Investigation to see how the voltage changes when we change the metals used in a cell
Investigation to see how the voltage changes when we change the metals used in a cell
Method
During this investigation I performed two different experiments. Both experiments were to see how the voltage changed when using two different metals as electrolytes in a battery. When using sulphuric acid as the electrolyte I set up the experiment as shown in the diagram. The other experiment used a grapefruit (therefore citric acid) as the electrolyte. We set it up in the same way as we set up the experiment when using sulphuric acid, except we inserted the pieces of metal (the electrodes) into the grapefruit. The wires were attached to the pieces of metal using crocodile clips. We then held the metals in the electrolyte until the voltage reading on the voltmeter remained steady.
I repeated the results once when using sulphuric acid, but did not have enough time to repeat the results when using citric acid.
Results
Sulphuric acid-1st set of results
Metal 1
Mg
Al
Zn
Sn
Pb
Cu
Metal 2
Mg
-------
Al
0.550
--------
Zn
0.130
0.330
---------
Sn
0.680
0.170
0.380
--------
Pb
0.870
0.170
0.640
0.240
--------
Cu
.050
0.410
0.830
0.720
0.410
--------
Sulphuric acid-2nd set of results
Metal 1
Mg
Al
Zn
Sn
Pb
Cu
Metal 2
Mg
-------
Al
x
--------
Zn
x
0.610
---------
Sn
.470
0.030
x
--------
Pb
0.720
0.230
x
0.050
--------
Cu
0.710
0.750
0.960
0.350
0.420
--------
Averages for sulphuric acid including anomalous results
st results with H2SO4 (Volts)
Metal 1
Mg
Al
Zn
Sn
Pb
Cu
2nd results with H2SO4 (Volts)
Metal 2
Mg
-------
x
x
x
x
x
Al
0.550
--------
x
x
x
x
Zn
0.130
0.470
---------
x
x
x
Sn
.075
0.100
0.380
--------
x
x
Pb
0.795
0.200
0.640
0.145
--------
x
Cu
0.880
0.580
0.895
0.535
0.415
--------
Grapefruit
Results using Grapefruit
Metal 1
Mg
Al
Zn
Sn
Pb
Cu
Metal 2
Mg
-------
x
x
x
x
x
Al
0.630
-------
x
x
x
x
Zn
0.200
0.560
-------
x
x
x
Sn
0.590
0.100
0.370
-------
x
x
Pb
.250
0.040
0.560
0.110
-------
x
Cu
.190
0.460
0.970
0.560
0.400
-------
Analysis
I have highlighted the anomalous results in red above. When using the sulphuric acid as the electrolyte, I repeated the experiment once. I have drawn charts that show the results from the experiments. I drew the charts by drawing a horizontal line across the page representing Magnesium's voltage as 0. Over this line I have drawn 6 vertical lines representing each of the metals I used. The further the points are from the horizontal line represents the larger the voltage produced.
When looking at the charts, except for a couple of the anomalous results that have been circled, it is easy to see that metals of the same type are on a very similar horizontal line. The different coloured lines I have drawn, represent the lines of best fit for the averages of each of the different metals. I have not drawn a line of best fit for Aluminium because the points for Aluminium on each of the lines vary over a very large range, and so it would not be accurate. I have also drawn a chart which ...
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When looking at the charts, except for a couple of the anomalous results that have been circled, it is easy to see that metals of the same type are on a very similar horizontal line. The different coloured lines I have drawn, represent the lines of best fit for the averages of each of the different metals. I have not drawn a line of best fit for Aluminium because the points for Aluminium on each of the lines vary over a very large range, and so it would not be accurate. I have also drawn a chart which shows the standard electrode potentials (stated later). The charts which show the results from this experiment do share similarities with the chart representing the standard electrode potentials. For example, the distance between Lead and Tin is very small compared to the distance with tin and zinc. Considering this is the same for each three of the charts, the results must be quite accurate. However, because the results for Aluminium are not accurate, it is difficult to prove how accurate it is. Also, the distance between magnesium and zinc is very large for the standard electrode potentials, where as with both of my experiments (when using sulphuric acid and the grapefruit) this distance is quite small.
It is definitely clear from looking at the charts I drew that the metals are organised in a very similar order to their reactivity. This is the order of reactivity of metals used obtained from 'GCSE Chemistry by Gallagher and Ingram'.
Magnesium
Aluminium
Zinc
Tin
Lead
Copper
The results from my experiment showed the metals appear in this order when drawn on the charts, which proves that the experiment must have been fairly accurate.
The results obtained showed that the further apart two metals are in the reactivity series, the larger voltage is produced. Voltage is also known as electromotive force (e.m.f.) or 'potential difference'. This is the difference in the energy of an electric charge at two different points. One volt is the potential difference between two points if one joule of energy is needed to take one coulomb of charge between the points. One volt of potential difference across a resistance of one ohm produces a current of one ampere. Electric charges flow, if they can, to the point where their energy is lowest so the potential difference, or voltage, is the driving force for currents. When we used two different metals in the experiment, the one that was more reactive would more readily give up electrons and the atoms of that metal would go into the acid as ions. The electrons given up would then flow along the wire to the strip of the less reactive metal.
