference in their standard electrode potential. However, due
to the limitations of the equipment in the school laboratory, the number of electrodes of a
certain metal and even the metals were very limited, which means that the optimum
electrodes could not be used. Furthermore, as it had to be made sure that the electrodes are
the same size and shape, this experiment used the most common metals available as the
electrodes.
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Miika Karjomaa
Candidate number: 000572-012
Bibliography
Brady, James E. and Senese, Fred (2004) Chemistry Matter and Its Changes. Wiley
International Edition Fourth edition. Von Hoffman Press. USA:
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