This is why there would be no voltage given if two of the same metals were used. Because they would have the same reactivity levels, neither would be more readily able to give up electrons, and so no electrons would flow along the wires and therefore no voltage would be produced.
If two metals that are very close in the reactivity series are used in the experiment, there will not be much difference as to which is more readily able to give up electrons. This would mean that the push of electrons would not be so strong, and therefore the voltage would not be so large.
The electrons that flowed along the wire to the less reactive metal join with ions in the solution and form additional metal onto the surface of that metal. The more reactive metal is gradually eaten away (oxidised as it is losing electrons) and the less reactive metal is being reduced (it is gaining electrons). This is why the voltage is higher when the metals used are further apart in the reactivity series. If one of the metals used is very reactive because of its electronic configuration it is much more able to lose electrons and therefore the flow of electrons is faster and a higher voltage is produced. When the other metal is very low, and so they are very far apart in the reactivity series, it is much more ready to gain electrons and so this also helps the electron flow to be increased and the voltage therefore also to be increased.
Although there were quite a few anomalous results obtained during this experiment, the charts clearly show the results in the order of their reactivity and some similarities to the standard electrode potentials in the differences between the points. However, I do not think that my results are significantly accurate to prove the metals electrode potentials. The standard electrode potentials obtained from 'Oxford Chemistry' for the metals used in this investigation are;
Magnesium: -2.6300
Aluminium: -1.6620
Zinc: -0.7628
Tin: -0.1360
Lead: -0.1620
Copper: +0.153
I have made a chart to show these electrode potentials. The differences between the actual electrode potentials and the differences between the results I obtained which are on the charts are not similar enough to make a significant conclusion stating the standard electrode potentials for this experiment.
However, the standard electrode potentials were obtained using standard conditions that were not the same conditions as the method I used during the experiment, which is why there are so many differences.
Evaluation
When we used Zinc and Magnesium in the sulphuric acid, the 2nd set of results (see results table above, one of the red highlighted results), the result was much lower than would be expected. I think that this was because we inserted only a small surface area of the Magnesium. This would have caused fewer ions to flow into the solution as ions and therefore fewer electrons to flow through the wires and so a smaller voltage produced.
The results table above shows that there were many anomalous results involving Aluminium. In most of the cases involving anomalous results concerning Aluminium, I think that the problem was that a layer of Aluminium oxide had formed on the surface of the metal. This would have meant that it was more difficult for the Aluminium to give up its ions into the solution and therefore make it less easy for its electrons to flow through the wires. This would therefore have meant that the push of electrons from Aluminium would have been less and therefore the voltage would have been much less.
Another reason for why there were quite a few anomalous results was because of the surface area of the metal inserted into the electrolyte. We did not measure this and I think that this must have caused the results to not have been so accurate. When a large surface area of metal was inserted into the electrolyte, many atoms went into the solution as ions and the electrons flowed through the wires. However, when we inserted only a small surface area of the metal, not so many of the atoms would go into the solution as ions and therefore, not so many electrons would flow through the wires which would have caused the voltage to have been less. If I repeated this experiment again, I would measure the surface area of the metals to be inserted making sure the areas of each metal were equal.
I repeated the experiment once when using sulphuric acid on most of the readings. The repeated readings are very similar which proves that the experiment must have been fairly accurate.
I think that to have made sure the results were very accurate I should have repeated both experiments at least once. This would then have accounted for most of the anomalous results and I would have had more accurate results that would have made the charts therefore more accurate.
When doing the experiment, I did not remove the oxide layer that could have formed on the metals. This is what caused the results involving Aluminium to be very inaccurate. It effected Aluminium more than the other metals because it forms an extremely stable oxide layer when exposed to oxygen. This caused it to be very difficult to remove the electrons from its aluminium atoms. The oxide layer on the other metals would also have caused the release of electrons to be more difficult. To have avoided this problem, I could have scraped the metals before I performed the experiment, which would have removed the oxide layer and also rid the metal of any impurities that were on its surface.
During the experiment, I did not make sure that I was keeping the temperature constant. If the temperature had increased or decreased this would have affected the voltage. If the temperature was increased, the electrons would vibrate faster and there would have been more collisions with atoms. This could have decreased the speed of the flow of electrons and reduced the voltage. The opposite effect would be produced if the temperature was decreased.
If I did the experiment again, I would have used the standard conditions for the experiment. The 'Heinemann Advanced Science Chemistry' textbook states that the conditions are as follows;
) All measurements are made at 25?C.
2) All solutions are to have unit activity (effectively 1.00 mol dm-3).
3) All measurements are made at 105 Pa pressure.
The diagram above shows how I would set up the experiment when using Zinc and Copper. Instead of having the pieces of metal (the electrodes) in one beaker I would prevent the solutions in each beaker from mixing by separating the metals, one in each beaker. The salt bridge which consists of agar jelly with added potassium chloride (potassium nitrate can be used) allows charge to pass through it to complete the circuit, but limits diffusion of the solution. I would also have used a high resistance voltmeter, which would have given more accurate results.
If I was to repeat this investigation I would also have made more repetitions of my results so I could have come to a better conclusion as to what the electrode potentials were for this experiment.
Eleanor Piercy 10SJ